Introduction
The pig-nosed turtle Carettochelys insculpta Ramsay, 1887 from New Guinea and Australia is the only
surviving representative of the clade Carettochelyidae (Pan-Carettochelys of
Joyce et al., 2004). Highly fragmentary remains document the early evolution
of this clade during the Cretaceous in Asia, but sometimes exceptionally
rich material attests to the spreading of carettochelyids to all northern
continents during the Paleogene. The Neogene record is restricted to
exceptionally rare finds from Europe, northern Africa, south Asia, and
Australia (Joyce, 2014).
The majority of carettochelyid fossil finds are shell fragments (Joyce,
2014), which can be diagnostic by their highly distinct surface texture
(e.g., Hutchison, 1996). Cranial remains have been reported from the Late
Cretaceous of Uzbekistan (Nessov, 1977a, b, c), the Eocene of China
(Danilov et al., 2017), England (Lydekker, 1889), France (Godinot et al.,
2018), Germany (Harrassowitz, 1922; Joyce et al., 2012), and Spain (Joyce,
2014), and the Miocene of Libya (Havlik et al., 2014), but only a few
specimens have been properly figured and described.
A mid-20th century expedition of the Field Museum of Natural History (FMNH)
to the Washakie Basin of Sweetwater County, Wyoming, yielded an isolated
pan-carettochelyid skull from the middle Eocene (early Uintan North American
land mammal “age”, NALMA) Adobe Town Member (Twka2 or Twka3) of the
Washakie Formation. Gaffney (1979, fig. 173) provided a reconstruction of
this fossil (FMNH PR966) under the name “Anosteira” or “Pseudanosteira”, whereas Havlik et al. (2014)
used it to score all cranial aspects of the taxon “Anosteira pulchra”, but a formal
description is still missing. Given that FMNH PR966 is the only known
pan-carettochelyid skull from North America, the primary purpose of this
contribution is to provide a description of this fossil.
Methods
We subjected FMNH PR966 to high-resolution X-ray microcomputed tomography (CT) using a Bruker SkyScan 2211 at the University of Fribourg, Switzerland,
with an exposure time of 37 ms, a voltage of 190 kV, a current of 50 mA, and
no filter. The 1800 projections that were acquired over 360∘ were
converted into 1160 coronal slices with a voxel size of 23 µm using
the native software of the machine. The slice stack is available upon
request at FMNH.
For comparative purposes, we obtained the CT scans of BMNH (Natural History Museum London) 1903.7.10.1, a skeleton of the extant
Carettochelys insculpta, which had been acquired by Serjoscha Evers using the in-house scanner with
an exposure time of 500 ms, a voltage of 180 kV, a current of 190 mA, and
a copper filter. The 3142 projections over 360∘ were converted
into 1935 coronal sections. This dataset will be made available to the
public in a separate publication that is currently in preparation.
Where possible, the basisphenoid, the right pterygoid, the carotid canals,
the canalis cavernosus, and the facial nerve canals were visualized for
Anosteira pulchra and C. insculpta using the software Amira (6.1.1). All reconstructions were obtained by
segmentation followed by production of isosurface models. The boundaries of
the bones and canals were delimited manually using the brush and lasso tools
of Amira in every second to fifth slice, depending on the complexity of the
contact. The remaining slices were then interpolated. The resulting 3-D
models are available upon request at FMNH.
As few formal descriptions of fossil carettochelyids are available, here we
compare FMNH PR966 mostly to Anosteira maomingensis (as described by Danilov et al., 2017) and
Allaeochelys spp. (as described by Havlik et al., 2014, and Godinot et al., 2018).
