Description

Skull. Features of the mummy QU 17279 previously recorded from the original T. gezei skull QU 17376 Complete and articulated, the skull of QU 17279 is almost as long as wide (Figure 3A–D). A dermal sculpture covers the frontoparietals, squamosals, nasals and maxillae, consisting of subcircular to suboval pits limited by low ridges of more or less constant thickness. A second diagnostic character is the lamella alaris of the squamosal, which has an anteriorly elongated projection; the latter rests on the dorsal margin of the maxilla, thus separating this bone from the orbit (Figure 3A, C). The prootic and palatine foramina open in the posteromedial wall of the orbit and a shallow groove for the vena jugularis interna runs posterolaterally to the two foramina. The ramus paroticus of the squamosal consists of a broad lamina solidly sutured to the crista parotica of the otic capsule, on its dorsal and anterodorsal surfaces. The posterolateral process of the squamosal, extending from the ramus paroticus to the crista parotica of the otic capsule, forms a lamina that makes up the lateral part of the posterior orbital wall. The internal ramus of the pterygoid is also united with both the otic capsule and the posterolateral process of the parasphenoid by a suture showing deep and narrow interdigitations. This complex configuration was noticeable in section during segmentation, but is extremely hard to model due to the slenderness of the structure. There is no contact between the squamosal and the frontoparietal. The latter lacks a visible sagittal suture, unless it remained unnoticed on the tomograms; however, the sculpture is weaker along the midline than elsewhere on the dorsal surface. On either side, the lateral margin of the frontoparietal extends as a tectum supraorbitale. Ventrally, the pars contacta is well developed (Figure 3C). A small portion of the dorsal surface of the sphenethmoid is exposed in a rhomboid space between the frontoparietal and the nasal (Figure 3A). The anteriormost part of the sphenethmoid is clearly damaged, but it probably did not extend anteriorly much beyond its preserved part. On the ventral face of the skull, the palatines are present and are not separated from one another by the anterior tip of the parasphenoid; however, the palatines do not make contact in the midline (Figure 3B). The posterior area of the skull closely resembles that of QU 17376. The condyloid fossae, in which the jugular foramina open, are partially hidden by the occipital condyles. The posterior openings of the canals for the occipital arteries open just below the sculptured frontoparietal table and the prominentia ducti semicircularis posterioris on each side is compressed mediolaterally and projects strongly posteriorly (Figure 3D). PPT PowerPoint slide

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larger image TIFF original image Download: Figure 3. Complete articulated skull of the mummy QU 17279. Shown in dorsal (A), ventral (B), left lateral (C), and posterior (D) views. Each bone or paired bone is identified with a single color for easy delimitation. Anatomical abbreviations: cr. par, crista parotica; exp. r. spheth, exposed rhomboid area of sphenethmoid; f. jug, foramen jugulare; fpar, frontoparietals; lam. al, lamella alaris; max, maxilla; nas, nasal; p. cont, pars contacta; pal, palatine; pmax, premaxilla; pr. plat, processus posterolateralis; prom. d. sc. post, prominentia ducti semicircularis posterioris; psph, parasphenoid; pt, pterygoid; qj, quadratojugal; r. par, ramus paroticus; spheth, sphenethmoid; sq, squamosal; st, stapes; t. so, tectum supraorbitale; vom, vomer. https://doi.org/10.1371/journal.pone.0074874.g003

Differences between the skull of QU 17279 and QU 17376. In spite of the many common characters shared by QU 17279 and QU 17376, there are a few notable differences. Although very elongate, the lamella alaris is shorter in QU 17279 than in QU 17376 and it does not separate the maxilla from the nasal [10]. The nasals, which are fused in the midline in QU 17376, are markedly separated in the midline in the scanned specimen (Figure 3A). The rhomboid area between the frontoparietal and the nasal is barely sculptured in QU 17279, whereas in QU 17376 the sculpture is clearly apparent although less marked than elsewhere. The palatine in QU 17279 does not reach the midline, nor does it contact the anterior tip of the parasphenoid (Figure 3B), contrary to QU 17376. On the posterolateral wall of the orbit, the groove for the vena jugularis interna is wider and not as well defined in QU 17279 as in QU 17376, and two unnamed foramina found in QU 17376 are not observed in QU 17279 ([10]: Fig. 7A). The condyloid fossae are larger in QU 17279 and not as well delimited as in QU 17376. QU 17279 also differs from QU 17376 in lacking a horizontal groove containing a minute foramen above the condyloid fossa QU 17376. However, based on comparisons with extant frogs [19], [20], [21], most or all of these differences can be attributed to ontogeny: we interpret QU 17279 as a post-metamorphic individual, yet younger than the presumably mature adult specimen represented by QU 17376. This conclusion is supported by the patent notochordal canal in presacral VIII of QU 17279.

