Stratigraphy of the Sierra Diablo (A. L. Titus, J. E. Harrell, L. L. Lambert)
Geological overview
The Carboniferous stratigraphy of the Sierra Diablo was recently
summarized by Harrell (2007). Virtually every major stratigraphic unit from
the Viséan to the Bashkirian has yielded ammonoids in this region
(Harrell, 2007), with the Viséan to late Serpukhovian section being
particularly fossiliferous and rich in ammonoids. All of the Viséan
material in this report came from the “Folks Member” of the Mississippian–Pennsylvanian Barnett Shale, which is exposed in only three places in
the northeast portion of the Sierra Diablo below an imposing
escarpment formed out of the Permian Hueco Formation (King, 1965), entirely
within the Figure 2 Ranch property. Although generally homoclinal and
south-dipping, the Folks Member is locally faulted, folded, and intruded by
small dikes and sills which caused local contact metamorphism. The entire
pre-Permian section was folded, faulted, and eroded into a peneplain during
the Ouachita Orogeny before being covered by the sediments of the Permian
Paw Paw Formation, which along with the Hueco Formation is the source for
most of the erosional debris that covers the non-resistant Folks Member.
The Folks Member is comprised mostly of mudstone and siltstone, with
prominent carbonate concretions, phosphatic pebble-rich lags, and
thin-bedded pelagic micritic limestone that all point towards a largely
sediment-starved depositional regime during the Late Viséan and early
Serpukhovian (Titus, 1999). This character is generally similar to that of
time-equivalent strata of the Barnett Shale exposed in the Llano Uplift,
which is even more condensed than the Sierra Diablo section. The Folks
Member is abundantly fossiliferous and yields a largely pelagic fauna, but
also a dwarf, low diversity benthic assemblage of gastropods and bivalves in
the phosphate pebble lag horizons. Well-preserved, uncrushed ammonoids occur
through much of the unit, preserved either as testiferous moulds in carbonate
concretions or beds, or as phosphatic steinkerns in the pebble lag rich
shales. Although there are three sections now known to yield Viséan
ammonoids in the northern Sierra Diablo (Harrell, 2007) only one
of these, herein referred to as King's section, had been studied and
sampled extensively enough to allow for detailed analysis when all access to
the Figure 2 Ranch to researchers was ended. Fortunately the other two
sections do not appear to preserve any fauna or significant horizons not
seen at King's section.
King's section
Our King's section (Fig. 2) is the same as Section 11 published by King (1965)
and from which all of the Mississippian ammonoids in his Barnett Shale
faunal list came from. The beds at King's section dip northeast about 15∘. Exposures through most of the approximate 46 m of Barnett
Shale section here are excellent.
The start of our measured section is near the slope break, in a drainage
located at UTM coordinates (NAD27): 13R 0512497E; 3482258N
(31.47504∘ N, 104.86844∘ W). The covered interval just
below our bed 1 has scattered pieces of large ex situ ammonoid-bearing
concretions. These concretion fragments were derived from our beds 9, 11,
and 13. The only ammonoids we collected from these loose pieces were
specimens of Goniatites eganensis Korn and Titus, 2011. The lowest 7 m of exposed
section did not yield in situ three-dimensional ammonoids, only rare
leiorhynchid and lingulid brachiopods and Nereites-type trace fossils.
Seven metres above the base of the section are three separate horizons of
1–1.5 m diameter concretions less than a half metre apart from each
other stratigraphically. The lowest of these three concretionary horizons
(bed 9) yielded an abundant ammonoid fauna dominated by the crenistriate
species G. eganensis, but also containing Girtyoceras hamiltonense Korn and Titus, 2011 (samples 00TXCU-21a and
00TXCU-21b). The next concretion horizon (bed 11) yielded uncommon specimens
of G. eganensis (sample 00TXUT-22), while the highest concretionary horizon (bed 13) in
this immediate grouping did not yield any diagnostic material.
Approximately 2 m higher is another single band of elliptical 0.3 m
diameter concretions (bed 15) that we obtained no diagnostic specimens from.
Above the bed 15 concretions is 2.5 m of non-concretionary
shale/siltstone, which ends at beds 17–19, two closely spaced concretionary
horizons separated by 40 cm of siltstone. The concretions of bed 17 yielded
an assemblage of Goniatites multiliratus Gordon, 1962 and Girtyoceras meslerianum (Girty, 1909) (sample 00TXCU-25).
Although we cannot make out the suture lines on the Goniatites specimens from this
horizon, we are confident of its assignment because its ornament is highly
diagnostic given other biostratigraphic constraints on the sample, which
include the co-occurring Girtyoceras. The upper concretionary horizon (bed 19), which
is highly septarian, did not yield diagnostic material.
The next 4 m of section is a highly condensed, fossiliferous
succession of glauconitic nodular phosphatic mudstone punctuated by a thin
oxidized silty/sandy bed (bed 21a) and three limonitic iron bands (beds 22,
24, 26). The phosphatic intervals are sediment-starved hardgrounds, some of
which exhibit extensive recycling. Phosphatic–glauconitic steinkerns of
Choctawites cumminsi (Hyatt, 1893) and Pachylyroceras cloudi (Miller and Youngquist, 1948) are common (sample
00TXCU-27), and occur with an impoverished benthic molluscan fauna that was
not collected. The highest iron band hardground (bed 26) is overlain by 1.3 m of phosphatic shale, which yielded no fossils.
Three thin, nodular packstone beds (collectively bed 28) only centimetres
apart yielded, along with the bivalve Caneyella wapanuckensis Girty,
1909, the following ammonoid taxa: Beyrichoceratoides sp.,
Tumulites cf. T. varians McCaleb et al., 1964,
Fayettevillea sp., Paracravenoceras cf. P. barnettense (Plummer and Scott, 1937) and ?Cravenoceras sp.
Large, but partially crushed limonite pseudomorph steinkerns of
Paracravenoceras ozarkense Gordon, 1960 and Syngastrioceras scotti (Miller and Youngquist, 1948) (sample 00TXCU-29) occur in the 1.6 m of shale above bed 28. Capping the 1.6 m thick shale interval is a
prominent thin yellow siltstone (bed 30) that is very similar to bed 21. Bed
30 is immediately overlain a 0.3 m thick bed (bed 31) that contains
phosphate clasts up to 3 cm in diameter.
Global correlation chart for upper Viséan and lowest
Serpukhovian ammonoid zones; after Ruzhencev and Bogoslovskaya
(1971), Korn and Horn (1996), Korn et al. (2007), Korn and Kaufmann (2009) and Korn and Titus (2011).
Two metres higher is a couplet of concretionary horizons bearing striate
goniatitids that look virtually identical to beds 17 and 19, but in reverse
stratigraphic order, with the septarian horizon (bed 35) being the lower of
the two. Although it was not detected in 2000 when the section was measured,
the ammonoid evidence, coupled with the repetition of the distinctive yellow
siltstone horizon and concretionary beds, seems to suggest there is a small-scale structural repetition of this portion of the section.
The highest ammonoids recovered from King's section are from bed 37, which
probably due to structural repetition, contains the same assemblage found in
bed 17. Because we did not observe an obvious structural break in the
exposed section, there is a small chance that beds 35–37 are in correct
stratigraphic position and therefore Serpukhovian in age. If this is the
case, then the lirate goniatitids probably belong to Lusitanoceras, which are known
from lower Serpukhovian strata of the Antler and Marathon–Ouachita foreland
basin systems (e.g. Drahovzal and Quinn, 1972). Unfortunately, without
sutural evidence we cannot assign specimens from bed 37 to a genus with
certainty and our bias, given the lithologic repetition and lack of
associated cravenoceratids or glaphyritids, is the structural hypothesis.
Beds 43–45 are not seen in any of the other Sierra Diablo sections and the
remaining 22 m of section up to the angular unconformity with the
overlying Paw Paw Formation is clearly from shallower water facies. Aside
from some sparse crinozoan debris, this upper interval lacks obvious
macrofossils.
Biostratigraphic interpretation
Late Viséan. – The ammonoids Goniatites eganensis and Girtyoceras hamiltonense were discovered from the base of the
measured section, as well as other localities to the south, indicating that
the Goniatites eganensis Biozone, which was first delineated in the Chainman Shale of Utah and
Nevada (Korn and Titus, 2011), is the lowest in the Sierra Diablo Barnett
section that can be diagnosed with in situ material (Fig. 2). Specimens of
Goniatites multiliratus from bed 17 indicate that the zone for that species can also be diagnosed
in that general part of the section. This same general succession can be
observed entirely, or in part, in the Chainman Shale of Nevada and Utah, the
Caney Shale of Oklahoma, and Moorefield Formation of Arkansas (Korn and
Titus, 2011).
The highest Viséan assemblages of the Sierra Diablo (from beds
23–25) contain Choctawites cumminsi and Pachylyroceras
cloudi. Previously, the entire post-Goniatites multiliratus
interval has been lumped into a single “Goniatites granosus” (now
referred to as Lusitanoceras Pereira de Sousa, 1923) Biozone (Gordon,
1965; Saunders et al., 1977; Webster et al., 1984). It is now clear to us
that both the taxonomy and biostratigraphy of this interval in North America
is much more complicated and can be subdivided in the eastern USA using
species of the characteristic genus Choctawites. As Ch. cumminsi is particularly common in Texas, we use it to name a new ammonoid
biozone, whose reference locality is the Chappel Hill section of the Barnett
Shale, San Saba, Texas. It is defined as the interval between the first
occurrence of the nominate taxon and the first appearance of
Cravenoceras. Associated ammonoids in Texas include
Sulcogirtyoceras ornatissimum, Pachylyroceras sp.,
Lusitanites sp., and various dimorphoceratids. At present, no
unequivocal younger Late Viséan ammonoid zones can be defined in North
America (Fig. 3), although the position of Lyrogoniatites georgiensis would presumably fall into this interval.