To investigate the phylogenetic placement of Anosteira pulchra, we updated the phylogenetic
analysis of Danilov et al. (2017), which in return is based on Joyce (2007)
and Havlik et al. (2014). We accepted all changes that Danilov et al. (2017)
made to the matrix of Havlik et al. (2014), with the exception of the following
updates: (1) A. pulchra was scored 1 not ? for character 148 (see Sect. 4);
(2) A. pulchra was scored 0 not ? for character 149 (see Sect. 4); (3) Allaeochelys libyca
was scored ? not 0 for character 149, as we find the relevant specimens not
to be preserved sufficiently to allow scoring; finally, (4) character 150
(distance of the foramen posterius canalis carotici interni from the
basisphenoid) was removed from the matrix, as we could not replicate the
scoring among derived carettochelyids because most extant taxa were
incorrectly scored as inapplicable, and because it appears to be correlated
with character 148 (presence of a pterygoid–pterygoid suture flooring the
foramen posterius canalis carotici interni). The revised matrix is available
in the Supplement.
The data matrix was subjected to a parsimony analysis using TNT (Goloboff et
al., 2008). Following Havlik et al. (2014), characters 7, 27, 33, 35, 54,
60, 61, 65, 68, 71, 85, 89, 98, 120, 133, 134, and 142 were run ordered.
Following Havlik et al. (2014), we also restricted the ingroup to
carettochelyids and utilized Adocus sp. as the outgroup. The matrix was subjected
to 1000 replicates of random addition sequences followed by a second round
of tree bisection-reconnection. The analysis was also performed using the
same parameters using implied weights with k values ranging from 1 to 12 at
full integers.
FMNH PR966, Anosteira pulchra, skull, middle Eocene Washakie Formation,
Sweetwater County, Wyoming, USA. A 3-D rendered model and illustrations in
(a) dorsal, (b) right lateral, (c) left lateral, and (d) ventral views.
Abbreviations are as follows: ap: antrum postoticum; bo: basioccipital; bs: basisphenoid;
dppf: descending process of the prefrontal; epi: epipterygoid; ex: exoccipital; fim: foramen
intermaxillare; fnt: foramen nervi trigemini; fpcci: foramen posterius canalis carotici
interni; fpp: foramen palatinum posterius; fr: frontal; ica: incisura columella auris; ju: jugal; mx: maxilla; op: opisthotic;
pa: parietal; pal: palatine; pf: prefrontal; po: postorbital;
pro: prootic; pt: pterygoid; qj: quadratojugal; qu: quadrate; so: supraoccipital; sq: squamosal; vo: vomer.
Description
Cranium
The preserved midline length of FMNH PR966 is only 38 mm
and, by comparison to Carettochelys insculpta, we estimate the original midline length to have been
only a few millimeters more (Fig. 1). The skull is generally well preserved, but much
dorsoventral crushing resulted in many cracks and significant distortion.
The CT imagery furthermore highlights areas fully replaced by glue. The
skull was likely buried completely prior to fossilization, but the
premaxillae, the right postorbital, much of the quadratojugals, the
articular process of the left quadrate, the posterior aspects of both
squamosals, and the posterior aspect of the supraoccipital were lost post
burial, likely during recovery. The dorsal skull roof shows fine texturing
that resembled hammered metal. This stands in stark contrast to the shell
sculpturing of Anosteira spp., which consists of fine raised ridges and nobs (Hay,
1908). The skull of FMNH PR966 is much narrower and lower than other
carettochelyids, even when crushing is taken into account.
Prefrontals
The prefrontals are relatively large bones that
contact the maxillae laterally, the frontals posteriorly, and one another
medially. The well-developed anterior process of the frontals partially
separates the prefrontals along the midline in both views (contra Gaffney,
1979). The descending process of the prefrontals is missing on the right
side, but the strut-like left descending process clearly documents a ventral
contact with the palatal bones, much as in C. insculpta, but the exact contacts are
unclear. The anterior margins of the prefrontals are intact and reveal that
the external nares were oriented anterodorsally. The interorbital bar formed
by the prefrontals and frontals is furthermore relatively narrow and the
orbits therefore face somewhat dorsolaterally. These observations contrast
the morphology of Anosteira maomingensis, Allaeochelys spp., and C. insculpta, which have anteriorly oriented external
nares, laterally oriented orbits, and a broad interorbital bar.
Frontals
The frontals are large elements that contact the
prefrontals anteriorly, the parietals posteriorly, and the postorbitals
posterolaterally, and they broadly contribute to the dorsal margin of the orbit.
