FRFossil RecordFRFoss. Rec.2193-0074Copernicus PublicationsGöttingen, Germany10.5194/fr-21-93-2018A new pachyrhizodontid fish (Actinopterygii, Teleostei) from the Muhi Quarry
(Albian-Cenomanian), Hidalgo, MexicoA new pachyrhizodontid fish (Actinopterygii, Teleostei)ArratiaGloriahttps://orcid.org/0000-0002-7363-1319González-RodríguezKatia A.katiag@uaeh.edu.mxHernández-GuerreroCitlalliBiodiversity Institute and Department of Ecology and Evolutionary Biology, The University of Kansas, Dyche Hall, Lawrence, Kansas 66045–7561, USAInstituto de Ciencias Básicas e Ingeniería, Museo
de Paleontología, Centro de Investigaciones Biológicas, Universidad
Autónoma del Estado de Hidalgo, Mineral de la Reforma, Hidalgo, MexicoDoctorado en Ciencias en Biodiversidad y Conservación, Universidad Autónoma del Estado de Hidalgo, Mineral de la Reforma, Hidalgo, MexicoKatia A. González-Rodríguez (katiag@uaeh.edu.mx)28March20182119310720October201715February201816February2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://fr.copernicus.org/articles/21/93/2018/fr-21-93-2018.htmlThe full text article is available as a PDF file from https://fr.copernicus.org/articles/21/93/2018/fr-21-93-2018.pdf
A new genus and species – Motlayoichthys sergioi (ZooBank registration:
urn:lsid:zoobank.org:pub:2C503741-2362-4234-8CE0-BB7D8BE5A236, urn:lsid:zoobank.org:act:EF5040FD-F306-4C0F-B9DA-2CC696CA349D) – from the
Cretaceous (Albian-Cenomanian) of the Muhi Quarry, Hidalgo, central
Mexico is assigned to the family Pachyrhizodontidae based on the presence of
the inner premaxillary tooth and a few other cranial characters. Its unique
suite of characters, that includes several autapomorphies supports its
assignment as a new genus and species, such as the presence of a triangular
head with three sides of similar lengths and with its
deepest part at the level of the supraoccipital region; the lower jaw
projecting anterior to the premaxilla; the ornamentation of cranial
bones – especially those of the cheek region – consisting of fine longitudinal
crests densely covering the bony surfaces; one or a few deep grooves on the
mandibular teeth, which in addition bear acrodin tips ornamented with fine
parallel striae; and the position of the dorsal fin, near the
posterior margin of the cranium. Motlayoichthys n. gen., together
with Michin, Pachyrhizodus, Rhacolepis, and
Notelops, is part of the early radiation of pachyrhizodontoids
during Albian times.
Map of Mexico showing the location of the Muhi Quarry and the
sites where pachyrhizodontids have been found. (1) Muhi Quarry
(Albian-Cenomanian), Zimapán, Hidalgo state, central Mexico; (2) Tlayúa Quarry (Albian), located in the Morelos District of Puebla,
central Mexico; (3) Mexcala Formation quarries (Turonian-Maastrichtian),
northern portion of the state of Guerrero; (4) La Mula Quarry (Turonian),
Múzquiz District, Coahuila, northern Mexico; (5) Vallecillo Quarry
(Lower Turonian), Nuevo León state, northern Mexico. Modified from
González-Rodríguez et al. (2013a).
Introduction
Pachyrhizodontidae is a family of now extinct fishes found throughout the Tethys
Ocean during the Cretaceous. The family belongs to the order
Crossognathiformes (Taverne, 1989, sensu Arratia, 2008a; Arratia and
Tischlinger, 2010) and the suborder Pachyrhizodontoidei (sensu Forey, 1977).
Ten genera are currently referred to Pachyrhizodontidae: Rhacolepis
Agassiz, 1841;
Pachyrhizodus Dixon, 1850 (=Thrissopater Günther,
1872); Elopopsis Heckel, 1856; Greenwoodella Taverne, 1973;
Platinx Taverne, 1980; Goulmimichthys Cavin, 1995;
Tingitanichthys Taverne, 1996; Michin Alvarado-Ortega et
al., 2008; Nardopiscis Taverne, 2008; and Aquilopiscis Cumbaa and Murray, 2008. Rhacolepis and Notelops Woodward, 1901 are
interpreted here as crossognathiforms following Arratia's (2008a) and
Arratia and Tischlinger's (2010) phylogenetic hypotheses. In contrast, both
genera were interpreted as elopomorphs by Maisey (1991a, b).
Notelops was included in its own family, Notelopidae, by Forey
(1977), an arrangement followed by Nelson et al. (2016). However, it appears
in an unresolved position within a non-monophyletic order Pachyrhizodontoidei
in the phylogenetic hypotheses of Cumbaa and Murray (2008: fig. 12) and
Arratia and Tischlinger (2010: fig. 12).
New records of the family have been found in some Cretaceous localities of
Mexico, including Michin csernai (Alvarado-Ortega et al., 2008), Rhacolepis sp. from the Albian
Tlayúa Quarry of Puebla (Espinosa-Arrubarrena and Alvarado-Ortega,
2010), Goulmimichthys roberti (Blanco and Cavin, 2003; González-Rodríguez et al., 2013a), Pachyrhizodus caninus (Giersch
et al., 2010b; González-Rodríguez et al., 2013a), and
Tingitanichthys sp. from the lower Turonian locality Vallecillo in Nuevo León State
(Giersch et al., 2010a; González-Rodríguez et al., 2016), as well
as the records of Goulmimichthys sp. from La Mula Quarry (Turonian), Múzquiz, Coahuila,
northern Mexico; Eagle Ford Formation (Blanco-Piñón and
Alvarado-Ortega, 2005), Pachyrhizodus sp. from Turonian-Maastrichtian sediments of
Guerrero state (Alvarado-Ortega et al., 2006; González-Rodríguez
et al., 2016), and the pachyrhizodontid of Muhi Quarry here described.
