Cranial osteology of the Eocene amphisbaenian reptile Spathorhynchus fossorium



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Cranial osteology of the Eocene amphisbaenian reptile Spathorhynchus fossorium (Reptilia, Squamata) , the oldest-known amphisbaenian with a completely preserved skull, based on high-resolution X-ray computed tomography, and the evolution of derived amphisbaenian characters
Johannes Müller, Christy A. Hipsley, and Jessica A. Maisano
Abstract: Modern Amphisbaenia, or worm lizards, are characterized by a suite of unique apomorphic traits in both skull and postcranium, whereas fossil evidence suggests that at least some of these shared derived characters evolved independently. However, the lack of detailed knowledge of many fossil taxa renders a more detailed interpretation difficult. ...Here wWe describe the cranial anatomy of the oldest-known well preserved amphisbaenian reptile, Spathorhynchus fossorium from the Eocene Green River Formation, Wyoming, U.S.A., using high-resolution X-ray computed tomography (HRXCT). The taxon possesses one of the most strongly reinforced crania known among amphisbaenians, with many dermal bones overlapping each other internally. In contrast to modern taxa, S. fossorium has a paired orbitosphenoid and lacks a true compound bone in the mandible, and retains a fully enclosed orbital rim. The latter feature represents a highly derived structure in that the jugal establishes an externally invisible contact with the frontal, which reinforces the posterior orbital margin internally. Also, S. fossorium possesses a strongly modified Vidian canal with a previously unknown, additional connection to the ventral surface of the parabasisphenoid. Comparison with the closely related fossil taxon Dityconastis rensbergeri reveals that these derived traits are also shared by the latter species and potentially represent synapopmorphies of an extinct Paleogene clade of amphisbaenians. The presence of a reinforced orbital rim suggests selection against the loss of a functional eye and indicates an ecology potentially different from modern taxa. Given the currently accepted phylogenetic position of Spathorhynchus and Dityconastis, we predict that supposedly “unambiguous” amphisbaenian apomorphies such as a fused orbitosphenoid and the reduction of the eye evolved at least twice within the crown clade, which is also evidenced by new data from other fossil amphisbaenians.
Keywords: Squamata, Amphisbaenia, Spathorhynchus, Eocene, osteology, micro-computed tomography
Introduction

Shouldn’t this be a Gans reference?
Amphisbaenians, or "worm-lizards,", are small fossorial reptiles highly specialized for life underground, making them morphologically and ecologically distinct from other squamates. All but three of their nearly 200 species are limbless, digging almost exclusively under loose or sandy soils in tropical and subtropical regions of the world (Kearney 2003). Their evolutionary origins have long remained obscure, but in recent years accumulating molecular and paleontological evidence suggests that the clade shares a common ancestry with Lacertidae, an Old-world family of fully terrestrial, four- legged lizards (Townsend et al. 2004, Vidal et al. 2008xx, Wiens et al. 2012xx, Müller et al. 2011a). In comparison to their non-fossorial relatives, amphisbaenians have undergone extreme modifications to the body and head, the latter of which is used as their primary digging tool (Gans 1969Kearney 2003). Most likely as a consequence of fossoriality, the skull of modern amphisbaenians is characterized, among other traits, by reduction of the eyes and loss of the bony orbital rim, reduction of the middle ear and absence of a tympanum, loss of several dermal roofing bones, fusion of the posterior skull roof with the braincase, and fusion of the ancestrally paired orbitosphenoids (Kearney 2003). Many evolutionary steps towards the modern amphisbaenian body plan remain unknown, however, which is likely due to the clade's poorly understood morphology, a problem that applies both to extant and fossil taxa.


The oldest-known well-preserved fossil amphisbaenian is Spathorhynchus fossorium from the Middle Eocene Green River Formation of Wyoming, U.S.A., known primarily from cranial material and first described by Berman (1973). Later, Berman (1977) described a slightly younger, second species of this genus, S. natronicus, from the Early Oligocene White River Formation of Wyoming. Since the original description, S. fossorium has never been studied in detail; only Kearney (2003), in her phylogenetic assessment of fossil and extant amphisbaenians, corrected a few characters described by Berman (1973, 1977). In addition, Gauthier et al. (2012) scored S. fossorium in their extensive morphological analysis of squamate relationships.