The frontals combined form a strong anterior process that substantially
reduces the midline contact of the prefrontals in both dorsal and ventral
view. The ventral aspect of this process is decorated by a ridge, which
delimits the sulcus olfactorius.
Parietals
The parietals are the largest skull roofing elements and
contact the frontals anteriorly, the postorbitals anterolaterally, and one
another on the dorsal surface. A lateral contact with the jugals or
quadratojugals appears to be absent, as in C. insculpta. The descending process of the
parietals appears to have been relatively short. Although cracks make it
difficult to interpret the contribution of the parietals to the upper
temporal fossa, it appears clear that the parietals contacted the prootics
anterolaterally and the opisthotics posterolaterally but did not contribute
to the foramen stapedio-temporale. Crushing and matrix obscure any potential
ventral contacts of the parietals in the trigeminal region. It similarly is
unclear if the trigeminal foramen is split.
Postorbitals
The right postorbital is missing, but the left
postorbital appears to be complete, though slightly displaced. The
postorbitals contact the frontals anteromedially, the parietals
posteromedially, and the jugals ventrolaterally, and they contribute to the orbital
margin anteriorly and the upper temporal emargination posteriorly. The
postorbital bar is much narrower than in C. insculpta and the jugals therefore appear to
contribute to the upper temporal emargination. The postorbitals, conversely,
are anteroposteriorly short and do not contact the quadratojugals, which
both contrast the condition seen in An. maomingensis, Allaeochelys spp., and C. insculpta.
Jugals
Some of the most notable differences from other
carettochelyids are apparent in the jugals of FMNH PR966. The jugals
contribute to the temporal bar, which must have been much lower
dorsoventrally than reconstructed by Gaffney (1979), even when crushing is
taken into account. The jugals otherwise contact the postorbitals medially,
possibly contribute to the upper temporal emargination posteriorly, contact
the quadratojugals posteriorly, contribute to the nuanced lower temporal
emargination ventrally, contact the maxillae below the orbit, and broadly
floor the posterior aspects of the orbit. In Allaeochelys spp. and C. insculpta, by contrast, the
quadratojugals have well-developed anterior processes that block the jugals
from contributing to the upper and lower temporal emarginations.
Quadratojugals
The left quadratojugal appears to be missing
completely, but three fragments of the right quadratojugal remain that
combined allow the deduction of much of the morphology of this element. The anterior
fragment is visible in right lateral view. This fragment broadly underlays
the jugal and is much larger than apparent from lateral view. The second
fragment contacts the quadrate posteroventrally and contributes to the cavum
tympani (contra Gaffney, 1979). The third fragment, finally, broadly
contacts the quadrate posteroventrally and clearly contacts the squamosal
posteriorly (contra Gaffney, 1979). It is unclear if a contact was present
with the postorbital. The most notable difference from Allaeochelys spp. and C. insculpta is the
lacking anterior contact of the quadratojugal with the maxilla, a feature
otherwise seen in An. maomingensis.
Squamosals
Both squamosals are present, but the posterior aspects
are heavily damaged. In lateral view, the squamosals frame much of the
posterior rim of the cavum tympani and contact the quadratojugals anteriorly,
the quadrates ventrally, and the opisthotics posteriorly. Well-defined antra
postotica are apparent on both sides that are highly reduced, though
somewhat larger than in An. maomingensis, Allaeochelys spp., and C. insculpta. Within the upper temporal fossa, the
squamosals narrowly define the lateral margin of the skull in form of a
ridge and otherwise broadly contact the quadrates and opisthotics medially.
The squamosal horns are damaged on both sides of the skull and their
posterior extent is therefore unclear.
Premaxillae
The premaxillae are not preserved.