Although not discernible in the Sierra Diablo section, we also erect the
Choctawites kentuckiensis Biozone with the Buffalo Wallow reference
section (Moorefield Shale; east of Batesville, Arkansas; Malinky and Mapes,
1982) for the interval between the first appearance of that taxon and the
appearance of Choctawites choctawensis. Key North American first
occurrences of ammonoids in this zone include Sulcogirtyoceras (as
S. limatum), Lusitanoceras, Neoglyphioceras,
Lusitanites, and Ruddellites (Gordon, 1965; Malinky and
Mapes, 1982). The interval between the Ch. kentuckiensis and
Ch. cumminsi biozones contains Choctawites choctawensis,
Neoglyphioceras caneyanum, Lusitanites sp.,
Sulcogirtyoceras cf. S. ornatissimum, and dimorphoceratids.
Based on this assemblage, we establish the Choctawites choctawensis
Biozone with Girty's station 2078 as the reference section (Caney Shale;
Pittsburg County, Oklahoma; Branson et al., 1959), defined as the interval
between that taxon's first appearance (Fig. 3) and the appearance of
Choctawites cumminsi.
Serpukhovian. – Although not the focus of this paper, Serpukhovian ammonoids
are abundant in the Sierra Diablo sections and it is clear that a
succession of multiple zones could be recognized at least as high as the
Reticuloceras Genus zone (Harrell, 2007). Earliest Serpukhovian
ammonoids are discussed here to give the Late Viséan succession a better
context. However, no high-resolution zonation for lower Serpukhovian
ammonoids equivalent to the Pendleian Stage of the British Isles currently
exists for North America and establishing one is beyond the scope of this
work. The oldest Serpukhovian ammonoids collected from the Sierra Diablo
(Tumulites, Paracravenoceras, Fayettevillea, Beyrichoceratoides, Eosyngastrioceras) are from beds 28–29 and are almost
identical to those found in the middle and upper Fayetteville Shale of
northern Arkansas. Collectively, they are diagnosed as the “Tumulites varians–Cravenoceras fayettevillae” zone of Saunders et al. (1977), probably
equivalent to the British E1b. As might be expected in a deeper-water
offshore section, earliest Serpukhovian assemblages (equaling British E1a)
characterized elsewhere in North America by a distinctive
Girtyoceras–Cravenoceras sensu stricto assemblage are not
preserved as three-dimensional material in the Sierra Diablo area.
Global correlations
High-resolution global correlations for the Late Viséan (Asbian
and Brigantian in the British subdivision scheme) ammonoids of North America
were proposed by Korn and Titus (2011). Our work on the Sierra Diablo
sections has not substantively changed this earlier interpretation and no
further discussion on correlation of pre-Choctawites kentuckiensis Biozone assemblages is warranted.
Choctawites cumminsi Biozone – Because the genus
Choctawites is endemic to North America, direct correlations are
difficult. However, Sulcogirtyoceras associated with
Choctawites cumminsi at San Saba (S. ornatissimum),
Texas, and Choctawites choctawensis (S. cf. S. ornatissimum)
in the Caney Shale of Oklahoma are very similar to the Rhenish Mountain
species S. burhennei, which co-occurs with Paraglyphioceras rotundum. In both Europe and North America, the earliest occurrence of
Neoglyphioceras is in the immediately underlying assemblage. We
think the similar pattern of occurrence for these two distinctive morphotypes
forms a better basis for correlation than the species or genus ranges of
various goniatitid taxa such as Lusitanoceras and
Dombarites, which appear to occur diachronously in various basins.
As a result, we equate the occurrence of Neoglyphioceras in the
Choctawites kentuckiensis assemblage of the Moorefield Formation
with the Neoglyphioceras spirale zone of western Europe (Fig. 3).
The overlying Choctawites choctawensis and Choctawites cumminsi zones thus correlate with the Paraglyphioceras rotundum
and Neoglyphioceras suerlandense–Lusitanoceras postriatum
intervals of the Rhenish Mountains sections (Fig. 3). We believe this
correlation is the most parsimonious solution as it synchronizes the first
appearance of S. burhennei-type Sulcogirtyoceras,
Neoglyphioceras, and advanced forms of more openly umbilicate
neoglyphioceratids like N. suerlandense (very similar to the more
evolute North American genus Pachylyroceras). Unfortunately
Choctawites is very rare in the US cordilleran region. Specimens
tentatively referable to Ch. cumminsi are known from the Chainman
Shale in west Utah, occurring in the upper portion of the range of
Lusitanoceras, associated with Lusitanoceras,
Lusitanites, Sulcogirtyoceras ornatissimum, and
Pachylyroceras utahense.
Descriptive terms for the conch morphology and suture
lines of the ammonoids described here.
Conch dimensions (in millimetres) and proportions for Girtyoceras
hamiltonense Korn and Titus, 2011, from the Sierra Diablo.
Specimen
dm
ww
wh
uw
ah
ww / dm
ww / wh
uw / dm
WER
NPL 68361
21.9
9.1
10.9
4.0
7.0
0.42
0.83
0.18
1.94
same
17.3
8.4
8.5
3.6
–
0.49
0.99
0.21
–
NPL 68362
14.8
6.7
6.9
3.7
–
0.45
0.97
0.25
–
UTSA 07062
10.7
5.3
4.3
3.0
–
0.50
1.23
0.28
–
Systematic descriptions (D. Korn, A. L. Titus)
Descriptive terminology for conch morphology is patterned after Korn (2010).
Abbreviations of conch dimensions (Fig. 4) are conch diameter (dm), whorl
width (ww), whorl height (wh), umbilical width (uw =
dm1 - wh1 - wh2) and aperture height (ah). The whorl expansion
rate (WER) is calculated using the algorithm of Korn and Klug (2007): WER =
[dm / (dm - ah)]2 or (dm1 / dm2)2. The imprint zone rate
(IZR) characterizes the whorl overlap, and it can be calculated by IZR =
(wh - ah) / wh. The terminology of the suture lines is taken from Korn et
al. (2003).
The material described in the following account is held in the following
collections with abbreviations as follows:
NPL – Non-vertebrate Paleontology Lab, The University of Texas in
AustinMB.C. – Cephalopod collection in the Museum für Naturkunde
BerlinUSNM – U.S. National Museum, Washington, D.C.SUI – State University of Iowa, Iowa City
Order Goniatitida Hyatt, 1884Suborder Goniatitina Hyatt, 1884Superfamily Girtyocerataceae Wedekind, 1918
Family Girtyoceratidae Wedekind, 1918Family diagnosis: Girtyocerataceae with ontogenetically changing conch
shape; inner whorls evolute, umbilicus closing during ontogeny, adult stage
often discoidal and oxyconic. Ornament with biconvex growth lines. Suture
line with V-shaped external lobe and rounded ventrolateral saddle; adventive
lobe V-shaped with slightly sinuous flanks.
Remark: For the genus composition of the family and a discussion of the
characteristics and limits, see Korn and Titus (2011).
Girtyoceras Wedekind, 1918
Type species: Adelphoceras meslerianum Girty, 1909 (original designation).
Genus diagnosis: Girtyoceratidae with moderately large conch that during
ontogeny passes through very different morphological stages: early juveniles
are widely umbilicate with crescent-shaped whorl cross section, later
thickly discoidal or pachyconic with subangular or rounded umbilical margin.
The intermediate stage has rounded flanks and venter and the adult stage is
lenticular and oxyconic. Without or with very faint ventrolateral grooves.
Suture line with V-shaped external lobe, moderately high median saddle,
narrowly rounded ventrolateral saddle, and V-shaped adventive lobe.
Remark: For the species composition of the genus and a discussion of the
characteristics and limits, see Korn and Titus (2011).
Girtyoceras
hamiltonense Korn and Titus, 2011 (Fig. 5)
1949 Girtyoceras aff. G. meslerianum. – Miller et al., p. 608, pl. 99, figs. 9,
10.1964 Girtyoceras meslerianum. – McCaleb et al., p. 13 (part), pl. 2, figs. 8,
9.2011 Girtyoceras hamiltonense Korn and Titus, p. 150, text-figs. 30, 31 [for more
synonymy].
Holotype: Specimen UMNH IP 3127; illustrated by Korn and Titus (2011,
fig. 30B).
Girtyoceras hamiltonense Korn and Titus, 2011 from the Figure 2 Ranch section,
Sierra Diablo. Specimen NPL 68361 from bed 9 (sample 00TXCU-21b);
× 2.5.
Type locality and horizon: Hamilton Canyon, bed 44; Camp Canyon Member of
Chainman Formation, Goniatites eganensis Biozone.
Material: Four specimens, NPL 68361 (sample OOTXCU-21q), NPL 68362 (sample
OOTXCU-21b), NPL 68363 (sample OOTXCU-21a), UTSA 07062.
Diagnosis: For a complete diagnosis see Korn and Titus (2011).