New information on the skull of Thaumastosaurus. Several cranial features found in QU 17279 were not preserved in QU 17376, most notably those of the snout region. The premaxillae are slightly disarticulated, and each bears pleurodont teeth all along its pars dentalis; the right premaxilla, on which they are more clearly seen, had twelve tooth loci, ten of which are filled (Figure 3B, Figure 4A). Located medially, the processus alaris is tall and straight, inclined posteriorly, concave lingually at its base and then flattening towards its dorsal narrower extremity. The short pars dentalis projects posterolaterally. Lingually, two processes originate from the lamina horizontalis. The medial process is slender and directed posteriorly, whereas the lateral one, longer and thicker, is directed posterolaterally. In anatomical position, the posterolateral margin of the pars dentalis was overlapped by the anterior tip of the maxilla. The latter, complete in the scanned specimen, is posteriorly in contact with the quadratojugal (Figure 3A–C). A processus zygomatico-maxillaris does exist, as well as a developed processus frontalis. Teeth are present, although not easily individually distinguishable and showing few details, preventing us from determining whether they were uni- or bicuspid, and whether or not they were pedicellate. The right maxilla, where they can be more easily counted, bears at least 62 teeth. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 4. Some of the new elements provided by the skull of the mummy QU 17279. A, right premaxilla in anterior view. B–C, left vomer in ventral and dorsal views. D, frontoparietals in ventral view. E, lower jaw in dorsal view. F, right prooticooccipital and stapes in ventral view. G, thyrohyal ossifications of the posteromedial processes of the hyoid (in anatomical position). Anatomical abbreviations: an, angular; cor. pr, coronoid process; den, dentary; f. c. ant, facies cerebralis anterior; f. c. post, facies cerebrales posteriores; mm, mentomeckelian bones; med. pr, medial process; lat. pr, lateral process; p. dent, pars dentalis; pr. al, processus alaris; pr. ant, processus anterior; pr. ch. ant, processus choanalis anterior; pr. ch. post, processus choanalis posterior; prootoc, prooticooccipital; st, stapes; t, teeth; vom. t, vomerine teeth. https://doi.org/10.1371/journal.pone.0074874.g004 The vomer is preserved and is articulated with the ventral face of the sphenethmoid (Figure 3B). It is paired, separated from the midline, moderately extended, and bears an oblique series of very small teeth on its ventral face (at least seven on the left vomer, where they are more clearly seen) (Figure 4B, C). The anterior process forms a small elongated, tapering lamina that extends anterolaterally toward the lamina horizontalis of the maxilla and contacts it; the anterior processus choanalis is markedly elongate and is directed laterally, but it did not reach the lamina horizontalis of the maxilla; the posterior processus choanalis is a flat lamina that is directed posterodorsally. Both anterior and posterior processes bear a crest on their dorsal face. Each vomer is posteromedially extended by another process consisting of a flat lamina whose posterior border is rounded and approaches the palatine without touching it. On the ventral face of the frontoparietal, the incrassatio frontoparietalis is comprised of an anterior, lanceolate unpaired facies cerebralis anterior and two small and circular facies cerebrales posteriores that are somewhat distant from the anterior facies (Figure 4D). The incrassations are very slight thickenings of the ventral face of the frontoparietal that insert into fenestrae of the endocranial roof [22]; therefore, they mirror the topography of the dorsal endocranium and may be phylogenetically informative. Unfortunately, their morphology and distribution within anurans are still poorly known. All constituents of the lower jaw are preserved: the paired angular and dentary, and mentomeckelian bones, the latter being disarticulated relative to the rest (Figure 4E). As is typical in anurans, the meckelian groove runs on the lateral face of the angular, except in the posterior portion of the bone where the groove passes on the dorsal face. The dentary lacks odontoids. The posterior extremity of the angular rises abruptly. A coronoid process is present on the angular; it is inclined mesially at ca 45°. The mentomeckelian bones are not conjoined, i.e. the symphysis is not fused. A strip of unidentified matter connects the posterior face of the mentomeckelian on both sides, but it is probably a remnant of sedimentary matrix that could not be distinguished from the bony structures during segmentation. The otic capsules are damaged and ventrally poorly ossified, but the stapes is preserved on both sides (Figure 3B, D). The right stapes is almost in functional position, ventral to the crista parotica of the capsule (Figure 4F). The stapes consists of a small thickened footplate (pars interna plectri) that was ossified; it is prolonged dorsolaterally by a long rod (pars media plectri), which is slightly curved anteriorly. The cartilaginous distal part (pars externa plectri) of the stapes and annulus tympanicus [23] are not preserved, assuming they existed, or may have not been visible during segmentation due to problems of contrast with surrounding materials having similar densities. The medial face of the footplate of the stapes appears to be transversely depressed; in addition, its rim is shallowly notched posteroventrally. The foramen ovale is larger than the footplate, which suggests that a cartilaginous operculum was present; this is supported by the posteroventral indentation of the footplate rim, the anterior border of the operculum generally being inserted into a stapedial notch like this.