Maxillae
In contrast to An. maomingensis, Allaeochelys spp., and C. insculpta, the maxillae are notably low in
lateral view, which supports the notion that the skull of FMNH PR966 was
relatively low, even prior to crushing. In lateral view, the maxillae form
the anteroventral margin of the orbit, contact the prefrontals dorsally, the
jugals posteriorly, the premaxillae likely anteromedially, and
broadly floor the anterior aspects of the orbit. In ventral view, the
maxillae form the low but blunt labial ridge, form a modestly broad and flat
triturating surface, medially contact the palatines, and posteriorly contact
the jugals and pterygoids. The maxillae finally frame a large foramen
intermaxillaris. A posterior contact with the quadratojugals is missing.
Vomer
As in all trionychians, the vomer is a highly reduced,
rod-like bone with a low ventral ridge. It lacks an anterior contact with
the premaxillae and thereby helps to create the large foramen
intermaxillaris. The vomer otherwise contacts the strut-like descending
process of the prefrontals anterolaterally and the palatines
posterolaterally.
Palatines
The palatines are large elements that roof much of the
primary palate. They contact the vomer and the strut-like descending process
of the prefrontal anteriorly, the maxillae and pterygoids laterally, and the
basisphenoid posteriorly. The palatines form small foramina palatinum
posterius near the lateral contact of the palatines with the pterygoids.
Crushing and matrix obscure the ascending processes of the palatines.
Pterygoids
The pterygoids are anteroposteriorly elongate elements
that broadly brace the braincase. The anterior plate delimits the lateral
margins of the broad nasal canal and contacts the maxillae anteriorly and the
palatines and basisphenoid medially. The mandibular process contacts the
quadrates posterolaterally but does not reach the articular condyles. The
broad posterior process broadly contacts the basisphenoid medially, the
basioccipital posteromedially, the quadrates and opisthotics laterally, the
exoccipital posteriorly, forms the margin of the reduced fenestra postotica,
and contributes to the tubera basioccipitalis, a broad set of morphological
characteristics seen in other carettochelyids. However, in contrast to An. maomingensis,
Allaeochelys spp., and C. insculpta, a deep, triangular pterygoid fossa is lacking and the foramen
posterius canalis carotici interni is not located within the pterygoid, but
rather at the contact with the basisphenoid, somewhat reminiscent of the
condition seen in paracryptodires.
Epipterygoids
The epipterygoids are preserved on both sides of the
skull but partially obscured by crushing and matrix. From what can be seen,
it appears that the epipterygoids are anteroposteriorly elongate elements
that contact the prootic posteriorly and the pterygoid ventrally and frame the
posterior portion of the trigeminal foramen dorsally. The anterior aspects
of this bone are unclear.
Quadrate
The quadrates are both relatively well preserved, but
various parts are missing or somewhat obscured by crushing. In lateral view,
the quadrates form the majority of the cavum tympani and contact the
quadratojugals anteriorly and the squamosals posterodorsally, but they do not
contribute to the margin of the upper temporal emargination. The articular
condyle is notably low, the incisura columella auris fully enclosed, and the
anterior rim of the highly reduced antrum postoticum is fully defined by the
quadrate, much as in other carettochelyids.
In ventral view the quadrates broadly contact the prootics and pterygoids
medially and the quadratojugals anterolaterally within the lower temporal
fossa. Posterior to the articular process, the quadrates furthermore contact
the squamosals posteriorly and the opisthotics posteromedially and frame the
lateral margin of the fenestra postotica. The quadrates are broadly exposed
in dorsal view within the upper temporal fossa where they contact the
prootics anteromedially and the opisthotics posteromedially and are narrowly
covered by the squamosals dorsolaterally. The foramen stapedio-temporale is
situated above the ear region at the suture between the quadrate and
prootic. The quadrates form the processus trochlearis oticum together with
the prootics, but only the quadrate portion of that structure is
decorated by fine crenulations reminiscent of a cartilaginous cap. Like
other basal branching carettochelyids, the quadrates possess a modest fossa
at the base of the articular process, in contrast to the deep cavities found
in Allaeochelys spp. and C. insculpta.