So farTo date, the only fossil amphisbaenian whose anatomy has been studied in detail is Rhineura hatcherii (Kearney et al. 2005) from the Oligocene Brule Formation of South Dakota, revealing that this taxon had a paired orbitosphenoid unlike any modern amphisbaenian. In comparison to the latter, S. fossorium is anatomically even more different from extant amphisbaenians and retains, for example, a complete orbital rim (Berman 1977). Given that Spathorhynchus has been assigned to the amphisbaenian crown clade Rhineuridae (Berman 1977, Kearney 2003, Hipsley & Müller 2014), which is sister to all other modern amphisbaenians (Kearney & Stuart 2004, Vidal et al. 2008, Longrich et al. 2015xx), the retention of such plesiomorphic traits may have important implications for our understanding of the origins of derived amphisbaenian characters. Potential events of parallel evolution in Amphisbaenia have already been discussed in light of limb loss and skull shape (Kearney & Stuart 2004, Kearney et al. 20055, Maisano et al. 20062008), suggesting that amphisbaenian character evolution is more complex than previously thought. However, there may be even more instances of parallel evolution in this clade, highlighting the necessity to also reinvestigate early fossil representatives. Therefore the aim of the present study is to reassess the cranial anatomy of S. fossorium using high-resolution X-ray micro-computed tomography (HRXCT), and to discuss the results with respect to the taxon's fossil and extant relatives, and the evolutionary origin and relationships of Amphisbaenia.
Material & Methods
We HRXCT-scanned the holotype specimen of Spathorhynchus fossorium (National Museum of Natural History, USNM 26317xxx), and for comparative reasons also the holotype specimen of the Oligocene amphisbaenian Dityconastis rensbergeri (University of California Museum of Paleontology, UCMP 76881xxx). Both specimens were scanned at the University of Texas High-Resolution X-ray CT Facility.

Spathorhynchus was scanned using the following parameters. A FeinFocus microfocal X-ray source operating at 180 kV and 0.133 mA with no X-ray prefilter was employed. An air wedge was used. Slice thickness corresponded to 2 lines in a CCD image intensifier imaging system, with a source-to-object distance of 69 mm. For each slice, 1400 views were taken with 2 samples per view. The field of image reconstruction was 22 mm and an image reconstruction offset of 5500 was used with a reconstruction scale of 700. The data set consists of 750 slices taken along the transverse axis of the skull, from the tip of the snout to the occiput. Each slice was gathered at 1024x1024 pixel resolution, resulting in an in-plane resolution of 21.5 µm, and represents a thickness and spacing of 47.2 µm.

The specimen was rotated to orthogonal and digitally resliced and rendered in VGStudioMax 2.0 (Volume Graphics, Heidelberg, Germany). An interactive, web-deliverable version of the HRXCT dataset keyed to the slice numbers cited below, as well as rotation and cutaway animations of the skull, can be viewed at http://www.digimorph.org/

specimens/Spathorhynchus_fossorium. Similar visualizations are available for Dyticonastis at http://www.digimorph.org/specimens/Dyticonastis_rensbergeri. The original full-resolution HRXCT data for both specimens are available from Maisano.