(a) Panoramic of the Muhi Quarry, Hidalgo, Mexico; (b) close-up view
of a sequence of the Muhi Quarry; (c) stratigraphic column of the Muhi
Quarry indicating the site where Motlayoichthys sergioi n. gen. et n. sp. was collected. Modified
from Bravo-Cuevas et al. (2009) and Hegna et al. (2014).
The Muhi Quarry (Fig. 1) is a fossil Lagerstätte that was discovered
almost 20 years ago in state of Hidalgo, central México (Bravo-Cuevas et
al., 2012). More than two thousand fossils have been recovered from the
site, which include ammonites, crustaceans, crinoids, numerous fishes and
two reptiles (González-Rodríguez et al., 2013a, 2016). The material
is deposited in the Museo de Paleontología of the Universidad
Autónoma del Estado de Hidalgo (UAHMP), and many groups are under study.
The quarry belongs to the La Negra Facies of the El Doctor Formation of
Albian-Cenomanian age (Fig. 2). The original section measured consists of a
7.2 m sequence of biomicrite and micrite limestones, with intercalated
bedded and nodular cherts, and scarce laminae of unconsolidated
siliciclastic clay and calcium carbonate (Bravo-Cuevas et al., 2009, 2012).
The paleoenvironment corresponds to an outer sea shelf, which occasionally
received both open-ocean and near-shore waters possibly due to storms
(González-Rodríguez and Bravo-Cuevas, 2005; Bravo-Cuevas et al.,
2009, 2012). This mixture of waters promoted the deposition of a diverse
pelagic and neritic fauna, such as the pachyrhizodontid fish. Fishes found in the locality include chondrichthyans (sharks
and rays), neopterygians incertae sedis (Pycnodontiformes; previously cited as
halecostomes in González-Rodríguez et al., 2016), actinistians
(Schultze and González-Rodríguez, 2016), and numerous and diverse
teleosts (González-Rodríguez et al., 2016) and the following
teleosts have been recovered: Crossognathiformes, Ichthyodectiformes,
Elopiformes, Tselfatiformes, Clupeiformes, Gonorynchiformes
(González-Rodríguez et al., 2013a), Aulopiformes (Fielitz and
González-Rodríguez, 2008, 2010), and Acanthomorpha
(González-Rodríguez and Fielitz, 2008; González-Rodríguez
et al., 2013b).
The goals of this paper are to describe and provide a taxonomic name
to a new pachyrhizodontid from the Muhi Quarry (that represents the first
specimen of the Muhi Quarry deposited in the Paleontological Collection of
the University of the State of Hidalgo, Mexico) and to characterize it
within other Cretaceous Mexican pachyrhizodontids, as well as worldwide
forms.
Material and methodsInstitutional abbreviations
The studied material is housed in the following institutions: UAHMP, Museo
de Paleontología, Universidad Autónoma del Estado de Hidalgo,
Mexico; IGM, “Ma. Carmen Perrilliat” Museum, Instituto de Geología,
Universidad Nacional Autónoma de México; KUVP, Division of
Vertebrate Paleontology, Natural History, Museum, University of Kansas,
Lawrence, Kansas, United States.
Material studied
The following specimens were used in comparative studies: Michin csernai from the Albian
Tlayúa Quarry of Mexico (IGM-9028 to IGM-9034); Pachyrhizodus sp. (KUVP-297, KUVP-271,
KUVP-49416, KUVP-69014, KUVP-276, KUVP-85127, KUVP-86210, KUVP-87698,
KUVP-98, KUVP-496, KUVP-391, KUVP-68976, KUVP-68986, KUVP-68987, KUVP-68994,
KUVP-68997, KUVP-69009, KUVP-69400, KUVP-69407, KUVP-392, KUVP-69408,
KUVP-69702, KUVP-13924, KUVP-16237, KUVP-59032) from the Niobrara Chalk and
Pachyrhizodus minimus (KUVP-300 and KUVP-36032).
Preparation and observation methods
The right side of the head was prepared first with needles under a
stereomicroscope and with acetic acid at 10 % to eliminate part of the
matrix in 2010. The specimen was then transferred to resin to observe the
left side, following Toombs' and Rixon's (1950) technique. Some vertebrae of
the specimen and an impression of part of the axial skeleton were kept in
the original slab. To obtain the positive of the impression, a cast was
done. Unfortunately, the specimen was damaged during preparation and some
bone fragments were lost. These are identified in the description below.
Photographs were taken before and after preparation of the specimen. The
head was also coated with ammonium chloride in order for photographs to be taken with a SONY
Cybershot DSC-H100 camera. Drawings were made using a Leica MZ6
stereomicroscope with a camera lucida attachment.
Terminology, measurements, and classification used
Highest teleostean ranks (e.g., infraclass) follow Betancur et al. (2017) and the understanding of the family Pachyrhizodontidae follows
current knowledge based on Forey (1977), Cavin (2001), Arratia (2008a), and
Arratia and Tischlinger (2010). Osteological nomenclature follows Arratia
(1997) for cranial bones, except those of the skull roof, which are based on
the homology terminology for fishes as proposed by Jollie (1962) and
Schultze (2008) for fishes. For other literature concerning this subject, see
Schultze (2008). Traditional skull roof terms are given with the homologous
ones in each figure.
Motlayoichthys sergioi n. gen. et n. sp. (a) Photograph of the right side of the holotype
UAHMP-600 included in wax during the transfer process to resin; (b) Photograph
of the left side of the holotype UAHMP-600 after preparation with
the transfer method.