Description: The largest specimen, nearly 22 mm conch diameter
(Table 1), is almost fully ornamented and has a thinly discoidal,
subinvolute conch shape (ww / dm = 0.42; uw / dm = 0.18). At the largest
diameter, it possesses an angular umbilical margin, subparallel flanks, and
a narrowly rounded venter. The fine growth lines are strongly biconvex with
a prominent ventrolateral projection and a deep ventral sinus. The last
volution displays four shell constrictions extending parallel to the growth
lines. They originate just outside the umbilical margin on the inner
flank area; they are deepest on the mid-flank but continue across the venter
(Fig. 5).
The next smaller specimen (NPL 68362) has a conch diameter of nearly 15 mm with
similar conch proportions (ww / dm = 0.45; uw / dm = 0.25). It is a partly
testiferous internal mould with predepositional breakage to the living
chamber. There are four constrictions, which are, as in all specimens,
visible externally. The fine rib density is about 12 per millimetre.
The smallest specimen (UTSA 07062) is approximately 10 mm in diameter,
thickly discoidal, and broken obliquely, but nearly in half, with the other
half missing. The umbilical shoulders are angular and about 90∘.
The umbilical width index is entirely within a narrow range. Ribs and constrictions are moderately sinuous, and just start to
develop a ventral sinus at about 8 mm diameter. There are three
constrictions.
Discussion: G. hamiltonense is the stratigraphically oldest Girtyoceras species currently known from
the Sierra Diablo section. It resembles G. gordoni Korn and Titus, 2011, G. welleri Gordon,
1965, and G. meslerianum (Girty, 1909). However, G. gordoni has less sinuous ribs and constrictions
at comparable diameters, while G. welleri has a slightly narrower umbilicus between 6
and 20 mm conch diameter and a more narrowly rounded ventral profile at
diameters between 6 and 15 mm. G. hamiltonense is now known from Nevada and Utah (Chainman
Shale) and the Sierra Diablo (bed 9). The near time-equivalent
species G. welleri occurs in the lower portion of Moorefield Formation near Batesville,
Arkansas, and the Caney Shale of the Arbuckle Mountains region. In every
case, these eastern occurrences are the oldest known horizons with a
Goniatites assemblage at their respective localities.
Conch ontogeny of Girtyoceras meslerianum (Girty, 1909); based on material from
Jackfork Creek, Oklahoma (Fig. 7).
dm
Conch shape
Whorl cross section shape
Aperture
1.5 mm
thickly discoidal; subevolute (ww / dm = 0.45–0.60; uw / dm = 0.40–0.45)
moderately depressed; moderately embracing (ww / wh 1.80–2.00; IZR = 0.20–0.25)
low (WER ∼ 1.65)
5 mm
thickly discoidal; evolute (ww / dm = 0.55–0.50; uw / dm = 0.50–0.55)
strongly depressed; moderately embracing (ww / wh = 2.25–2.50; IZR = 0.25–0.30)
low (WER = 1.50–1.60)
10 mm
thickly discoidal; subevolute (ww / dm = 0.50; uw / dm = 0.35)
weakly depressed; strongly embracing (ww / wh = 1.25; IZR = 0.35–0.40)
moderate (WER = 1.75–1.90)
20 mm
thinly discoidal; subinvolute (ww / dm ∼ 0.45; uw / dm ∼ 0.20)
weakly compressed; strongly embracing (ww / wh = 0.80; IZR ∼ 0.40)
high (WER ∼ 2.10)
Girtyoceras meslerianum (Girty, 1909) from the Figure 2 Ranch section, Sierra
Diablo. Specimen NPL 68364 from bed 17 (sample 00TXCU-25); × 4.0.
Girtyoceras meslerianum (Girty, 1909), conch cross sections and ontogenetic
trajectories of material from the Caney Shale of Jackfork Creek near Ada,
Oklahoma (a–d). (a) Specimen MB.C.25466; × 2.5. (b) Specimen
MB.C.25467; × 2.5. (c) Specimen MB.C.25468; × 2.5. (d)
Specimen MB.C.25469; × 2.5. (e–g), ontogenetic development of the
conch width index (ww / dm), umbilical width index (uw / dm), and WER of the sectioned specimens and type material.
Girtyoceras meslerianum (Girty, 1909) (Figs. 6 and 7)
1909 Adelphoceras meslerianum. – Girty, p. 66, pl. 12, figs.
1–3c.1918 Girtyoceras meslerianum. – Wedekind, p.
140.1964 Girtyoceras meslerianum. – McCaleb et al., p. 13 (part), pl. 1, figs.
4–6.1965 Girtyoceras meslerianum. – Gordon (part), pl. 24, Figs. 17–22, 30–32, text-figs. 63A, B,
64D.
Holotype: Specimen USNM 119594; illustrated by Girty (1909, pl. 12,
figs. 1–1c).
Type locality and horizon: USGS locality 2083, Sec. 14, T. 2 S., R. 7 E.,
northwest of Wapanucka, Johnston County, Oklahoma; Delaware Creek Member of
Caney Shale, Goniatites multiliratus Biozone.
Conch dimensions (in millimetres) and proportions for Girtyoceras meslerianum (Girty,
1909) from the type area and the Sierra Diablo.
Specimen
dm
ww
wh
uw
ah
ww / dm
ww / wh
uw / dm
WER
IZR
USNM 119594 (holotype)
16.0
6.0
7.6
3.9
4.6
0.38
0.79
0.24
2.00
0.40
NPL 68364
9.5
5.1
4.5
3.1
2.5
0.54
1.10
0.33
1.85
0.44
NPL 68365
6.8
3.8
1.9
2.5
–
0.56
1.90
0.37
–
–
Material: Three immature specimens (NPL 68364 through NPL 68366) from bed 25
(sample 00TXCU-25).
Diagnosis: Species of Girtyoceras with a thickly discoidal, widely umbilicate conch at
6–8 mm diameter (ww / dm = 0.50–0.60; uw / dm = 0.45–0.55), and a thinly
discoidal, narrowly umbilicate conch at 20 mm diameter (ww / dm = 0.45;
uw / dm = 0.20). Whorl cross section strongly depressed in juveniles (5 mm
diameter) with rounded trapezoidal whorl cross section, followed by an
intermediate stage with rounded venter and angular umbilical margin (above 7 mm diameter), and a discoidal stage (above 20 mm diameter) with acute
venter. Ornament with sharp riblets in the juvenile stage; coarse
prorsiradiate growth lines with concavo-convex course in the intermediate
stage and fine biconvex growth lines in the adult stage. Strong shell
constrictions in intermediate growth stages.
Description: The three specimens are all mostly testiferous. No sutures are
visible. The specimen that represents the largest diameter (NPL 68366) is
only a whorl fragment and poorly preserved. However, it represents a
specimen that is clearly advanced over NPL 68364 (at least 12 mm in
diameter) and shows the prominent tongue-shaped salient formed in the ribs
and constrictions that are typical of this stage.
Conch dimensions (in millimetres) and proportions for reference
specimens of Goniatites eganensis Korn and Titus, 2011 from the Sierra Diablo.
Specimen
dm
ww
wh
uw
ah
ww / dm
ww / wh
uw / dm
WER
IZR
NPL 68431
47.3
31.7
23.9
2.9
10.6
0.67
1.33
0.06
1.66
0.56
NPL 68471
37.5
26.4
18.4
2.0
7.1
0.70
1.43
0.05
1.52
0.61
NPL 68493
26.9
21.5
13.8
1.9
5.7
0.80
1.56
0.07
1.61
0.59
NPL 68389
18.4
15.5
9.7
0.9
3.9
0.84
1.60
0.05
1.61
0.60
The better preserved of the two larger specimens (NPL 68364) is nearly 10 mm
in diameter (Fig. 6). By 8 mm diameter the umbilical margin forms a strong
right angle and the ribs and constrictions (which number four per whorl, are
moderately deep and visible on both the internal moulds and external shell)
are very strongly deflected anteriorly. The ribs and constrictions also form
a distinct ventral sulcus at this stage. Faint longitudinal lines can be
seen in the ventral area.
The smallest specimen (NPL 68365) is transitioning from the widely
umbilicate acutely marginated form to the almost parallel-sided thickly
discoidal form with a 90∘ umbilical shoulder. The flanks
and venter are gently rounded. Fine thread-like riblets originate from the
umbilical margin and deflect anteriorly at the ventrolateral margin. The
ribs are straight across the venter.
Discussion: Girtyoceras meslerianum is a rare taxon in the Sierra Diablo sections mostly because
well-preserved three-dimensional material is difficult to obtain from the
concretions that bear its assemblage. Although interpreted rather broadly in
the past, in our view, G. meslerianum is a morphological intermediate between G. hamiltonense and the
stratigraphically younger taxon Sulcogirtyoceras limatum. It bears the more compressed conch form,
strongly developed anterior deflection of the ribs and constrictions on the
ventrolateral shoulder, and wider umbilicus of the latter (uw / dm = 0.31),
but has the weaker ornament and lack of ventrolateral sulcus of the former.
Thus G. meslerianum might be confused with G. hamiltonense and small specimens (smaller than 12 mm) of
S. limatum. However, G. meslerianum can be readily differentiated from G. hamiltonense because the latter has a
consistently narrower umbilicus (uw / dm = 0.25 at 16 mm diameter). Early
members of the genus Sulcogirtyoceras develop a prominent ventrolateral sulcus at
approximately 12–15 mm diameter, but also have slightly stronger ribs and
constrictions at comparable stages (less than 20 mm diameter). Smaller
specimens of more advanced species of Sulcogirtyoceras have more depressed whorls.