Anatomical abbreviations: a – angular; aar – anterior ampullary recess; afvc – additional foramen into Vidian canal; aif - anterior inferior alveolar foramen; amf – anterior mylohyoid foramen; appr – alar process of prootic; asc – ascending process of supraoccipital; asf – anterior surangular foramen; avsc – anterior vertical semicircular canal; bo – basioccipital; c – coronoid; cc – cranial cavity; d – dentary; dt – dentary tooth; ec – ectopterygoid; ecl – extracolumella; f – frontal; fc – frontal canal; fct – foramen chorda tympani; ff – facial foramen; fll – frontal lateral lamina; fo – fenestra ovalis; fon – optic nerve foramen; fpm – fused posterior mandibular bones; fvl – frontal ventral lamina; fvo – fenestra vomeronasalis; gf – Gasserian foramen; glf – glenoid fossa on mandible; hf – hypoglossal foramen; hsc – horizontal semicircular canal; ims – intramandibular septum; j – jugal; jr – jugular recess; m – maxilla; mc – Meckelian canal; mf – mandibular fossa; mfo – mental foramen; mt – maxillary tooth; n – nasal; ooc – oto-ooccipital complex; os – orbitosphenoid; p – parietal; pbs – parabasisphenoid; pc – parietal canal; pd – perilymphatic duct; pf – prefrontal; pl – palatine; pm – premaxilla; pmf – posterior mylohyoid foramen; pof – postorbitofrontal; pop – paroccipital process; pra – prearticular; pro – prootic; psf – posterior surangular foramen; pt – pterygoid; pvsc – posterior ventral semicircular canal; q – quadrate; rp – retroarticular process; s – stapes; sa – surangular; slt – sella turcica; sm – septomaxilla; soc – supraoccipital; sp – splenial; sq – squamosal; sy – mandibular symphysis; v – vomer; vb – vestibule; vc – Vidian canal; vdf – ventral dentary foramen; vf – vagus foramen; vlp – ventrolateral process; vnc – vomeronasal chamber; “x” – element “x” (sphenoccipital ossification)....


Description
Skull (Figs. 1-45)
The skull of Spathorhynchus fossorium measures 35.4 mm from the tip of the snout to the posterior tip of the occipital condyle, and 19.3 mm at its widest expanse, at the level of the external opening of the jugular recess. In comparison to modern Rhineura floridana (http://digimorph.org/specimens/Rhineura_floridana/), but also to fossil taxa like Rhineura hatcherii (http://digimorph.org/specimens/Rhineura_hatcherii/), Spathorhynchus fossorium is distinguished by very strong craniofacial angulation. The angulated facial surface is formed by the premaxilla, maxilla, nasal, frontal, prefrontal, postorbitofrontal, and the anteriormost portion of the parietal (Fig. 1). These bones are characterized by a remarkably rugose dorsal surface with many pits and grooves. Pits are particularly numerous on the frontal bones, but largest on the maxilla and the anterior part of the nasals. Many bones of the skull are very thick in cross-section, and show a low degree of vascularization (e.g., Fig. 2).
Premaxilla (Tra 0042-14033). The premaxilla forms the anterior tip of the snout and together with the nasals contributes to the blade-like structure that overhangs the mandible (Fig. 1B, D), a characteristic feature of ‘shovel-headed’ amphisbaenians. It also forms the medial margin of the ventrally facing external naris (Fig. 1B). The bone, which is not completely preserved on its left side, is an azygous element contacting the septomaxilla, nasal, maxilla, and vomer (Fig. 3). Anteriorly, the bone has a somewhat half-rounded margin and is much broader than further posteriorly, where the bone narrows into a well-developed nasal process (Fig. 1A). In contrast to Rhineura hatcherii, in which the nasal process is largely covered by the nasals and thus appears superficially short (Kearney et al. 2005), the nasal process of Spathorhynchus fossorium remains visible dorsally and, increasingly tapering, extends well between the two nasals for more than halfway of their entire length, which and the latter in turn embrace clasp the sides of the premaxilla nasal process both dorsolaterally, laterally and ventrally (Figs. 2B, 4B, C). In transverse section, it becomes apparentcan be seen that the nasal process is also thickened at its midpoint (Fig. 2B), which is not visible dorsally, so that the process appears dorsally narrower in dorsal view than it actually is.