Prootic
Within the upper temporal fossa, the prootics contact the
quadrates laterally, the parietals medially, and the opisthotics posteriorly
and
form the medial margin of the foramen stapedio-temporale and the medial
portions of the low processus trochlearis oticum. A low protrusion that is
formed by the prootics and parietals defines the lateral aspect of the
processus trochlearis oticum. Within the lower temporal fossa, the prootics
contact the quadrates laterally, but the medial aspects are obscured by
matrix and crushing. It nevertheless seems apparent that the prootics
subdivide the foramen nervi trigemini into two distinct foramina, much as in
other carettochelyids.
Opisthotic
The opisthotics are broadly exposed in dorsal view
within the upper temporal fossa. They contact the prootics anteriorly, the
quadrates and squamosals laterally, and the supraoccipital medially. In
ventral view, the opisthotics contact the quadrates anterolaterally, the
squamosals posterolaterally, and the pterygoids and exoccipitals medially, form
the lateral margin of the reduced fenestra postotica, and support the base
of the short tubera basioccipitalis.
Supraoccipital
The contacts of the supraoccipital within the upper
temporal fossa are partially obscured by cracks, but it is clear that this
bone contacts the prootics and opisthotics above the otic capsule, the
parietals anterodorsally, and the exoccipitals ventrolaterally, as in most
turtles. The supraoccipital otherwise forms the majority of the crista
supraoccipitalis, which is notably T-shaped in cross section, as in other
carettochelyids. The horizontal portion of the crista is damaged at its
posterior end, but the vertical portion is intact. By comparison to C. insculpta we
therefore conclude that the full length of the crista is likely preserved
and that the horizontal plate is notably narrower than in C. insculpta.
Exoccipital
In posterior view, the exoccipitals form the lateral
margins of the foramen magnum, contact the supraoccipital dorsally and the
basioccipital ventrally, contribute to the occipital condyle and the tubera
basioccipitalis, and form two pairs of foramina nervi hypoglossi. A broad
contact between the opisthotic dorsally and the pterygoid and exoccipital
ventrally fully separates the small fenestra postotica from the enclosed
posterior jugular foramen, which is located between the exoccipital and the
opisthotic.
Basioccipital
In ventral view, the basioccipital contacts the
basisphenoid anteriorly and the pterygoids laterally and contributes to the
formation of the occipital condyle and the tubera basioccipitalis. A broad
semicircular depression is apparent on the basioccipital in ventral view
that is fully restricted to this bone.
Basisphenoid
The basisphenoid is an elongate element that broadly
contacts the pterygoids laterally and the basioccipital posteriorly. The
basisphenoid apparently forms a thin sheet of bone, perhaps the homolog of
the parasphenoid (Sterli et al., 2010), that partially underlaps the
basioccipital, but much of this sheet of bone is now lacking on the right
side. The foramen posterius canalis carotici interni is unusual by being
situated at the contact of the basisphenoid with the pterygoid. A small knob
with uncertain function or homology finally adorns the midline of the
basisphenoid. Such a knob has not been reported for other carettochelyids.
FMNH PR966, Anosteira pulchra, skull, middle Eocene Washakie Formation,
Sweetwater County, Wyoming, USA. A 3-D rendered model of the skull, internal
carotid, and facial nerve canals in ventral view. Abbreviations are as follows: cci: canalis caroticus internus;
cnv: canalis nervus vidianus; facci: foramen anterius canalis carotici interni; fpcci: foramen posterius
canalis carotici interni.