Species that are stratigraphically older than G. hamiltonense in North America have less
sinuous constrictions and narrower umbilici at comparable diameters (e.g.
G. gordoni), while younger Serpukhovian species (e.g. Girtyoceras jasperense Gordon) are more narrowly
umbilicate and more robust (higher-value ww / wh ratios). Girtyoceras meslerianum appears to be
limited to the Goniatites multiliratus Biozone in both the mid-continent USA and the western
Cordillera (Chainman Shale). It is rather uncommon outside of its type area,
with the only other definitive occurrences we know of at San Saba, Texas in
the base of the Barnett Formation and in the Chainman Shale at the classic
Duckwater locality (not mentioned by Korn and Titus, 2011). It should occur
in the Goniatites multiliratus Biozone of the upper Pool Creek Member of the Moorefield Formation,
but specimens from this interval are all crushed (Gordon, 1965).
Superfamily Goniatitaceae de Haan, 1825Family Goniatitidae de Haan, 1825
Goniatites eganensis Korn and Titus, 2011 from the Figure 2 Ranch section,
Sierra Diablo. (a) Specimen NPL 68431 from bed 9 (sample OOTXCU-21b);
× 1.0. (b) Specimen NPL 68471 from bed 9 (sample OOTXCU-21b);
× 1.25. (c) Specimen NPL 68493 from bed 11 (sample OOTXCU-22);
× 1.75. (d) Specimen NPL 68389 from bed 9 (sample OOTXCU-21a);
× 2.5.
For the subfamily composition of the family and a discussion of the
characteristics and limits, see Korn and Ebbighausen (2008) and Korn et
al. (2010).
Subfamily Goniatitinae de Haan, 1825
For the genus composition of the subfamily and a discussion of the
characteristics and limits, see Korn and Titus (2011).
Goniatites de Haan, 1825
Type species: Conchiliolithus Nautilites (sphaericus) Martin, 1809 [nomen nudum], equal to Ammonites sphaericus Sowerby, 1814 (Opinion 420 ICZN,
1956).
Diagnosis: Goniatitinae with subinvolute inner whorls; the umbilicus becomes
closed in early ontogeny. External lobe usually V-shaped and rarely
Y-shaped, very narrow or narrow, usually with slightly curved flanks.
For the species composition of the genus and a discussion of the
characteristics and limits, see Korn and Titus (2011).
Goniatites eganensis
Korn and Titus, 2011 (Fig. 8)
2011 Goniatites eganensis Korn and Titus, p. 175, text-figs. 51–54 [for more
synonymy].
Holotype: Specimen UMNH IP 4021; illustrated by Korn and Titus (2011, fig. 51B).
Type locality and horizon: Trough Springs Canyon, bed 6 (sample 93NVLI-3);
Camp Canyon Member of Chainman Formation, Goniatites eganensis Biozone.
Material: 127 specimens with conch diameters up to 48 mm from samples
00TXCU-20a–00TXCU-20f; 00TXCU-21a, and 00TXCU-21b.
Diagnosis: Goniatites with a globular conch between 2 and 8 mm diameter (ww / dm
0.85–0.90) and thickly pachyconic conch (ww / dm 0.75–0.80) at 20 mm
diameter. Umbilicus moderate in early ontogeny (uw / dm = 0.20–0.30 at 2 mm
diameter) and very narrow in all stages larger than 4 mm diameter (uw / dm
0.02–0.10). Umbilical wall convexly rounded in all stages. Aperture low or
moderately high (WER = 1.70–1.80 at 20 mm diameter). Ornament with
crenulated, slightly biconvex, and rectiradiate growth lines; external sinus
shallow. Suture line at 25 mm conch diameter with narrow external lobe (0.55
of the external lobe depth; 1.25 of the ventrolateral saddle width), and
moderately low median saddle (0.40 of the external lobe depth). External
lobe V-shaped with narrow, V-shaped prongs; ventrolateral saddle subangular.
Description: Four specimens are selected to outline the morphology of the
material from the Sierra Diablo. Unfortunately, inner whorls are
largely recrystallized and thus do not contain solid information about the
conch ontogeny.
Specimen NPL 68431 is a fully chambered specimen of 47 mm conch diameter; it
is representative for a larger growth stage (Fig. 8a). The pachyconic conch
(ww / dm = 0.67; uw / dm = 0.06) is very narrowly umbilicate and possesses a
broadly rounded, wide venter and a steep umbilical wall. Parts of the
specimen are covered with shell remains. These show fine and crenulated
growth lines, which extend with weakly biconvex course across flanks and
venter, forming equally high dorsolateral and ventrolateral projections and
shallow lateral and ventral sinuses.
The smaller specimen NPL 68471 (38 mm dm) shows very similar conch
proportions but differs in the slightly coarser, irregularly spaced growth
lines (Fig. 8b). Specimens NPL 68493 (27 mm dm) and NPL 68389 (18.4 mm dm)
are thickly pachyconic conchs with stronger depressed whorl cross section
(Fig. 8c, d). The larger of the two possesses well-preserved shell ornament
with biconvex, strongly crenulated growth lines. These form a shallow
lateral sinus and a very shallow ventral sinus. The smaller of the two
possesses weakly crenulated growth lines standing in wide distances.
Goniatites multiliratus Gordon, 1962 from the Figure 2 Ranch section, Sierra
Diablo. Specimen NPL 68494 from bed 17 (sample OOTXCU-25); × 2.0.
Goniatites multiliratus
Gordon, 1962 (Fig. 9)
1909 Goniatites choctawensis. – Shumard, 1863; Girty, p. 59 (part), pl. 13,
fig. 1.1962 Goniatites multiliratus Gordon, p. 356,
text-fig. 1A.1964 Goniatites choctawensis. – Branson, figs. 1–2.1965 Goniatites multiliratus. – Gordon, p. 186, pl. 18, figs. 8–12 [for more
synonymy].
Holotype: Specimen USNM 119499; illustrated by Gordon (1965, pl. 18, figs. 9–12).
Type locality and horizon: USGS locality 6619A, 2 km south of Buckhorn
(Murray County, Oklahoma); Caney Shale, Goniatites multiliratus Biozone.
Material: Three specimens (NPL 68494 through NPL 68496 from sample
00TXCU-25).
Character matrix of the genera within the subfamily
Dombaritinae for the cladistic analysis presented here.
0. Conch ontogeny of whorl width index: biphasic (0), triphasic (1)
1. Distance of prongs of external lobe: very short (0), moderate or wide (1)
2. Median saddle higher than 45% of ventrolateral saddle: no (0), yes (1)
3. Median saddle higher than 55% of ventrolateral saddle: no (0), yes (1)
4. Median saddle higher than 65% of ventrolateral saddle: no (0), yes (1)
5. Median saddle higher than 75% of ventrolateral saddle: no (0), yes (1)
6. Median saddle higher than 85% of ventrolateral saddle: no (0), yes (1)
7. External lobe V-shaped (0), Y-shaped (1)
8. Ventrolateral saddle subacute: no (0), acute (1)
9. Ventrolateral saddle tectiform: no (0), yes (1)
10. External lobe wider than 0.60: no (0), yes (1)
11. External lobe wider than 0.85: no (0), yes (1)
12. External lobe wider than 1.10: no (0), yes (1)
13. External lobe wider than 1.35: no (0), yes (1)
14. External lobe with tridentate branches: no (0), yes (1)
15. Adventive lobe: flanks simple curved (0), sinuous or with deflexion (1)
16. Adventive lobe: flanks not angular (0), angular or tridentate (1)
17. Adventive lobe tridentate: no (0), yes (1)
18. Inner whorls involute (0), subevolute or evolute (1)
19. Drastic size reduction of the conch: no (0), yes (1)
20. Whorl expansion rate: low (0), moderate (1)
21. Preadult whorls triangularly coiled: no (0), yes (1)
22. Apertural shape in juveniles: rectiradiate (0), prorsiradiate (1)
23. Adult conch shape pachyconic or globular (0), discoidal (1)
24. Shell constrictions in intermediate stage: no (0), yes (1)
25. Course of growth lines: with projections and sinuses (0), linear (1)
Progoniatites pilus
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
?
0
Goniatites crenistria
1
1
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Arnsbergites falcatus
1
1
1
0
0
0
0
0
0
0
1
0
0
0
0
1
0
0
1
0
0
0
1
0
0
0
Hibernicoceras hibernicum
1
1
1
0
0
0
0
0
0
0
1
0
0
0
0
1
0
0
1
0
0
0
1
0
0
0
Paraglyphioceras rotundum
1
1
1
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
Lusitanoceras poststriatum
1
1
1
0
0
0
0
1
0
1
1
0
0
0
0
1
0
0
1
0
0
1
1
0
1
0
Goniatitella agricola
?
1
0
0
0
0
0
?