In ventral view (Fig. 1B), the premaxilla consists of a broad alveolar plate of roughly hour-glass shape, which forms the internal margin of the external nares and carries three conical premaxillary teeth. The central tooth is of the typically enlarged amphisbaenian morphology and positioned slightly more rostrally than the remaining adjacent two. Roughly halfway between the teeth and the anterior premaxillary margin, the alveolar plate possesses two foramina, each positioned lateral to the midline. These foramina are part of the rostral canal, and complementary openings can also be found on the dorsal side of the premaxilla, as indicated by the preserved right half of the dorsal portion (Figs. 1DA, B, 2A). Further posteriorly, on the ventral side, the premaxilla is markedly stepped dorsally (Fig. 4), transforming extending into a palatal process with posterolaterally diverging branches (Figs. 1B, 3DB). These branches processes are expanded posterolaterally and contact the anteromedial portion of the maxilla in a somewhat undulating suture, and anteriomedially clasp the rostral tip of the paired vomer (Figs. 1B 2B, 3B3D).


Maxilla (Tra 04229-3876). The maxilla is of a somewhat triangular shape in lateral view due to the presence of a distinct, pointed posterodorsal frontal process (Figs. 1A, C). At its posterior end, the bone is also distinctly flared laterally (Fig. 1A). The maxilla forms the posterior and posteromedial margin of the external naris (Fig. 1B), and makes a small contribution to the anteroventral margin of the orbit (Fig. 1C)in the latter's anteroventral area. The external surface of the bone presents several distinct foramina that can be seen in section to communicate with the maxillary canal that runs longitudinally above the maxillary teeth (Figs. 2D, E, 3B), similar to the condition in Rhineura hatcherii (Kearney et al. 2005)and grooves In life this canal conveys the superior alveolar nerve, a sensory branch of the maxillary branch of the trigeminal nerve (Oelrich 1956)., whereas in cross section, the bone is thick and dense. On the dorsal side the nasal intervenes to preclude there is no contact of the maxilla with the premaxilla (Fig. 1A) as the maxilla is anteromedially exclusively in touch with the nasal. Further posteriorly it makes extensive contact with the frontal, whereas the anteromedial margin of the maxilla eventually turnsextending posterodorsally into the posterodorsal frontal process. In transverse section it can be seen that the maxilla, in fact, dorsolaterally overlaps both nasal and frontal extensively on the dorsal side, and that the latter two bones extend thin flanges along the medial side of the maxilla's dorsal portion (Figs. 2C, D). Thus, the dorsal expression of the "suture" between maxilla and nasal and frontal, respectively, is only superficial.

Posterior to the posterodorsal frontal process, the maxilla is in touch withcontacts the prefrontal, which projects into the maxilla along the ventrolateral edge of the posterodorsal frontal process (Fig. 1C), but similarly to nasal and frontal also underlies the medial surface of the bone by a narrow flange (Fig. 2E), similar to the nasal and frontal. Ventrally to the contact with the prefrontal, the posterior end of the maxilla forms a small part of the orbital margin, and then touches meets the jugal in an zigzag-shapedinterdigitating suture (Fig. 1C). The posteroventral edge of the bone clasps the tiny lateral exposure of the ectopterygoid. The contact between maxilla and ectopterygoid becomes more extensive and complex further posteriorly, where the ectopterygoid covers the complete posterior margin of the maxilla (Figs 2F, 3C).