Carotid canals
A diagonal grove is present on the ventral side of
the pterygoids that leads to a relatively large foramen located halfway along
the contact of the basisphenoid and pterygoid. CT images indicate that the
foramen is the posterior opening of a canal that penetrates the basisphenoid
anteromedially and that opens close to its counterpoint within the sella
turcica (Fig. 2). A small canal branches from the large canal halfway along its
path through the basisphenoid. The second canal mostly penetrates the
palatines (Fig. 2). These canals can be found in the extant C. insculpta as well, with the notable difference that the large canal
penetrates the pterygoids further towards the back (Fig. 3). We identify the
small canal in both animals by reference to the known canals in trionychids
(Albrecht, 1967) as the canalis nervus vidianus and the large canal as the
canalis caroticus internus (sensu Rabi et al., 2013) prior to its split into
the cerebral and palatal branches. Our full rationale for these
identifications is explained below (see Sect. 5). The primary difference
between the fossil Anosteira pulchra and the extant
Carettochelys insculpta is the relative placement of the foramen
posterius canalis carotici interni. The foramen is located halfway along the
contact of the basisphenoid and pterygoid in A. pulchra. This
superficially resembles the condition seen in paracryptodires (Gaffney,
1975). The same foramen is typically positioned further to the back in
C. insculpta within the pterygoid, although much variation is
apparent among figured material with some specimens displaying a condition
that resembles A. pulchra (e.g., Waite, 1905; Joyce, 2014; Godinot
et al., 2018). The pterygoid furthermore forms a suture with itself along the
floor it forms of the canalis caroticus internus. To our knowledge, this
suture is unique among turtles.
BMNH 1903.7.10.1, Carettochelys insculpta, skull. The 3-D rendered models of the
basisphenoid, right pterygoid, internal carotid, and facial nerve canals in
(a) dorsal, (b) ventral, and (c) left lateral views. Abbreviations are as follows: bs: basisphenoid; cci:
canalis caroticus internus; ccv: canalis
cavernosus; cnf: canalis nervus facialis; cnv: canalis nervus
vidianus; cprnv: canalis pro ramo nervi vidiani; facci: foramen
anterius canalis carotici interni; fpcci: foramen posterius canalis
carotici interni; pt: pterygoid.
Facial nerve canals
A thick, laterally oriented canal connects the
fossa acustico-facialis in C. insculpta with the short, enclosed portion of the sulcus
cavernosus. A small, poorly traceable canal branches from the large canal.
This small canal leads from the large canal to the ventral surface of the
skull near the foramen posterius canalis carotici interni (Fig. 3). The
skull of A. pulchra is badly crushed in the relevant region, but the canals apparent in
C. insculpta appear to be present here as well, but are too fragmentary to allow
visualizing in 3-D. We identify the smaller canal as the canalis pro ramo
nervi vidiani (sensu Rollot et al., 2018). The medial portion of the large
canal is therefore the canalis nervus facialis and the lateral portion the
canalis pro ramo nervi hyomandibularis. Our rationale for these
identifications is explained below (see Sect. 5).
Discussion
Alpha taxonomy
Two pan-carettochelyids are currently recognized as valid from the Eocene of
North America: Anosteira ornata Leidy, 1871 from the early–middle Eocene (Bridgerian NALMA)
of Wyoming and Anosteira pulchra (Clark, 1932) from the middle Eocene (Uintan NALMA) of Utah
(Joyce, 2014). No cranial material associated directly
with either taxon has yet been found. Joyce (2014) noted that it is unclear if the apparent
differences between the two recognized species are due to intraspecific
variation, poor preservation, or true taxonomic differences. It is therefore
plausible that both taxa are synonymous or chronotaxa. As no morphological
data are available that would allow referring FMNH PR966 to either taxon,
Joyce (2014) identified this specimen as Anosteira indet. Havlik et al. (2014), by
contrast, referred this specimen to Anosteira pulchra using temporal considerations and
utilized it to score cranial characters for cladistic analysis. We follow
this assessment herein as well.
Phylogeny
Our phylogenetic analysis retrieved 41 most parsimonious
trees with 48 steps. The strict consensus replicates the results of Danilov
et al. (2017, fig. 5b) by finding a paraphyletic Kizylkumemys, a fully unresolved,
paraphyletic Anosteira, and a fully unresolved clade consisting of Allaeochelys spp. and
Carettochelys insculpta. In contrast to Danilov et al. (2017), however, this analysis did not
demand omitting Anosteira lingnanica to achieve this level of resolution. Utilizing implied
weight with k values ranging from 1 to 12 does not impact the outcome of the
analysis. As our result is identical to previous results, we do not figure a
tree herein.