Measurements were taken with a digital caliper Starret 799. The head length
used in descriptions and comparisons was measured from the anterior margin of
the lower jaw to the posterior margin of the opercular region. Orbital
diameter and preorbital length were also expressed as percentages of the
head length. The preorbital length was measured from the anterior border of
the premaxilla to the anterior margin of the orbit. Orbital diameter was
measured as the maximum width of the orbit.
Systematic Paleontology
Infraclass Teleostei Müller, 1845, sensu Arratia, 1999
Order Crossognathiformes Taverne, 1989, sensu Arratia 2008a
Suborder Pachyrhizodontoidei Forey, 1977
Family Pachyrhizodontidae Cope, 1872, sensu Forey 1977
Genus Motlayoichthys n. gen.
Content: Type and only species, Motlayoichthys sergioi n. sp.
Diagnosis: The same as for species.
Motlayoichthys sergioi n. sp.
Figures 3–9.
Synonyms
2005 Salmoniform, González-Rodríguez and Bravo-Cuevas, p. 36, fig. 5E.
2016 Pachyrhizodus sp., González-Rodríguez et al., p. 114, fig. 3E.
Diagnosis: Pachyrhizodontid characterized by the following unique
combination of characters (autapomorphies are identified with an asterisk in
brackets): Head almost triangular-shaped with nearly equal sides and its
maximum depth at the supraoccipital level [*]; small orbit, approximately
15 % of head length; a short preorbital length, about 15 % of head
length; cranial bones densely ornamented with long, narrow longitudinal
crests; pterotic with a well-developed posterior spine; large and
squarish-shaped extrascapular bone [*]; large orbitosphenoid forming an
almost complete septum separating both orbital cavities [*]; expanded
postero-dorsal infraorbital plate reaching the anterior region of the
opercle; lower jaw extending anterior to premaxilla [*]; supramaxilla long
and narrow, covering a length equivalent to ca. 50 % of the maxilla
length; narrow maxilla with curvature along its ventral border [*] and
posterior region of maxilla postero-ventrally inclined [*];
quadrate-mandibular articulation posterior to the posterior margin of orbit;
premaxillary, maxillary, and dentary teeth with acrodin tips irregularly
ornamented with fine, short striae and deep, elongated grooves; and dorsal
fin anteriorly placed, closer to the posterior margin of head than middle
body region.
Etymology: Genus name comes from the word “decorated, ornamented” (= Motlayolo) in the
Mexican Náhuatl language; and the species name is sergioi in honor of the worker of the Muhi
Quarry who discovered the specimen (Sergio Yáñez).
Holotype: UAHMP-600 (Figs. 3–9) consists of the head (ca. 150 mm length),
the pectoral fins, several vertebrae and the impression of the anterior
portion of the dorsal fin support, as well as several ribs and neural spines
of the abdominal region. A section of the digestive tract is preserved
(Figs. 3, 4).
Formation and age: The La Negra Facies of the El Doctor Formation.
Albian–Cenomanian age (Bravo-Cuevas et al., 2009).
Description:
As a result of preparation, some fragments were lost; thus, there are slight
differences in the length of some bones between the left and right sides of the
head, for instance the anterior tip of the dentary. The head outline is
nearly an equilateral triangle, with a sharp tip anteriorly at the level of
the premaxilla and dentary, with the latter slightly projected rostrad
(Figs. 3–5).
Skull: The head is preserved in lateral view (Figs. 3, 4). The cranial bones
are completely ornamented with striations that make the identification of
articulations difficult. The parietal bone (= frontal bone of traditional
terminology) is elongated and widens posteriorly, whereas the post-parietal
(= parietal bone of traditional terminology) is a small bone with a
rectangular shape. Due to conditions of preservation, it is not possible to
observe if the post-parietals are separated by the supraoccipital or not.
The small supraoccipital is incompletely preserved dorso-medial to the
extrascapula (see Figs. 4a, b, 5). The massive, slightly triangular
autosphenotic projects latero-ventrally as a prominent spine forming the
postero-dorsal margin of the orbit. The pterotic is rectangular in shape and
possesses a sharp postero-ventral process or spine extending posterior to
the articular facet for the hyomandibula. The posterior border of the
pterotic articulates with a large, square extrascapula, which is
triangular-shaped in the majority of the family members. The surface of the
right extrascapula presents short longitudinal crests of different lengths
that radiate from the center of the bone. Its medial border is in contact
with the supraoccipital. No sensory canals and pores are visible on the
skull roof bones due to their thickness and ornamentation.
Motlayoichthys sergioi n. gen. et n. sp. (a) Drawing of the right side of the head, some
abdominal vertebrae, and a section of the digestive duct of the holotype
UAHMP-600. (b) Drawing of the left side of the head and part of the pectoral
girdle. Dotted sections indicate an area covered by sediment.
Abbreviations: ang, angular; asp: autosphenotic; b.ri: broken ribs; cl:
cleitrum; de: dentary; dig.d: section of digestive duct; dsp: broken and
displaced dermosphenotic; ent: entopterygoid; hy, hyomandibula; io:
infraorbital bones; l.exc: slightly displaced left extrascapula; l.pmx: left
(broken) premaxilla; mx: maxilla; orb: orbitosphenoid; pa[= fr]: parietal
bone; ppa[= pa]: postparietal bone; par: parasphenoid; pec.r: pectoral
rays; pop: preopercle; pmx: premaxilla; pt: pterotic; ptt: displaced
posttemporal; qu: quadrate; r.exc: slightly displaced right extrascapula;
r.pmx: right (broken) premaxilla; scl: supracleitrum; so: supraorbital bone
(broken and displaced in a); soc: supraoccipital; smx: supramaxilla;
vc: abdominal vertebral centra.