The dental laminamaxillary palatal process is greatly expanded, giving the maxilla an L-shaped outline in transverse section (Fig. 2C, D), and posteromedially touches the palatine and, to a very small extent, the anterolateral portion of the pterygoid (Fig. 1B). Anteromedially, the dental laminapalatal process is partly in contact both with the lateral flange of the vomer and with a miniscule part of the latter’s rostral process bone's anterior tip, immediately posterior to the sutural contact with the premaxilla and septomaxilla (Figs. 2 B, C). In fact, it prominently insertstersects between the latter three bones through a slender but well-developed bilobed process (Fig. 2B), t. Thus also, the maxilla also formings the lateral margin of the choana. The dental laminaalveolar margin carries 7 teeth of conical and slightly recurved shape (Fig. 1 B, C). The tooth at the 2nd position is notably larger than the remaining ones, and there is a decrease in size posteriorly. Anterolaterally, the ventral side portion of the maxilla is greatly laterally expanded and turns into the dorsal flangeextends anteriorly to form the rostral process that overlaps the nasal (Figs. 1D, 3A).
Nasal (Tra 0194-220195).. The paired nasal is a flatslightly, dorsally arched bone that is almost as long as the frontal (Fig. 1A). Together with the premaxilla it forms the spatulate anterior edge of the snout and the anterior margin of the external naris (Fig. 1B). As mentioned above, the lateral side of the nasal is externally overlain by the maxilla. Except for its anteriormost part, where the bone's overlain obscured portion is thickened and fits, at its lateralmost extent, into a pocket-like embayment of the medial side of the dorsal flangefacial process of the maxilla (Fig. 2B), the covered obscured area of the nasal (the facial lamina sensu Kearney et al. 2005) is notably thinner than the dorsally exposed part, the latter becoming also increasingly thicker posteriorly (Fig. 2D). Also anteromedially, the nasals are thick and suturally clasp the nasal process of the premaxilla; at the same time, each nasal establishes an initially small sutural contact with the septomaxilla anteroventrally (Fig. 2B), which becomes increasingly broader posteriorly (Fig. 3C). The aforementioned overlap of by the maxilla is continued by the frontal further posteriorly;, i.e., while in dorsal view the nasal seems appears to become increasingly narrowernarrow posteriorly due to the frontal interveningsecting between maxilla and nasal, in but in transverse view it becomes apparent that the frontal participates in the dorsal overlap of the nasal (Fig. 4A). In addition, the frontals extensively underlie the posterior portion of the nasals by narrow sheets laminae that even meet each other in at the midline (Fig. 2D), eventually becoming thicker and exposed dorsally between the nasals at the latter bones' posteromedial edges (Fig. 2D1A, D). Internally, the nasal is pierced by several canals which that are dorsally expressed as distinct foramina, being among the largest found on the external surface of the skull (Fig. 1D).
Septomaxilla (Tra 04930-202191). The paired septomaxilla consists of a thickened anteromedial portion that transforms into a posteriorly elongate vertical lamina, and a posteriorly (or better: posterodorsally , due to the oblique anteroposterior orientation of the bone) into an even longer horizonal lamina (Figs. 2 B-D, 3C). The vertical lamina forms the ossified lateral wall of the internasal septum and remains separated from its opposite counterpart throughout its length (Fig. 2D). At its anteriormost portion it forms a sharp rostral spine, similar to Rhineura hatcherii (Kearney et al. 2005). In the anterior area, the septomaxilla is in sutural contact with the maxilla and the vomer ventrally, and with the nasal dorsally (Fig. 2B).

Further posteriorly/posterodorsally the bone septomaxilla loses contact to with all surrounding elements (Fig. 2D). As mentioned above, the anteromedial portion of the bone is thickened due to the vertical lamina incorporating a prominent canal (Fig. 2B), serving for the ethmoid passageway that carries various branches of the opthalmic nerve and running along the bone's entire anteroposterior extent. Further posteriorly/posterodorsally, the canal is no longer completely enclosed by bone due to the vertical lamina becoming increasingly thinner, taller, and more strictly vertically oriented (Fig. 2C). Only close to the posteriormost area the right septomaxilla presents a small foramen at the turning point of the vertical to the horizontal lamina, which may be related to the above passageway (Fig. 2D). The horizontal lamina consists of a thin sheet of bone that anteriorly is straight and ventrolaterally directed but further posteriorly/posterodorsally bends dorsolaterally. It contributes to the floor of the nasal chamber and forms the roof of the vomeronasal chamber, similar to Rhineura hatcherii (Kearney et al. 2005).