0
0
0
0
0
0
0
1
0
0
0
1
1
0
0
0
0
1
Neogoniatites milleri
1
1
1
1
1
0
0
0
0
0
1
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
Hypergoniatites exiguus
1
0
0
0
0
0
0
1
1
0
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
Dombarites tectus
1
1
1
1
1
1
0
1
1
1
1
1
0
0
0
1
1
0
1
0
0
0
1
0
1
0
Proshumardites delepinei
1
1
1
1
1
1
1
1
0
1
1
1
0
0
0
1
1
1
1
0
0
1
1
0
1
0
Platygoniatites molaris
1
1
1
1
1
1
0
1
1
1
1
1
1
0
0
1
0
0
1
0
0
1
1
1
1
0
Delepinoceras bressoni
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
0
0
1
0
0
1
1
1
1
0
Choctawites choctawensis
1
1
1
1
0
0
0
1
0
1
1
0
0
0
0
1
1
0
1
0
0
0
1
0
0
0
Revilloceras granofalcatum
1
1
1
1
0
0
0
1
1
1
1
0
0
0
0
1
0
0
1
0
0
1
1
0
1
0
Dombarites falcatoides
1
1
1
1
1
0
0
1
1
1
1
0
0
0
0
1
0
0
1
0
0
1
1
0
1
0
Cladogram of selected genera, represented by well-known
species, of the superfamily Goniatitaceae.
Diagnosis: Goniatites with a globular conch between 2 and 8 mm diameter (ww / dm
0.85–1.10) and thickly pachyconic to globular conch (ww / dm 0.80–0.85) at
20 mm diameter. Umbilicus moderately wide in early ontogeny (uw / dm =
0.30–0.35 at 2 mm diameter) and very narrow in all stages larger than 4 mm
diameter (uw / dm 0.02–0.10). Umbilical wall convexly rounded in all stages.
Aperture low or moderately high (WER = 1.60–1.80 at 20 mm diameter).
Ornament with more than 200 spiral lines and crenulated, slightly biconvex,
and rectiradiate growth lines; external sinus shallow. Suture line at 25 mm
conch diameter with narrow external lobe (0.55 of the external lobe depth;
1.00 of the ventrolateral saddle width), and moderately low median saddle
(0.45 of the external lobe depth). External lobe Y-shaped with very narrow,
V-shaped prongs; ventrolateral saddle subacute.
Description: The largest specimen (NPL 68494) has a partly crushed body
chamber, but the phragmocone appears to be uncrushed (Fig. 9). The conch is,
at 30 mm diameter, thinly pachyconic (ww / dm = 0.70) with a very narrow
umbilicus. Shell fragments are visible at some places. They show fine spiral
lines, which in accompany with crenulated growth lines form a spider
web-like pattern. The steinkern is smooth except for wide, shallow
constrictions.
The smallest specimen (NPL 68495) is partly crushed and has a diameter of
15 mm. Faint traces of spiral ornament show that this feature has developed by
this diameter. Like both of the other specimens, the phragmocone is
extensively recrystallized. The medium sized specimen was difficult to
extract and the only useful data that can be obtained is that it shows
strong spiral lines, with a density of 13 per 2 mm, at an estimated diameter of
25 mm.
Discussion: This species can easily be confused with the stratigraphically
older species G. deceptus, which occurs in Nevada and Utah sections of Chainman Shale.
The main differences between the two species are the generally wider conch
in G. multiliratus (ww / dm = 0.80–0.85 at 20 mm diameter in G. multiliratus in contrast to 0.65–0.70 in G. deceptus)
and the wider external lobe with higher median saddle in G. multiliratus.
G. multiliratus is the stratigraphically youngest species of Goniatites known from North America, and
certain features of its conch ontogeny (prolonged subevolute juvenile stage)
and suture line (relatively high median saddle and angular flanks of the
adventive lobe) may foreshadow the evolution of a Choctawites-like form, although there
would clearly be other intermediate stages.
Family Delepinoceratidae Ruzhencev, 1957
Family diagnosis: Representatives of the superfamily Goniatitaceae with
advanced suture lines possessing a Y-shaped external lobe and a tectiform
ventrolateral saddle. The adventive lobe and also the prongs of the external
lobe show the tendency to become tridentate.
Discussion: There exist three diverging concepts about the classification of
the advanced goniatitid species and genera. Ruzhencev and Bogoslovskaya
(1971, 1978) separated the families Delepinoceratidae (including
Platygoniatites and Delepinoceras) and Agathiceratidae
(including Dombarites, Pericleites, Proshumardites, and Agathiceras) on the basis of the subdivision
mode of the external lobe, thus delineating two independent evolutionary
lineages. Leonova (2002, 2011) doubted the assignment of Agathiceras in the order
Goniatitida and transferred it into the order Tornoceratida (which has
usually only been accepted as a suborder within the Goniatitida). Finally,
Kullmann et al. (2007) and Kullmann (2009a, b) separated Agathiceras and closely
related forms in the independent superfamily Agathiceratoidea within the
suborder Goniatitina. In Kullmann's concept, the genera Dombarites and Proshumardites are grouped in the
subfamily Dombaritinae within the family Delepinoceratidae.
If Agathiceras is excluded from the discussion of the phylogeny of the family
Goniatitidae and its descendants (for instance, if the central or sub-central
position of the siphuncle in the juvenile stage is been taken as a reason
for separation on higher taxonomic levels), then the phylogenetic analysis
(see below) speaks for the extension of the family Delepinoceratidae to also
include the subfamily Dombaritinae (Kullmann, 2009a). This subfamily,
however, also requires re-definition to include the genera Revilloceras, Lusitanoceras, and
Choctawites.
For resolving the phylogenetic relationships of the goniatitid ammonoid
genera, we performed a cladistic analysis based on the character matrix of
Korn (1997a). In contrast to that analysis, we used Progoniatites uncus Korn, Bockwinkel and
Ebbighausen, 2010 as the outgroup species and included three more ammonoid
species in the new analysis, Choctawites choctawensis, Revilloceras granofalcatum, and Dombarites falcatoides. We included a new character, the
presence of shell constrictions, in the matrix.
The heuristic cladistics analysis performed with PAST (Hammer et al., 2009)
led to 35 most parsimonious trees of 35 steps length, of which a majority
consensus tree was formed (Fig. 10). This tree shows some unresolved
branches but demonstrates that a crown group is well established with the
genera Choctawites, Lusitanoceras, Revilloceras, Dombarites,
Proshumardites, Platygoniatites, and Delepinoceras, which are all characterized by the apomorphies of a
Y-shaped external lobe and a tendency towards a tectiform ventrolateral
saddle. This clearly defined clade can best be described as the family
Delepinoceratidae. One has, however, to take into account that a similar
tendency of sutural development also occurs in the more conservative lineage
leading to Neogoniatites.
The cladogram also shows that Choctawites is more plesiomorphic than the other of the
genera, which are all defined by the presence of outer shell constrictions
in the juvenile stage. The crown group is subdivided into two independent
clades, one represented by the advanced Dombarites species D. tectus and Proshumardites and the other
represented by the genera Platygoniatites and
Delepinoceras.
Subfamily Dombaritinae (in Kullmann et al.,
2007)
Subfamily diagnosis: Delepinoceratidae with advanced, moderately wide
external lobes (usually between 0.60 and 0.90, measured at half height, of
the external lobe depth), and median saddles with a height of 0.50 to 0.75
of the external lobe depth. External lobe Y-shaped and undivided.
Included genera:Choctawites n. gen.Lusitanoceras Pereira de Sousa, 1923Revilloceras Wagner-Gentis, 1980Dombarites Librovitch, 1957Proshumardites Rauser-Tschernoussowa, 1928Deleshumardites Kullmann, 2007
Discussion: Distinctive Late Viséan to early Serpukhovian goniatitids
with advanced lobes (especially with broader, more deeply divided external
lobe) have an almost cosmopolitan distribution (Librovitch, 1957; Gordon,
1965; Ruzhencev and Bogoslovskaya, 1970, 1971; Drahovzal and Quinn, 1972;
Wagner-Gentis, 1980; Webster et al., 1984; Korn et al., 1999; Nikolaeva and
Konovalova, 2005; Klug et al., 2006). Until recently, it was thought that
this morphological complex formed an essentially monophyletic clade that
dispersed during the Late Viséan, evolving from the advanced goniatitid
Lusitanoceras (Korn, 1988, 1997b; Kullmann, 2009a). Cross sections of various specimens
belonging to Lusitanoceras were shown by Kullmann and Pitz (1980), Kusina (1987), Korn
(1988, 1997b), Kusina and Yatskov (1999) and Nikolaeva and Konovalova
(2005); all sections possess a rather long subevolute juvenile stage.
Nikolaeva and Konovalova (2005) figured cross sections of a series of
specimens from the Urals that were all referred to as the genus Dombarites by Ruzhencev
and Bogoslovskaya (1971) and concluded that the shapes of the early whorls
in this are very different to those of the genus Lusitanoceras. Consequently, Nikolaeva
and Konovalova (2005) postulated that Dombarites sensu stricto did not evolve from Lusitanoceras, but directly
from the genus Goniatites. Indeed, the figured cross sections of Dombarites with subinvolute
inner whorls suggest that the umbilical width in the inner whorls are more
similar to Goniatites; however, one has to keep in mind that there is no reason to
assume that evolutionary pathways always proceed in the same trend (e.g.
from involute to evolute inner whorls); Dombarites, with its less widely umbilicate
inner whorls may also have been derived from more evolute forms.
Furthermore, the suture lines of all of the genera assembled in the
subfamily Dombaritinae are very similar in great detail, speaking for very
close relationships and obviously ruling out homoplasy.