Reconstruction of Motlayoichthys sergioi n. gen. et n. sp. Abbreviations: ang: angular;
asp: autosphenotic; de: dentary; dsp: dermosphenotic; exc: extrascapula; io:
infraorbital bone; iop: interopercle; mx: maxilla; orb: orbitosphenoid; op:
opercle; pa[= fr]: parietal bone; par: parasphenoid; pmx: premaxilla; pop:
preopercle; ppa[= pa]: postparietal bone; pt: pterotic; qu: quadrate; smx:
supramaxilla; so: supraorbital bone; soc: supraoccipital; sop: subopercle.
Photographs showing the ornamentation of bones in the head of
Motlayoichthys sergioi n. gen. et n. sp. Arrows point to the ornamentation on (a) the parietal bone
(pa[= fr]); (b) pterotic; (c) preopercle; and (d) narrow crests extending
along the bony surface of the infraorbital bones.
Motlayoichthys sergioi n. gen. et n. sp. (a) Close-up view of a section of the infraorbital
plate io3 + io4 + io5 (right side of head); (b) close view of a section of
the infraorbital plate io1 + io2 (right side of head). Arrows point to the
narrow crests that extend along the bony surface.
Part of the parasphenoid is preserved at the ventral portion of the
braincase. The preserved portion is elongated, thin, and toothless. A thin
interorbital septum (Figs. 4a, b, 5) formed by the orbitosphenoid separates
both orbits almost completely. A well-developed basisphenoid is present, but
a basipterygoid process is absent.
Circumorbital series: The orbit is small, ca. 15 % of the head length. It
is located in an anterior position in the head (preorbital length is ca. 15 % of the head length). The circumorbital series (Figs. 3–5) seems to
be closed anteriorly, but apparently has a small opening at the level of the
autosphenotic, because its spine projects latero-ventrally, separating the
antero-dorsal and postero-dorsal circumorbital regions. The dermosphenotic
is incompletely preserved over the left autosphenotic, whereas its posterior
portion is preserved on the right side of the cranium (Fig. 4). The region
between the lateral margin of the cranium and the dorsal margin of the
infraorbital plate 2 + 3 + 4 + 5 (left side) is partially destroyed leaving
a broad space, suggesting a large dermosphenotic. The circumorbital series
is comprised of a supraorbital bone and two large plates (Figs. 4, 5) that
are interpreted as the result of fusion of infraorbitals 1 + 2 and 3 + 4 + 5
by comparison to pachyrhizodontids with unfused infraorbitals, such as
Goulmimichthysarambourgi (after Cavin, 2001) and Pachyrhizodussalmoneus (after Wenz and Fricot, 1985). However, the
opposite side of the specimen (Figs. 4b, 5) suggests a broader fusion
including infraorbitals 2–5. The lower part of the infraorbital series
reaches the anterior margin of the preopercle, whereas the upper part
extends onto the preopercle and reaches the anterior region of the opercle
(Figs. 4b, 5). The two infraorbital plates are densely covered by long, very
narrow crests (Figs. 3–7). The trajectory of the infraorbital canal (and
its pores) is not visible due to the thickness of the bones and their
ornamentation.
The supraorbital bone is triangular in shape, with a broad antero-ventral
portion and a narrow postero-dorsal portion; ventro-anteriorly it is in
contact with the plate formed by infraorbitals 1 + 2 or infraorbital 1 alone
and with the dermosphenotic posteriorly. An independent antorbital bone is
not present, as in other crossognathiforms; it is unclear whether the
antorbital is fused with infraorbital 1 or is absent. The sclerotic bones
are not preserved.
Upper jaw: The premaxilla (Figs. 3, 4, 8) is somewhat rectangular-shaped,
and no ascending process is observed. The lateral surface of the bone is
ornamented with thin crests of various lengths. A row of a few large,
slightly recurved teeth is preserved in the oral margin. This row's teeth
are slightly different in size. The premaxillary internal tooth (Fig. 8a),
characteristic of pachyrhizodontids, is located medial to the external
teeth; it is larger, approximately twice the size of the external teeth, and
more curved.
The tips of the teeth, where the preservation permits observation, have
their acrodin cups ornamented with longitudinal striae. This kind of tooth
has not been previously reported in the jaws of any pachyrhizodontoid
species. Additionally, one to three deep grooves are observed in the lateral
surface of some of premaxillary teeth.
Drawing of a region of the premaxilla of Motlayoichthys sergioi n. gen. et n. sp.,
showing premaxillary teeth with acrodin tips and ornamentation, the inner
premaxillary tooth (a), and a dentary tooth (b). Abbreviations:
i.pmxt:
inner premaxillary tooth; mx: maxilla; pmx: premaxilla.
Motlayoichthys sergioi n. gen. et n. sp. (a) Impression of the abdominal vertebral region
partially preserved in the slab, and of the dorsal and pectoral fins;
(b) drawing of the left pectoral fin. Abbreviations: d.pt: dorsal
pterygiophores; epl: epipleurals; epn: epineurals; hs: haemal spines; m.pt:
middle radials; ns: neural spines; p.r: pectoral ray; p.ra: pectoral
proximal radials; r: dorsal rays; ri: ribs; vc: vertebral centra.
The maxilla (Figs. 3–5) is approximately rectangular in shape with its
posterior region curved ventrally and a concave ventral margin in its
posterior half. Parallel longitudinal striae apparently cover the whole bony
surface, although they are incompletely preserved or damaged in the
holotype. A single row of about 10 straight and conical teeth, slightly
smaller than those of the premaxilla, are observed in the left side of the
head; three teeth are present anteriorly, and nine teeth posteriorly. The
posterior teeth are closer to each other and wider than the anterior ones.
The rest of the teeth are not preserved. The right side of the head shows
about 12 teeth slightly smaller than those of the premaxilla in the anterior
portion. Each tooth has an acrodin tip similar to the one in the
premaxillary teeth. The supramaxilla (Figs. 4a, b, 5) is an elongated bone
at the postero-dorsal margin of the maxilla and occupies about 50 % of the
length of the maxilla.