Our description highlights consistent differences between Anosteira pulchra and all other
properly described carettochelyids. The vast majority of differences pertain
to the fact that the skull of Anosteira pulchra is narrower, that the orbits and external
nares are oriented more laterodorsally and anterodorsally, respectively,
that the postorbital bar is more gracile, and that the jaws and triturating
surfaces are narrower.
The skull of the closest unambiguous outgroup to the
Anosteira–Allaeochelys clade, Kizylkumemys schultzi, is not well known (see scoring of Danilov et al., 2017, based on
first-hand observation of the available material) and it is therefore not
possible to correctly polarize these character complexes. The overall
gracile skull of Anosteira pulchra may therefore be an apomorphic feature of this North
American taxon.
Cranial circulation and innervation
To our knowledge, the cranial
circulation and innervation has not yet been described for Carettochelys insculpta and we therefore
identify the canals of the carettochelyids for which we have CT datasets by
reference to published dissections, mostly Albrecht (1967, 1976).
Anosteira pulchra and Carettochelys insculpta possess an enlarged canal that diagonally penetrates the basisphenoid
(Figs. 2, 3). The canal originates near the pterygoid–basisphenoid suture in
An. pulchra, but further posterior, within the pterygoid only, in C. insculpta. In both cases, the
canal terminates within the sella turcica. In both cases, a small canal
branches from the large canal, which mostly crosses the palatine and
terminates in numerous diffuse foramina on the dorsal and ventral sides of
the palate. These two canals are broadly consistent with the internal
carotid and vidian nerve canals described by Albrecht (1967)
for the trionychid Apalone spinifera (his Trionyx spinifer), the closet living relative for which data
are
available. The primary difference we note is that Albrecht (1967) reported
that the internal carotid of Apalone spinifera splits into two branches, of which one exits
anteromedially in the sella turcica and the other anterolaterally in the sulcus
cavernosus. As the medial canal feeds the brain and the palate and the
lateral canal the mandible, Albrecht (1967) named the medial canal the
pseudopalatine artery and the lateral canal the mandibular artery, but we
here follow Rabi et al. (2013) by naming them the cerebral and palatine
arteries to emphasize their topological homology with similar canals in
other turtles. The lack of a palatal canal either reflects the reduction of
the palatine artery in carettochelyids, as seen for instance in some
paracryptodires (Rollot et al., 2018) or may indicate that the split occurs
after the internal canal exits the sella turcica. As the split occurs close
to the sella turcica in Apalone spinfera, we find the second hypothesis to be more
plausible, but only the dissection of a Carettochelys insculpta cranium will be able to resolve this
question unambiguously. Albrecht (1976) arrived at a similar conclusion by
reference to an unpublished dissection of the extant trionychid Lissemys punctata, for which
the mandibular artery was reported to originate from the cerebral artery.
The two carettochelyids resemble Apalone spinfera in that the vidian canal mostly traverses
the palatine, in contrast to all other turtles, in which the canal traverses
the pterygoid (Albrecht, 1967, 1976; Rollot et al., 2018). This arrangement
may therefore serve as yet another synapomorphy for Trionychia.
A relatively thick, straight canal connects the brain cavity with the
canalis cavernosus in Anosteira pulchra and Carettochelys insculpta. A second, much thinner canal that is hard to
trace exits this canal, transverses the pterygoid, and surfaces within or
near the canal of the internal carotid artery. The thick canal is broadly
consistent with canalis nervus facialis and the small canal with the canalis
pro ramo nervi vidiani (sensu Rollot et al., 2018) of other turtles
(Albrecht, 1967, 1976). If the split of the facial nerve into the
hyomandibular and vidian branches occurred within the thick canal, the
lateral portion of the thick canal should be interpreted as the canalis pro
ramo nervi hyomandibularis. However, as the location of the geniculate
ganglion is unclear, it is equally likely that (1) the two nerves always
split within the cavum cranii and traverse the medial part of the facial
canal together, (2) the two nerves split within the facial canal, or (3) that
the nerves split within the canalis cavernosus and that the vidian nerves
utilizes the facial canal again on the way to the surface. Too few
comparative data are available to enable us to restrict our options.