A more detailed subdivision of the various forms often assembled in the
genus Dombarites requires the separation of genera on the base of morphology of the
inner whorls, ornament and suture line details (Table 6). For this reason we
describe the new genus Choctawites to accommodate the North American species “Goniatites choctawensis Shumard,
1863”, “G. kentuckiensis Miller, 1889” and “G. cumminsi Hyatt, 1893” and revive the genus
Revilloceras Wagner-Gentis, 1980 for superficially similar species known from the
Cantabrian Mountains of Spain and the Anti-Atlas of Morocco.
Characteristics of genera within the subfamily
Dombaritinae.
Choctawites
Lusitanoceras
Revilloceras
Dombarites
General shape of innerwhorls
subevolute to evolute
subevolute
evolute
subinvolute
Whorl profile of innerwhorls
whorls widest at some distance from umbilical margin, flanks diverging or subparallel,trapezoidal
whorls widest at umbilical margin, flanks converging
whorls widest at umbilical margin, flanks converging
variable
Constrictions
only on steinkern
on shell surface
on shell surface
often on shell surface
Coiling of inner whorls
rounded
usually triangular
triangular
triangular at somespecies
Ornament in the subadult stage
fine spiral lines
coarse spiral lines
coarse spiral lines
variable
Ornament in the adultstage
fine spiral lines, falcate transverse ornament
coarse spiral lines, occasionally falcate transverseornament
coarse spiral lines, falcate transverse ornament
variable
Height of median saddle
0.60 of external lobe depth
0.50–0.55 of externallobe depth
0.50–0.55 of externallobe depth
0.65–0.75 of external lobe depth
Shape of external lobe
weakly Y-shaped
weakly Y-shaped
strongly Y-shaped
weakly Y-shaped
Width of external lobe at half depth
0.80 of E lobe depth
0.65 of E lobe depth
0.70 of E lobe depth
0.70–0.85 of E lobe depth
Shape of external lobeprongs
V-shaped
V-shaped
lanceolate
V-shaped or lanceolate
Flanks of adventive lobe
sinuous
slightly sinuous
with inflexion
with inflexion
Choctawites n. gen.
Type species: Goniatites choctawensis Shumard, 1863.
Genus diagnosis: Dombaritinae with subevolute to evolute inner whorls; the
umbilicus becomes closed in early ontogeny. Whorl profile with parallel or
subparallel flanks, conch in juveniles widest at some distance from the
umbilicus. Externally expressed constrictions extremely rare or absent.
Growth lines slightly biconvex and rectiradiate; external sinus shallow.
External lobe Y-shaped, very narrow or narrow, usually with slightly curved
flanks.
Included species:
choctawensis: Goniatites choctawensis Shumard, 1863, p. 109;
Oklahoma.cumminsi: Glyphioceras cumminsi Hyatt, 1893, p. 467;
Texas.kentuckiensis: Goniatites kentuckiensis Miller, 1889, p. 439;
Kentucky.
Discussion: All Late Viséan–early Serpukhovian goniatitid ammonoids
with prominent adult spiral ornament found in North America can be readily
divided into two major groups (Gordon, 1965), (1) those with external shell
constrictions and (2) those in which constrictions are developed as internal
shell thickenings and thus only visible on the internal mould. This
subdivision largely parallels the whorl cross sections, of which the first
group possesses more or less trapezoidal early whorl profiles (in which the
conch is widest at some distance from the umbilicus), and the second group
shows “normal” whorl profiles with flanks converging from the umbilical wall
towards the venter.
Examinations of collections from the Moorefield Formation, Caney Shale, and
Barnett Shale show that the two morphological groups have very little
morphological or ornamental overlap. Trapezoidal forms are consistently
crenistriate or cross lirate to about 15 mm diameter. In contrast, the
normal forms develop strong spiral ornament already by 10 mm conch
diameter and can exhibit sub-triangular coiling. Furthermore, at any given
locality or horizon in the eastern USA, either trapezoidal or rounded forms
can totally dominate a sample.
Three possible hypotheses could explain this dimorphic pattern: (1) broad
intraspecific variation; (2) sexual dimorphism, and (3) two different
taxonomic groups are present in North America. Given the lack of
morphological intermediates between trapezoidal and normal forms, we
reject the first hypothesis. Regarding the second hypothesis, the
stratigraphically lower normal forms associated with
Sulcogirtyoceras limatum (Miller and Faber, 1892) are readily referable to Lusitanoceras because they possess the
characteristic early ontogeny, sutural shape, and ornament (particularly
with granulated spiral lines) of that genus. In fact, these forms are so
close to the European species of Lusitanoceras that they have been referred
to as “Goniatites granosus Portlock, 1843” since the early 1960s (Gordon, 1965). While we doubt the
species assignment, the referral of the North American material to the genus
Lusitanoceras is correct. If the trapezoidal forms are simply sexual dimorphs of
Lusitanoceras, it would be expected that this same dichotomy would exist in the European
and North American occurrences. Instead, the trapezoidal forms are found
almost exclusively in Marathon–Ouachita–Appalachian foreland basin system,
being unknown from Europe and extremely rare in the Antler Foreland Basin.
This pushes us to accept that the trapezoidal forms are a distinct taxonomic
entity endemic to Laurentia (North America). Since they cannot be
accommodated in any existing genus, we erect the new genus Choctawites for them.
Choctawites cumminsi (Hyatt, 1893) (Figs. 11a, b and 12)
1893 Goniatites cumminsi Hyatt, p. 467, pl. 47, figs.
33–43.1965 Goniatites choctawensis. – Gordon, pl. 19, figs. 1–12, text-fig. 44E, F
[only].
Lectotype: Specimen UT 12031 (designated by Cloud and Barnes, 1948);
illustrated by Gordon (1965, pl. 19, figs. 1–6).
Type locality and horizon: 5 miles west of Lampasas (Lampasas County, Texas);
Barnett Shale.
Diagnosis: Choctawites with a thickly pachyconic to globular conch between 2 and 12 mm
diameter (ww / dm = 0.85–0.95) and thickly pachyconic conch (ww / dm =
0.70) at 20 mm diameter. Umbilicus moderately narrow to moderately wide in
early ontogeny (uw / dm = 0.25–0.35 at 1.5–2 mm diameter) and very narrow
in all stages larger than 4 mm diameter (uw / dm 0.05–0.08). Umbilical wall
convexly rounded in all stages, very short early juvenile stage with
slightly trapezoidal whorl cross section. Aperture low (WER = 1.65–1.70
throughout ontogeny). Ornament with about 120 spiral lines and crenulated,
slightly biconvex, and rectiradiate growth lines; external sinus shallow.
Falcate ornament begins at 40 mm dm.
Material: 43 specimens (NPL 68497 through NPL 68539), all from sample
00TXCU-27. The material consists of brown to black coloured phosphatic
steinkerns, which have varying amounts of shell preserved on them. Shell
occasionally exhibits iridescent colour in small areas. Most of the inner
phragmocones (diameter less than 10 mm) were not filled with minerals and
weathered out rapidly or were not preserved, leaving a hollow space in the
specimen. Some of these small interior phragmocones are exceptionally well
preserved when freshly exposed and show the fine details of the septa and shell.
The largest intact specimens were about 20 mm in diameter. Only fragments of
larger specimens were collected.
Choctawites cumminsi (Hyatt, 1893) from bed 25 (sample 00TXCU-27) of the
Figure 2 Ranch section; both × 2.5. (a) Specimen NPL 68497. (b)
Specimen NPL 68498.
Choctawites cumminsi (Hyatt, 1893) from San Saba (a–d) and the Figure 2
Ranch section (e, f). (a) Cross section of specimen 26-13 from San Saba;
× 2.5. (b) Cross section of specimen 26-2 from San Saba; × 2.5. (c) Cross section of specimen 26b from San Saba; × 2.5. (d)
Cross section of specimen 26-10 from San Saba; × 2.5. (e) Cross
section of specimen NPL 68499 from bed 25 (sample 00TXCU27) of Figure 2
Ranch; × 2.5. (f) Suture line of specimen NPL 68500 from bed
25 (sample 00TXCU27) of Figure 2 Ranch, at 14.4 mm diameter, 11.3 mm ww, 7.2 mm wh; × 5.0. (g–i), Ontogenetic development of the conch width index
(ww / dm), umbilical width index (uw / dm), and WER of
all available specimens (the cross sections of specimens (a–d) from San Saba
were produced by R. Kant; the original specimens are stored in the
collections of the USNM, acetate peels are stored in the collections of the
GPI Tübingen).
Description: The conch cross section of the small specimen NPL 68499 (10 mm
conch diameter) shows a globular shape (ww / dm = 0.86) and a very narrow
umbilicus (uw / dm = 0.06) with broadly rounded flanks and venter (Fig. 12e). The inner whorls show crescent-shaped whorl profiles and a moderately
wide umbilicus; the highest uw / dm ratio with 0.34 occurs at 1.2 mm conch
diameter. In the growth interval between 5 and 10 mm conch diameter, the
conch is widest at some distance from the umbilicus.
The conch cross section from the Sierra Diablo specimen is very similar to
four cross sections (specimens 26-13, 26-2, 26b, 26-10) produced by R. Kant
(Tübingen) of material from San Saba (original specimens identified by
M. Gordon in the USNM; acetate peels in the collections of the GPI
Tübingen). The San Saba topotypes only differ in the slightly more
pronounced trapezoidal whorl profile and the more subparallel flanks (Fig. 12a–d).