Lower jaw: The position of the anterior tip of the lower jaw – anterior to
the premaxilla – gives the fish a characteristic aspect (Fig. 5) that
differentiates it among pachyrhizodontids where the anterior ends of the
dentary and premaxilla are at (or nearly at) the same level. Laterally, the
lower jaw is formed by the dentary and the angular. The dentary (Figs. 3–5)
is narrower close to the symphysis and almost rectangular posteriorly, with
the oral and ventral borders almost parallel along their lengths so that a
coronoid process is not developed. The dentary is approximately 58 % of
the head length. Its lateral surface is ornamented with fine, longitudinal
crests running parallel to the bone borders. The dentary presents a single
row of teeth that are incompletely preserved in both jaws. The right dentary
has eight conical teeth preserved, whereas the left jaw has seven preserved;
they are larger (Fig. 8b) than those of the maxilla and exhibit acrodin tips
that are not well exposed in the specimen. They may bear a deep groove on
their exposed surfaces. The lateral portion of the angular with a
well-developed postarticular process is preserved. The suture between the
angular and dentary is partially obscured by the longitudinal crests
extending along the bony surface. The presence of a retroarticular bone has
not been observed, because the postero-ventral corner of the jaw is damaged.
Hyopalatine series: The hyomandibula has a simple and broad articular head
with the cranium, but its posterior opercular process is not preserved; its
ventral region is long and narrow (Fig. 4b), and forming an angle of about
90 degrees with respect to the body axis. The quadrate, that is incompletely
exposed, is triangular in shape, with a large articular condyle. The
entopterygoid is partially exposed in the right side of the specimen, no
teeth or sockets for teeth are visible. Other bones of the hyopalatine
series are not visible, because they are covered by the infraorbitals.
Opercular series, branchiostegal rays, and gular plate: The opercular series
is not clearly visible on either side of the specimen, because it is
partially covered by the distal tip of postero-dorsal infraorbital plate,
and the posterior margins of the opercle and subopercle are weathered and
partially covered by sediment. The shape of the preopercle is unclear. Part
of the ventral limb of the preopercle is observed with the postero-ventral
portion moderately expanded and markedly inclined postero-ventrally (Figs. 3–5).
There are at least four branches of the preopercular sensory canal in
the ventral limb (Fig. 4). The preserved sutural region between opercle and
subopercle (Fig. 4b) and the restoration of the region show that the
subopercle seems to be larger than the opercle (Fig. 5). The interopercle is
not preserved. No branchiostegal rays are preserved. A gular plate is not
present between the lower jaws.
Vertebral column: Forty-seven well-ossified, narrow vertebral centra with a
height of ca. 10 mm are preserved (Figs. 4, 9). They were kept in the
original slab during preparation, and 22 vertebrae remained articulated with
the head (Fig. 4). The centra are smooth with no ridges or grooves on their
lateral surfaces. The neural arches and spines seem to be particularly
narrow, because there is not a marked difference between them in the
preserved vertebral column. The neural arches seem to be autogenous;
however, it is not possible to clarify whether the neural spines are simple
or double. The impression of long and narrow neural spines is seen behind
the head and along the vertebral series. Although ribs are not preserved,
their impressions are observed associated with the first 43 vertebrae. The
ribs are slender with their proximal bases slightly expanded. Remains of
some epineurals lie above centra 28 to 36. They are slender and long,
extending about the length of five centra. Some remains of epipleurals are
present in the abdominal region; they are more evident from vertebrae 26 to
38. A few long narrow bones lie below and lateral to some centra. We
interpret these elements as displaced ribs.
Pectoral girdle and fin: During the chemical and mechanical preparation of
the specimen, some fragments of the posttemporal, cleithrum and
supracleithrum were lost; nevertheless, it is still possible to observe that
incomplete cleithrum has a boomerang form. The antero-dorsal border of the
cleithrum is covered by the supracleithrum, which is a straight, elongated,
and narrow bone (Fig. 4b), whose dorsal region is incompletely preserved.
Some striae running parallel to the postero-ventral portion of the
supracleithrum are observed. Dorsal to the supracleithrum is a displaced
posttemporal lying on the extrascapula (see Fig. 4). Although the bone is
missing part of its anterior region, it is possible to see that the
posttemporal is an elongate bone.
The pectoral fin, which has a low position in the flank (Figs. 3, 4, 9),
consists of 10 unbranched rays distally segmented. The outermost ray is
wider than the rest and is spine-like. It is a compound element like that
present in many primitive teleosts and a few recent ones (e.g.,
elopiforms) – as described by Arratia (2008b, 2009). Four pectoral proximal
radials (Fig. 9) are incompletely preserved, they are displaced near the
pectoral fin.
Dorsal fin: The dorsal fin is incompletely preserved so that its total
length and size is unknown. At the level of the 13th vertebra, the
impression of 12 long and slender proximal radials is observed on the
original slab (Fig. 9), suggesting an anterior position of the dorsal fin in
the trunk. The first pterygiophore presents a slight expansion proximally;
the rest are narrow bones. There is an equal number of small middle
radials lying above the proximal ones, but at least seven more middle
radials are located behind these associated with preserved proximal
radials. It is possible that the fin continued posteriorly, as occurs
in Pachyrhizodus caninus (Shimada, 2015) and Tingitanichthysheterodon (Taverne, 1996) that have a similar, anteriorly
positioned dorsal fin. The pelvic, anal, and caudal fins are not
preserved.