Prefrontal (Tra 15409-34117). The prefrontal is a prominent element that forms the anterior and anterodorsal portion of the orbital wall and the anterdorsal orbital rim (Fig. 1C). In external view, the bone has a rugose dorsal surface and projects into the body of the maxilla in the form of a triangular antorbital process (Figs. 1A, C); medially, the prefrontal meets withabuts the frontal along its entire extent;, posteriorly it establishes a pointy narrow contact with the parietal;, and posterolaterally it makes extensive contact with the postorbitofrontal. In cross-transverse section it becomes apparent that the articulation with the frontal is of a distinctively tongue-and-groove type (Fig. 2E2F). Also, the bone’s antorbital process is actually much larger than what is superficially exposed, and internally forms a thin sheet of bone that is extensively overlain by the maxilla (Fig. 2E). Like As in Rhineura hatcherii (Kearney et al. 2005), the lacrimal duct enters between the latter bonemaxilla and the prefrontal, i.e. at the ventrolateral edge of the bone’s prefrontal’s palatine process, which also forms the anterior wall of the orbit. The infraorbital foramen is found at the junction of the palatine, prefrontal palatine process of prefrontal, and ectopterygoid maxillary process of the ectopterygoid. However, in contrast to Rhineura hatcherii, the prefrontal establishes only little a small contact with the palatine posterior to the foramen. By contrast, the ventral contact with the ectopterygoid is well developed, and in the anteroventral area of the orbital rim the palatine process of the prefrontal touches the anteromedial edge of the orbital process of the jugal (Fig. 2F).

F


All of the bones are dense
Frontal (Tra 1204-44126). The paired frontal forms a significant portion of the facial area but remains is shorter than the parietal (Fig. 1A, C). In dorsal view, the bone projects anteriorly deeply between nasal and maxilla through the development of a prominent triangular process (Fig. 1A). As mentioned above, the frontals also develop flat sheets laminae in their anterior portion that ventrally underlie the posterior parts of the nasals (Fig. 2D). In external view, these sheets are only modestly exposed and appear as a short median process that intersects between the latter bones. Posteromedially, the parietal extends deeply between the frontals along almost halfway of their median suturelength (Fig. 1A). The parietal also clasps the entire posterior and posterolateral margin of the frontal (Fig. 4). Midway aAlong the centralits lateral area margin the frontal is in sutural contact with the prefrontal, and further anterolaterally, with the maxilla (Fig. 1A, C). In transverse section, it becomes apparent that the frontal is extremely thick and dense, surpassing all other cranial bones in this respect (Figs. 2D-G). Also, as mentioned above, the bone possesses a lateral flange in its anterior portion that is externally overlain by the maxilla, and . Especially further posteriorly this flange is fairly thick and forms a complex sutural contact with the latter bone further posteriorly. Medially, a

As is the case also known in other amphisbaenians and Cryptolacerta (Kearney et al. 2005, 2007?; Müller et al. 2011), the frontals meet each other in a characteristic tongue-and-groove articulation, which is especially well developed in the more central areanear the midpoint of the suture (Figs. 2E, F). A somewhat similar, though much smaller more limited, sutural contact is present between the frontal and the prefrontal. Posteriorly, the transverse sections reveal a complicated internal sutural pattern between frontal and parietal; similar to Rhineura hatcherii (Kearney et al. 2005), frontal and parietal deeply interdigitate in a complex three-dimensional fashion through numerous small flanges and processes, which extend beyond the externally visible dorsal suture in both anterior and posterior direction (Figs. 2G, H, 4).

Especially anteriorly, the anterior area of the frontal’s dorsal surface presents the external openings of several primarily anteroposteriorly- directed canals with branch-offs opening dorsally. These are endpoints of the most prominent of these canals is the frontal canal (Fig. 2E, F), situated dorsal to the descending process of the frontal, which begins originates posterolaterally as a foramen in the dorsomedial area of the orbit and then continues straight rostrally, situated dorsally to the descending process of the frontal, to eventually terminatinge in the anterior area portion of the bonefrontal.