Conch ontogeny of Choctawites cumminsi (Hyatt, 1893); mainly based on
specimens from San Saba (Elias Coll; cross sections by R. Kant,
Tübingen).
dm
Conch shape
Whorl cross section shape
Aperture
1.5 mm
thickly pachyconic; subinvolute or subevolute (ww / dm = 0.75–0.85; uw / dm = 0.25–0.35)
strongly depressed; very strongly embracing (ww / wh = 2.00–2.25; IZR = 0.45–0.50)
low (WER = 1.55–1.65)
5 mm
globular; involute (ww / dm = 0.90; uw / dm = 0.05–0.10)
moderately depressed; very strongly embracing (ww / wh = 1.65; IZR = 0.55–0.60)
low (WER = 1.60–1.75)
10 mm
thickly pachyconic or globular; involute (ww / dm = 0.85; uw / dm = 0.05)
moderately depressed; very strongly embracing (ww / wh = 1.55; IZR = 0.50–0.60)
low (WER = 1.60–1.75)
25 mm
thinly pachyconic; involute (ww / dm = 0.70; uw / dm = 0.05)
weakly depressed; very strongly embracing (ww / wh = 1.30; IZR = 0.55)
low (WER = 1.60–1.70)
50 mm
thickly discoidal; involute (ww / dm = 0.58; uw / dm = 0.08)
weakly depressed (ww / wh = 1.15)
Conchs are thickly pachyconic at 20 mm (e.g. specimen NPL 68497; Fig. 11a),
with a very narrow umbilicus and moderately depressed whorls (ww / dm =
0.74; ww / wh = 1.36). Flanks and venter are continuously rounded and the
aperture is low (WER = 1.64). The specimen is an internal mould without
shell remains; the body chamber has nearly the length of a volution. The
internal mould shows shallow constrictions with irregular distances; they
extend linearly across the flanks and turn forward for a low and wide ventral
projection.
The slightly smaller specimen NPL 68497 (17 mm dm) largely resembles the
previous specimen. The internal mould has four prominent constrictions, which
probably follow the growth lines in their course, are almost rectiradiate,
bearing only a slight forward inflection or are salient over the ventral area
(Fig. 11b).
The smallest specimens (e.g. specimen NPL 68501) are 7 mm in diameter. By
that stage, the conch is starting to change from sub-trapezoidal (strongly
recurved flanks) to rounded whorl profiles. As a result, the trapezoidal
phase, which is only weakly developed in this taxon, was only seen on
cross-sectioned conchs.
Specimen NPL 68500 shows the suture line of a specimen in the intermediate
growth stage (at 14.4 mm dm). It possesses a Y-shaped external lobe with
sinuous flanks. The external lobe has, measured at half depth, 0.64 of the
external lobe depth and is 1.25 times wider than the ventrolateral saddle.
The median saddle reaches a height of 0.42 of the external lobe depth. The
ventrolateral saddle is tectiform and subacute (Fig. 12f).
Ornament is only commonly preserved on the larger whorl fragments, but
smaller specimens show crenistriate ornament up to diameters of
approximately 15 mm, after which it changes to strong longitudinal lirae
numbering about 19–20 per 5 mm.
Choctawites kentuckiensis (Miller, 1889). (a) Cross section of specimen 57a from
Moorefield, Arkansas; × 2.5. (b) Cross section of specimen
MB.C.25470 from Moorefield, Arkansas; × 2.5. (c) Suture line of
specimen MB.C.25471 from Moorefield, Arkansas, at 14.1 mm diameter, 10.2 mm ww, 6.6 mm wh; × 5.0. (d–f) Ontogenetic development of the conch
width index (ww / dm), umbilical width index (uw / dm), and WER of all available specimens (cross section (a) was produced by R. Kant;
the original specimen is stored in the collections of the University of
Iowa, acetate peels are stored in the collections of the GPI Tübingen).
Choctawites choctawensis (Shumard, 1863). (a) Cross section of topotype USNM
119504 from Brushy Creek, Oklahoma; × 2.5 (from Gordon, 1965). (b)
Cross section of specimen 71 from Ada, Oklahoma; × 2.5. (c) Cross
section of specimen 26-1 from San Saba; × 2.5. (d–f), Ontogenetic
development of the conch width index (ww / dm), umbilical width index (uw / dm),
and whorl expansion rate (WER) of all available specimens (the cross
sections of specimens (b) and (c) from Ada and San Saba were produced by
R. Kant; the original specimens is stored in the collections of the USNM,
acetate peels are stored in the collections of the GPI Tübingen).
Conch dimensions and ratios for Choctawites cumminsi (Hyatt, 1893) from the
type area and the Sierra Diablo.
Specimen
dm
ww
wh
uw
ah
ww / dm
ww / wh
uw / dm
WER
IZR
Paratype-UT 12032
50.0
29.0
26.0
4.0
–
0.58
1.12
0.08
–
–
Holotype-UT 12031
20.8
14.6
11.5
1.8
4.7
0.70
1.27
0.09
1.67
0.59
NPL 68502
19.8
14.5
11.4
1.6
4.5
0.73
1.27
0.08
1.67
0.61
NPL 68497
19.6
14.6
10.7
2.0
4.3
0.74
1.36
0.10
1.64
0.60
NPL 68503
17.6
13.5
9.6
1.9
3.9
0.77
1.41
0.11
1.65
0.59
NPL 68498
17.2
12.9
8.9
1.9
4.1
0.75
1.45
0.11
1.72
0.54
NPL 68504
15.3
12.5
8.4
1.8
–
0.82
1.49
0.12
–
–
NPL 68505
14.8
11.9
8.0
1.8
3.5
0.80
1.49
0.12
1.72
0.56
NPL 68500
14.5
11.6
7.7
2.1
3.3
0.80
1.51
0.14
1.68
0.57
NPL 68499
12.4
9.8
6.0
1.4
2.8
0.79
1.63
0.11
1.67
0.53
NPL 68501
6.9
6.2
–
–
–
0.90
–
–
–
–
Discussion: Forms we refer to Choctawites have historically been attributed to three
species; “Goniatites choctawensis Shumard, 1863”, “Goniatites cumminsi Hyatt, 1893”, and “Goniatites kentuckiensis Miller, 1889”. Gordon
(1965) regarded G. cumminsi as a junior synonym of G. choctawensis, a view supported in nearly every
subsequent paper. Recently, Work and Mason (2009a) synonymized G. kentuckiensis with G. choctawensis and
followed Ruzhencev and Bogoslovskaya (1971) in assigning G. choctawensis to Dombarites, which left
G. choctawensis as the only valid species in this group. After reviewing the literature and
numerous specimens, we have concluded that all three established species of
the “trapezoidal” group (G. kentuckiensis, G. choctawensis, and G. cumminsi) are valid and represent a stratigraphic
and possibly evolutionary succession.
The stratigraphically oldest species is Choctawites kentuckiensis, which is associated with
Sulcogirtyoceras limatum and can be differentiated from the other species because it possesses a
moderately well-developed trapezoidal shape (Fig. 13a, b) and late
development of falcate ornament (typically appearing at diameters greater
than 50 mm). This taxon was referred to as Dombarites choctawensis by Work and Mason (2009a).
However, type specimens of Choctawites choctawensis consistently develop falcations by 40–45 mm
diameter. Also, the early ontogenies of Ch. choctawensis and Ch. kentuckiensis are similar, but the former
has a much stronger expression of the trapezoidal shape. These two criteria
can be used to readily differentiate between the two species. There is an apparent
facies difference in the occurrence pattern of the two species, with
Choctawites kentuckiensis being known from shallower water deposits including a carbonate platform and
pro-deltaic mudstone facies in the Appalachian and Ouachita foreland basins
(Gordon, 1965; Work and Mason, 2009a), and Ch. choctawensis occurring in the deeper water
sequences of the Caney and Barnett shales. The exclusion of the latter from
shallow water facies may have resulted from the sea level fall recorded by
the Batesville Sandstone wedge in Arkansas.
The stratigraphically next species we can recognize in succession is the
type species, Ch. choctawensis, which has the unique combination of early appearance of
falcations and a strongly developed trapezoidal phase (Fig. 14a–c). Ch. choctawensis is
associated with a more advanced species of Sulcogirtyoceras, S.. cf. S. ornatissimum. It is known widely from
the Caney Shale of Oklahoma, e.g. specimen USNM 119505 from USGS locality 2078A
(Gordon, 1965, fig. 44G, H) and specimen Kant71 (Elias Collection) from 6 miles
SE of Ada along Highway 99 (Pontotoc County, Oklahoma), the Barnett Shale of
central Texas (specimen Kant26-1), and tentatively, the Batesville Sandstone
of Arkansas (Gordon, 1965).
The stratigraphically youngest species, Ch. cumminsi, is a form whose distinct, but
reduced, trapezoidal phase (Fig. 12a–e), more narrowly umbilicate early
whorls, and early appearance of falcations are highly definitive. The
species can still be differentiated from the other species of Choctawites using the
brief or even absent trapezoidal stage. Ch. cumminsi is known mostly from the Barnett
Shale of Texas, but almost certainly occurs in the Caney Shale of Oklahoma
and Arkansas (e.g. specimen USNM 119502; Gordon 1965, fig. 44e, f). This
species is associated with Sulcogirytoceras ornatissimum (Miller and Youngquist, 1948) and species of
Pachylyroceras, which together comprise one of the youngest Viséan ammonoid
assemblages we can recognize in the eastern USA. Since the Sierra Diablo
specimens all exhibit the characteristic reduced trapezoidal stage, we
refer
to them as Ch. cumminsi.