Taxonomic affinities and comparisons
Even though up until now only one incomplete specimen of the taxon described
here has
been found in the Muhi Quarry, the study of the skull bones and postcranial
skeleton of the specimen allowed its taxonomic assignation as a new genus
and species. Motlayoichthys sergioi n. gen. et n. sp. presents the following characters that are
interpreted as synapomorphies of the order Crossognathiformes: a large and
well-developed extrascapular bone, toothless parasphenoid, and an absent gular plate (Arratia, 2008a; Arratia and Tischlinger, 2010). Other features such
as the roofing of the posttemporal fossa, the retroarticular excluded from
the articular facet for quadrate, and an acuminate dorsal fin cannot be
determined because of incomplete preservation. Motlayoichthys sergioi n. gen. et n. sp. shares
numerous characters with members of the suborder Pachyrhizodontoidei e.g.,
the presence of an orbitosphenoid and a basisphenoid, a hyomandibula with a
single articulatory head, a premaxilla with small dorsal process, a single
row of maxillary teeth, a single, splint-like supramaxilla, a well-developed
postarticular process in the lower jaw, absence of a basipterygoid process,
no separate antorbital, and a pectoral fin positioned low on the flank
(Forey, 1977).
Motlayoichthys sergioi n. gen. et n. sp. shares with members of the family Pachyrhizodontidae
(Forey, 1977) a long parietal bone (= frontal of traditional terminology)
with the lateral margin excavated above the autosphenotic spine and in
having an internal premaxillary tooth present. Although the fusion of
certain infraorbital bones has been interpreted as a pachyrhizodontoid
character (Forey, 1977), not all genera described and/or illustrated in
the literature share this character (see Fig. 10a, b). It also shares with
other pachyrhizodontoids a quadrate-mandibular articulation placed posterior
to the posterior margin of the orbit (Arratia and Tischlinger, 2010).
Heads in lateral view of certain pachyrhizodontoids.
(a)Pachyrhizodus salmoneus (modified from Wenz and Fricot,
1985), (b)Goulmimichthysarambourgi (modified
from Cavin, 2001), (c)Notelops brama (slightly modified
from Forey, 1977), and (d)Rhacolepis buccalis (slightly
modified from Forey, 1977). Abbreviations: de: dentary; dsp: dermosphenotic;
io1–5: infraorbitals 1–5; mx: maxilla; op: opercle; pa[= fr]: parietal bone
[= frontal of traditional terminology]; pmx: premaxilla; pop: preopercle;
qu:
quadrate; ppa[= pa]: postparietal bone [= parietal of traditional terminology];
so: supraorbital bone; smx: supramaxilla; sop: subopercle.
A revision of the pachyrhizodontid genera was done to compare with the new
pachyrhizodontid from Muhi Quarry, and though we found some similarities,
the new genus does exhibit a unique combination of characters and several
autapomorphies (see Diagnosis); for instance: the head shape is nearly an
equilateral triangle, whereas other pachyrhizodontids (and also Notelops) have an
elongate head (Fig. 10a–d), as do the oldest crossognathiforms,
Chongichthys (Jurassic: Oxfordian; Arratia, 1982, 2008a) and Bavarichthys (Jurassic:
Kimmeridgian?;
Arratia and Tischlinger, 2010). An exception is Tingitanichthys (Cretaceous) with a short and
deep head (Taverne, 1996). The lower jaw extending anterior to the premaxilla
is a unique feature of Motlayoichthys sergioi n. gen. et n. sp. In the remaining
pachyrhizodontoids, the jaws end at the same level (compare Figs. 5 and 10).
The characteristic maxilla of Motlayoichthys sergioi n. gen. et n. sp. has a concavity in the
posterior half of its ventral border, and its posterior tip is inclined
ventrally, whereas in other pachyrhizodontoids the maxilla is narrow, with
straight dorsal and ventral margins.
Motlayoichthys sergioi n. gen. et n. sp. possesses an almost complete interorbital septum (Figs. 4a, b, 5),
separating both eyes. Apparently, Michin scernai also has a nearly complete
septum (Citlalli Hernández-Guerrero, personal observation, October 2013). A reduced interorbital septum (Fig. 10d) is
present in Rhacolepis buccalis (Forey, 1977: figs. 13, 20) and also in the Jurassic genera
Chongichthys (Arratia, 1982: fig. 4) and Bavarichthys (Arratia and Tischlinger, 2010: fig. 5). It is
unclear whether the bone is reduced in size in other pachyrhizodontids,
because it has not been mentioned or illustrated in the literature – the
bone is frequently damaged because of its thinness. The circumorbital ring
seems to be completely closed in pachyrhizodontids such as Pachyrhizodus salmoneus (Fig. 10a), Rhacolepis buccalis
(Fig. 10d), and Goulmimichthys (Fig. 10b) species, as well as in the notelopid Notelops brama (Fig. 10c),
whereas the ring is open at the level of the autosphenotic in
Motlayoichthys sergioi. The condition is unclear in Michin due to incomplete preservation.
Motlayoichthys sergioi has a small orbit occupying ca. 15 % of its head length, the smallest among
pachyrhizodontids such as Goulmimichthysarambourgi (ca. 19 %; see Fig. 10b) and Goulmimichthysgasparini
(Páramo-Fonseca, 2001), Rhacolepis buccalis (ca. 20 %; Fig. 10d), Pachyrhizodus salmoneus (ca. 25 %; Fig. 10a),
and Michinscernai (ca. 25 %; Alvarado-Ortega et al., 2008), and the notelopid
Notelops brama (about 24 %; see Fig. 10c). Additionally, the preorbital length of
Motlayoichthys sergioi n. gen. et n. sp. (ca. 15 % of head length) and of Pachyrhizodus salmoneus (ca. 17 %) are
considerably shorter than that of Aquilopiscis wilsoni, Goulmimichthys species,
Rhacolepis buccalis, and Michinscernai (which are about 25 %
of head length).