Starting roughly at the midpoint-level of its anteroposterior extent, the frontal develops a prominent ventral downgrowthdescending process (Fig. 2F), which increasingly gradually turns bends in medially direction, to forming a medial lamina that, and eventually meets its opposite counterpart in a tongue-and-groove suture to form a tube-like cavity and a significant portion of the cranial floor (Figs. 2F, G, H, 3A, 4B). Along the complete ventral edge surface of this suture, the frontal medial laminae are in a complex contact with the parabasisphenoid, actually . When the latter bone widens posteriorly, the medial lamina still claspings the parabasisphenoid latter dorsally and laterally for along a fair amount of the latter bone’sits anteroposterior extentlength (Fig. 2G-I). The lateral contact with the parabasisphenoid becomes is contact is eventually replaced posteriorly by a prootic/parabasisphenoid contact further posteriorly, at the anterior level end of the braincase, with the prootic also being suturally attached tocontacting the frontal medial lamina;. At the same time the frontal medial lamina becomes dorsally covered by the orbitosphenoid (Fig. 2H, I). At this level, the frontal and anterior portion of the orbitosphenoid areis transversely pierced by a well- developed canal that also pierces the anterior portion of the orbitosphenoid, the function of which is currently unknown. Further posteriorly, and similar to Rhineura hatcherii and other shovel-snouted amphisbaenians (Kearney et al. 2005), the frontal downgrowth descending process develops a temporal wing that forms part of the lateral wall of the braincase (Fig. 1C). It extends far posteriorly and eventually clasps the alar process of the prootic along much of the latter bone’s lateral surface, and dorsomedially extends along the ventrolateral portion of the descending processtemporal lamina of the parietal; it thus, thereby remainsing excluded from the endocranial wall (Figs. 2I, J).

Further laterally, the ventral surface of each downgrowth frontal descending process closely approaches the dorsally convex surface of the palatine (Fig. 2G), but suturally touches the latter bone only further posteriorly. Along its anterolateral edge, the frontal downgrowth descending process meets the prefrontal and together with this boneto forms the anterior wall of the orbit, whereas the frontal downgrowthformer also forms the medial wall, similar to Rhineura hatcherii (Kearney et al. 2005). Another distinctive and highly derived feature of Spathorhynchus fossorium is a sutural contact with the jugal through a dorsolateral extension of the medial laminadescending process at the level of the posterior orbital rim; t. This extension runs ventral to the dorsal roofing bones and also covers the ventral surface of the postorbitofronal (Fig. 2G).

But that is exactly the inference made by calling it a postorbitofrontal....
PPoostorbitofrontal (Tra 260-352). Noeither jugal, postorbital or postfrontal are is present in modern amphisbaenians; however, Spathorhynchus fossorium is different in that it still retains separate bones around the dorsal orbital rim. In the his original description, Berman (1973) described a separate postfrontal and postorbital; however, as already noted by Gans (1978), Kearney (2003) and Gauthier et al. (2012), Berman’s so-called ‘postorbital’ is, in fact, the jugal. Given there are currentlythat no amphisbaenians are known in whichto possess both a separate postorbital and postfrontal are present, it remains problematic to designate the bone posterior to the prefrontal as either postfrontal or postorbitaldorsal to the orbit as either; therefore, this element will be here preliminarilyis here described as postorbitofrontal, whereas no inference is made about a potential fusion of the two elements, or the respective loss of one of them (contra Kearney 2003).

The postorbitofrontal forms the posterodorsal portion of the orbital rim (Fig. 1A, C). Much of its anterior/anteromedial margin is in sutural contact with the prefrontal, whereas the bone’s increasingly gradually tapering posteriorolateral extension is clasped by the parietal (Fig. 1A, C). In transverse section, it becomes apparent that theis contact is between postorbitofrontal, frontal and prefrontal is interdigitating andalso vertically complex (Fig. 2G), with the two bones being highly intertwined. The posterolateral edge of the postorbitofrontal shares an extensive, interdigitating suture with the postorbital process of the jugal. As mentioned above, the ventral surface of the bone is underlain by a lateral extension of the medial lamina of the frontal (Fig. 2G).




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