Superfamily Neoglyphiocerataceae Plummer and Scott,1937Family Neoglyphioceratidae Plummer and Scott, 1937
Pachylyroceras Ruzhencev and Bogoslovskaya, 1971
Type species: Lyrogoniatites cloudi Miller and Youngquist, 1948 [original designation].
Genus diagnosis: Representatives of the family Neoglyphioceratidae with
subevolute conch, which becomes more evolute in the adult stage. 20–30
coarse spiral lines, growth lines, and shell constrictions concavo-convex
with shallow ventral sinus. Suture line with rather narrow external lobe,
usually with slightly sinuous flanks.
Pachylyroceras cloudi (Miller and Youngquist, 1948) from bed 25 (sample
00TXCU-27) of the Figure 2 Ranch section; both × 2.5. (a) Specimen
NPL 68540. (b) Specimen NPL 68541.
Included species:cloudi: Lyrogoniatites cloudi Miller and Youngquist, 1948, p. 660;
Texas.utahense: Lyrogoniatites utahensis Miller, Youngquist and Nielsen, 1952, p. 154;
Utah.
Discussion: When they introduced the genus Pachylyroceras, Ruzhencev and
Bogoslovskaya
(1971) assumed that certain Uralian specimens and the types for
Lyrogoniatites cloudi from the Barnett Shale were conspecific. As a result, they based
Pachylyroceras on the types of L. cloudi from Texas, rather than the Uralian material they had in
their collections and erected three new species, P. angustum, P. consequens, and P. constrictum. We see
significant differences in the course of growth lines in all of the Uralian
specimens (ventral salient) versus the Texas types of Pachylyroceras (ventral sulcus), and
conclude that the genus, which is closely related to Neoglyphioceras, is currently known
only from North America.
This excludes all of the Uralian material from Pachylyroceras, and given that there is no
existing genus to accommodate these widely umbilicate Neoglyphioceras-like forms with
ventral salient in their growth lines and constrictions, we erect the new
genus Uralyroceras for the Uralian species (see below).
According to its conch morphology, suture line, and ornament,
Pachylyroceras has an intermediate position between Neoglyphioceras and Lyrogoniatites. It possesses the type of shell
constrictions with ventral sinus of Neoglyphioceras but the subevolute conch morphology of
Lyrogoniatites. In the suture line, Pachylyroceras shows the sinuous flanks of the external lobe like
Neoglyphioceras, but at the same time, the external lobe is rather narrow with weakly
diverging flanks like in Lyrogoniatites.
Pachylyroceras cloudi (Miller and Youngquist,
1948) (Figs. 15a, b and 16c)
1909 Goniatites newsomi. – Girty, pl. 12, figs. 11,
11a.1948 Lyrogoniatites cloudi Miller and Youngquist, p. 660, pl. 94,
figs. 1–3.1965 Neoglyphioceras cloudi. – Gordon, pl. 20, figs. 12–14, text-fig. 59D, E,
J.1965 Neoglyphioceras cloudi. – King, p. 42.1971 Lyrogoniatites newsomi cloudi. – Furnish and Saunders, pl. 2,
fig. 5.2009b Pachylyroceras cloudi. – Kullmann, p. 65, text-fig.
38.5a–d.
Conch dimensions and ratios for Pachylyroceras cloudi (Miller and Youngquist,
1948) from the type area and the Sierra Diablo.
Specimen
dm
ww
wh
uw
ah
ww / dm
ww / wh
uw / dm
WER
IZR
Holotype
30.4
16.8
10.1
12.7
5.8
0.55
1.66
0.42
1.53
0.43
NPL 68541
21.7
10.7
8.2
6.8
–
0.49
1.30
0.31
–
–
NPL 68540
21.2
11.4
8.1
6.4
4.3
0.54
1.41
0.30
1.57
0.47
UTSA 07023
19.9
10.3
6.0
6.0
4.0
0.52
1.72
0.30
1.57
0.33
NPL 68543
14.0
7.9
5.0
4.6
2.5
0.56
1.58
0.33
1.48
0.50
Holotype: Specimen USNM 113011; illustrated by Miller and Youngquist (1948,
pl. 94, figs. 1–3).
Type locality and horizon: 3.5 km south of San Saba, Texas (Chappel Hill
locality); lower portion of Barnett Shale, probably Choctawites cumminsi Biozone.
Diagnosis: Species of Pachylyroceras with rounded umbilical shoulder and 23–30 spiral
lines.
Material: A total of 22 specimens; 21 specimens from sample 00TXCU-27 (NPL
68540 through NPL 68560) and 1 specimen from NMC 1/03-14 (UTSA 07023). All
of the specimens are from the same starved interval and are nearly all black
to dark brown coloured phosphatic steinkerns with rare phosphatized shell
remains preserved. All show various states of corrosion and dissolution.
Description: The earliest whorls are not preserved in the material, but in
specimen NPL 68540 the conch has achieved a discoidal shape with moderately
depressed whorls and a medium width umbilicus (ww / dm = 0.54; uw / dm =
0.30) by 21 mm diameter (Fig. 15a). The whorl cross section is slightly
depressed (ww / wh = 1.41) with a broadly rounded umbilical shoulder and
broadly rounded ventral region. The internal mould shows rather prominent
constrictions, being rectiradiate at the umbilical margin, and bending
rather sharply forward at the ventrolateral shoulder forming a pronounced
ventral salient and a shallow ventral sinus. Longitudinal lirae are
relatively coarse; 28 can be counted from umbilical shoulder to umbilical
shoulder.
The suture line of specimen NPL 68542 (phragmocone whorl width of 13.2 mm,
whorl height of 8.0 mm) shows a V-shaped external lobe with sinuous flanks
and slightly asymmetric prongs. The median saddle has a height of one-third
of the external lobe depth. On the flanks follow a bell-shaped ventrolateral
saddle and a symmetric adventive lobe with slightly convex flanks (Fig. 16c).
Discussion: The Sierra Diablo material of Pachylyroceras appears to represent only a
single species, with all specimens being very close to each other in
ornament and morphology. They are also close, in their conch width/diameter
and umbilical width ratios, to several of the paratypes in the type lot from
San Saba. As has been pointed out by Gordon (1965), the holotype is atypical
for the type material in that it possesses five fewer spiral lines and a
much wider umbilicus, rather approaching the morphology of “Neoglyphiceras hyatti Gordon,
1960”
than the other types. However, the holotype is the largest specimen in the
type lot by about 4 mm, and these differences may be the result of later
ontogenetic changes or strong variation rather than of taxonomic value.
Unfortunately, because the type lot was collected from ex situ nodules and
has no detailed stratigraphic context, it is possible that the type lot is
from a different horizon.
Occurrence: Ruzhencev and Bogoslovskaya (1971) reported the species from the
South Urals, but we consider this assignment erroneous because the conch
shape, suture, and ornament of the Uralian forms are fundamentally different
from the type of P. cloudi. We propose the new name Uralyroceras arquatum (named after the arched course
of the constrictions) for the Uralian species (see below). Pachylyroceras cloudi appears to be
characteristic of the Choctawites cumminsi Biozone of the eastern USA and equivalent age strata
in Nevada and Utah.
Uralyroceras n. gen.
Type species: Pachylyroceras constrictum Ruzhencev and Bogoslovskaya, 1971.
Genus diagnosis: Representatives of the family Neoglyphioceratidae with
subevolute to evolute conch, which becomes more evolute in the adult stage.
20–30 coarse spiral lines, growth lines, and shell constrictions nearly
linear or with low ventral projection. Suture line with narrow external
lobe, usually with slightly sinuous flanks.
Included species:angustum: Pachylyroceras angustum Ruzhencev and Bogoslovskaya, 1971, p. 246; South
Urals.consequens: Pachylyroceras consequens Ruzhencev and Bogoslovskaya, 1971, p. 245; South
Urals.constrictum: Pachylyroceras constrictum Ruzhencev and Bogoslovskaya, 1971, p. 247; South
Urals.arquatum: Uralyroceras arquatum new species; South Urals. (new name for Pachylyroceras cloudi in Ruzhencev and Bogoslovskaya, 1971,
p. 244).esini: Pachylyroceras esini Nikolaeva, 1994, p. 83; Darvaz
(Tajikistan).? hyatti: Neoglyphioceras hyatti Gordon, 1965, p. 139;
Texas.
Suture lines of representatives of Pachylyroceras from localities in
Utah, Nevada, and Texas; all × 6.0. (a) Pachylyroceras utahensis (Miller et al., 1952), specimen NPL 68561 from Skunk Spring, Utah, at 11.6 mm
diameter, 8.3 mm ww, 5.6 mm wh. (b) Pachylyroceras utahensis (Miller et al., 1952),
specimen MB.C.25472 from Hamilton Canyon, Nevada, at 9.3 mm ww, 6.1 mm wh.
(c) Pachylyroceras cloudi (Miller and Youngquist, 1948), specimen NPL 68542 from bed 25 of the
Figure 2 Ranch, Texas, at 13.2 mm ww, 8.0 mm wh.
Discussion: Uralyroceras differs from Pachylyroceras in the course of the shell constrictions, which
extend with a low and wide projection across the venter in the new genus but
with a ventral sinus as in Pachylyroceras.