Heavily ornamented cranial bones covered by long striae and/or longitudinal
crests characterize Motlayoichthys sergioi n. gen. et n. sp. (see Figs. 4–7). Similar striae are
present in Pachyrhizodusminimus (KUVP-300), Rhacolepis buccalis (Voltani and Bertini, 2012), and P. caninus (FHSM VP-218;
Citlalli Hernández-Guerrero, personal observation, November 2013), with the ornamentation located only on the jaw bones and
parietals. Apparently, this is a character unique to these species among
pachyrhizodontids. The premaxilla of P. caninus from Vallecillo, Mexico is almost
completely covered by well-defined striae (Giersch et al., 2010b: fig. 4,
photograph).
Motlayoichthys sergioi n. gen. et n. sp. is interpreted here as having the infraorbital bones
fused into two asymmetric plates (infraorbital 1 + 2 and 3–5 on the right
side of the head and io1 and io2–5 on the left; Fig. 4). Infraorbitals 2 + 3
and 4 + 5 are interpreted as fused in the notelopid Notelops brama by Forey (1977; see
Fig. 10c), and in the pachyrhizodontids Tingitanichthys by Taverne (1996) and Michinscernai by
Alvarado-Ortega et al. (2008: fig. 2c). However, a suture between
infraorbitals 4 and 5 was observed during examination of the holotype of
Michin scernai (Citlalli Hernández-Guerrero, personal observation, October 2013).
In contrast, all infraorbital bones are independent in
pachyrhizodontids such as Goulmimichthys (Fig. 10b); Cavin, 2001; Páramo-Fonseca,
2001), Greenwoodella tockensis (Taverne, 1973), Pachyrhizodus megalops (Forey, 1977),
Pachyrhizodus salmoneus (Fig. 10a: Wenz and Fricot, 1985),
and Pachyrhizodus etayoi (Páramo-Fonseca, 2001).
The occurrence of conspicuous longitudinal crests on the infraorbital bones
is a feature shared with Goulmimichthys arambourgi (Cavin, 2001), Pachyrhizodus salmoneus (Wenz and Fricot, 1985),
Pachyrhizodus etayoi (interpretation of photographs kindly provided by María Eurídice Páramo Fonseca),
and the crossognathiform Crossognathus sabaudianus (Taverne, 1989). Moreover, we have observed that
the infraorbital crests can be tenuous and narrow or pronounced and wide,
running along the bone, or running only from the middle portion of the bone
to the posterior part. Wide crests in the posterior half of infraorbitals 3,
4 and 5, which are different from those present in Motlayoichthys sergioi n. gen. et n. sp., have
been observed in specimens assigned to Pachyrhizodus minimus (KUVP-300 and KUVP-36032). A review
of the literature revealed the presence of wide interdigitations in
Goulmimichthys arambourgi (Cavin, 2001), Pachyrhizodus salmoneus (Wenz and Fricot, 1985), Pachyrhizodus etayoi (Páramo-Fonseca, 2001), and the
crossognathiform Crossognathus sabaudianus (Taverne, 1989).
Most pachyrhizodontoids possess jaws bearing many teeth of similar sizes,
generally small (see for instance Fig. 10a–d), a condition also observed in
the Jurassic forms (see Arratia, 1982: figs. 6, 8 for Chongichthys dentatus; Arratia and
Tischlinger, 2010: fig. 5 for Bavarichthys incognitus). In contrast, teeth of different sizes are
present in Michin scernai (Alvarado-Ortega et al., 2008: fig. 2), and apparently in
Aquilopiscis wilsoni (Cumbaa and Murray, 2008: figs. 6B, 7–9). By comparison, the new fish here
described presents fewer teeth, but they are larger in size (Figs. 4, 5).
Acrodin tips bearing some characteristic striations and deep grooves along
certain teeth seem to be unique to Motlayoichthys sergioi n. gen. et n. sp. However, Michin scernai seems to
present some striae on the surface of the teeth (Citlalli Hernández-Guerrero, personal observation, October 2013). Tooth
ornamentation seems to be more common in pachyrhizodontids than previously
thought, as observed in Michin scernai
(Citlalli Hernández-Guerrero, personal observation, October 2013) and in Aquilopiscis wilsoni (Cumbaa and Murray,
2008).
Apparently, a general pachyrhizodontoid condition is the presence of teeth
ordered in one main row. Motlayoichthys sergioi n. gen. et n. sp. shares this character with other
pachyrhizodontoids (see for instance Figs. 4, 5, 10). In contrast, Michinscernai bears three rows of teeth in the lower jaw (Alvarado-Ortega et al., 2008);
however, only one row of teeth is observed in the holotype IGM-9028 (Citlalli Hernández-Guerrero, personal observation, October 2013).
The dentary of the Late Jurassic Bavarichthys incognitus has a platform-like oral
margin covered with many small conical, villiform teeth (Arratia and
Tischlinger, 2010: figs. 4C, 5), whereas the dentary teeth of the basal
crossognathiform Chongichthys dentatus (from the Oxfordian of Chile) are small and conical in the
outer row close to the symphysis and become larger posteriorly and in the
inner rows (Arratia, 1997).
Motlayoichthys sergioi n. gen. et n. sp. has an elongate, styliform-like supramaxillary bone (Fig. 4)
that is a diagnostic feature of pachyrhizodontoids after Forey (1977).
The bone is about 50 % of the length of the maxilla as in Goulmimichthys arambourgi (Fig. 10b), but
it is shorter in other pachyrhizodontoids (e.g., Fig. 10). Two supramaxillae
of different sizes were illustrated in some specimens of the notelopid
Notelops by Maisey (1991b). Two supramaxillae are known in the Late Jurassic
Bavarichthys, with supramaxilla 2 smaller than supramaxilla 1 (Arratia and Tischlinger,
2010: fig. 5).
The dorsal fin position varies among pachyrhizodontoids; the fin can be
located close to the head or far back and can extend almost the entire
length of the trunk or be short. Although in Motlayoichthys sergioi n. gen. et n. sp., the dorsal
fin is not preserved, it is possible to observe it as an impression (see
Fig. 9a) that starts at the level of the 13th vertebra, close to the head as
occurs in Pachyrhizoduscaninus (Shimada, 2015), in which the dorsal fin starts at the level of
the 10th vertebra. This anterior position of the fin is also observed in
Tingitanichthysheterodon (Taverne, 1996) in which it starts about the 11th vertebra. Moreover,
Pachyrhizoduscaninus and Tingitanichthysheterodon have long dorsal fins, with about 36 rays in the former, and three
spines and 27 rays in the latter. The remaining pachyrhizodontoids present a
triangular, short dorsal fin positioned about the middle length of the trunk
or more posteriorly, for example, Michincsernai (Alvarado-Ortega et al., 2008: fig. 1)
has a dorsal fin composed by 15 rays originated at about the 20th
vertebra; Elopopsis microdon (Taverne, 1994) has 16 dorsal fin rays with the fin originating
at the 27th vertebra; Platinx macropterus (Taverne, 1980) has a dorsal fin with two spines
and 22 rays, originating at the 44th vertebra. Therefore, we suggest
that Motlayoichthys sergioi n. gen. et n. sp. had a long dorsal fin according to its anterior
position and the 22 medial radials preserved, perhaps similar to
Pachyrhizoduscaninus. Finally, we consider that the combination of characters present in
Motlayoichthys sergioi strongly support its assignment as a new genus and species within the
family Pachyrhizodontidae.
Pachyrhizodontoids during the Lower Cretaceous (Albian)
The oldest crossognathiforms are known from the Oxfordian of Chile (Arratia,
2008a, 2015) and from the Kimmeridgian of Germany (Arratia and Tischlinger,
2010), which certainly represent very distant locations and are interpreted
as belonging to the Tethys realm. Biogeographic hypotheses suggest a
connection between these distant places in the Northern and Southern
Hemispheres through the so-called Hispanic Corridor that was formed by the
separation of North and South America (Hallam, 1983; Aberhan, 2002; Porter
et al., 2013). From that beginning, the pachyrhizodontids and relatives,
apparently underwent a significant radiation during the Cretaceous
with one genus known from the Aptian of Germany (Greenwoodella; Taverne, 1973) and five
genera from the Albian time: the notelopid Notelops from Brazil (South America), and
the pachyrhizodontids Rhacolepis from Brazil (South America) and Mexico (North
America), Pachyrhizodus from England and France (Europe), as well as Queensland
(Australia), and Michin and Motlayoichthys n. gen. from Mexico (North America). Of these genera,
only Pachyrhizodus reached the Cenomanian (of United States; Shimada, 2015) and Turonian
times (of United States, Mexico and Colombia; Páramo-Fonseca, 2001;
Alvarado-Ortega et al., 2006; Giersch et al., 2010b; González-Rodríguez et al., 2016). Tingitanichthys was first recovered in the Cenomanian of
Morocco (Taverne, 1996) and more recently in the lower Turonian of Mexico
(Giersch et al., 2010a; González-Rodríguez et al., 2016).
Conversely, Goulmimichthys, is the youngest pachyrhizodontid, reported from the Turonian of
Mexico (Blanco and Cavin, 2003). It is also known from the Turonian of
Colombia (Páramo-Fonseca, 2001) and Morocco (Cavin, 2001). Another
Turonian pachyrhizodontid, Aquilopiscis, is known only from the Northwest Territories of
Canada (Cumbaa and Murray, 2008). According to the available information,
the notelopid Notelops is endemic to Brazil (Forey, 1977; Maisey, 1991b), whereas
the pachyrhizodontids Michin and Motlayoichthys n. gen. are endemic to Mexico. Rhacolepis, a genus that was
previously considered as endemic to Brazil (e.g., Forey, 1977; Maisey,
1991a), was recently reported in Mexico by Espinosa-Arrubarrena and
Alvarado-Ortega (2010). Currently, the Muhi Quarry is an important locality
from where several fishes belonging to a broad array of teleostean groups
are known from this locality: primitive teleosts such as the
crossognathiform pachyrhizodontid Motlayoichthys sergioi n. gen. et sp., euteleosts such as the
aulopiforms Ichthyotringa mexicana (Fielitz and González-Rodríguez, 2008) and Enchodus zimapanensis (Fielitz and
González-Rodríguez, 2010), the stem acanthomorph Muhichthys cordobai
(González-Rodríguez and Fielitz, 2008), and the miniature armored
acanthomorphs, Handuichthys interopercularis, Pseudomonocentris microspinosus and Dalgoichthys tropicalis (González-Rodríguez et al., 2013b).
All material included in the paper is accessible in the listed museums and all data are
included in the descriptions.
The authors declare that they have no conflict of
interest.
Acknowledgements
The authors thank Sergio Yañez, worker of the quarry for collecting the
specimen that was named to honor his effort to conserve the fossil patrimony
of the Muhi Quarry. We thank especially Jesús Alvarado-Ortega and María Eurídice Páramo-Fonseca
who kindly made available photographs of Michin csernai and
Pachyrhizodusetayoi, respectively. We thank María Carmen Perrilliat for permitting the third author to
study fossil material deposited in the “Ma. Carmen Perrilliat” Museum of
the Instituto de Geología, Universidad Nacional Autónoma de
México and Hans-Peter Schultze for helping Citlalli Hernández-Guerrero with the pachyrhizodontoid
fish collection of the Division of Vertebrate Paleontology, Natural History
Museum, University of Kansas, Lawrence. Erika Alonzo helped with the editing
of figures. Terry J. Meehan checked the manuscript for grammar and style. Alison Murray
and an anonymous reviewer provided interesting and helpful comments
and criticisms.
Edited by: Torsten Scheyer
Reviewed by: Alison Murray and one anonymous referee
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