Keuperotesta limendorsa, Szczygielski & Sulej, 2016

Szczygielski, Tomasz & Sulej, Tomasz, 2016, Revision of the Triassic European turtles Proterochersis and Murrhardtia (Reptilia, Testudinata, Proterochersidae), with the description of new taxa from Poland and Germany, Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 177 (2), pp. 395-427 : 413-417

KEUPEROTESTA LIMENDORSA GEN. ET SP. NOV.

( FIGS 9 , 10I, L, M , 11C )

Proterochersis robusta Fraas, 1913 : Joyce et al., 2013: 1, figs 1 – 3.

Occurrence and distribution: Lower Lowenstein Formation (?lower or middle Norian) of Rudersberg, Baden-W urttemberg €, Germany.

Etymology: From Keuper, the geological unit where the animal lived, and ‘testa’, Latin for shell. ‘Limen’ from the Latin for threshold; ‘dorsum’ from the Latin for back (referring to the surface of its shell).

Diagnosis: Differing from Proterochersis in: acromion and coracoid forming 140 ° angle; bone distinctly thickened at the level of the posterior ends of vertebral scutes II – IV (even three times thicker than at their anterior parts); anterior marginals spiky; marginal series lacking the element corresponding to the first marginal of Proterochersis ; contact between the cervical scute and the first marginal dorsally very narrow, with marginal positioned laterally instead of anterolaterally relative to cervical scute; first vertebral scute contacting the first pleural posteromedially rather than anteromedially; the last cervical vertebra not coossified with carapace nor with the first thoracic vertebra.

Holotype: SMNS 17757 (the only specimen).

The shell ( Figs 9 , 10M ) and scapulocoracoids ( Fig. 11C ) of the specimen described recently by Joyce et al. (2013) turn out to be somewhat different than those of Proterochersis robusta . The shell is fragmentary, but the arrangement of preserved sulci ( Fig. 9 ) is nearly the same as in Proterochersis spp. There are some differences nonetheless: the first vertebral reaches farther posterolaterally, resulting in posteromedial rather than anteromedial contact with the first pleural. There is no equivalent of the first marginal of Proterochersis spp. , instead the cervical scute contacts the first marginal (equivalent of the second marginal of Proterochersis spp. ) via a narrow lateral lamella (the anterior margin of that region is tapered and does not seem to be broken). The rest of the marginals visible in the specimen occupy the same positions as marginals III – V and XI – XIII of Proterochersis . The first two marginals are pointed, in contrast to the slightly sinuous anterior carapace rim in Proterochersis robusta (SMNS 17561) and the almost straight carapace rim in specimens of Proterochersis porebensis sp. nov. The whole posterior margin of the fifth vertebral is missing, so there is no natural rim of the caudal notch present and therefore its shape, or even presence, cannot be established. There are no unambiguous sutures visible on the shell, but slight depressions on the visceral surface of plastron (corresponding to the sutures seen in Proterochersis robusta ) indicate that two pairs of mesoplastra were present. In lateral view distinct bulges are visible in the posterior parts of the second, third, and fourth vertebral scute areas, caused by much higher bone thickness there: whereas the carapace thickness in the anterior part of the third vertebral area is 0.5 cm, in the posterior part it reaches 1.5 cm ( Fig. 10M ). Although some gradual bone thickening is visible in the vertebrals of Proterochersis robusta specimen SMNS 17561, the resulting shape is different (more round, in contrast to the almost straight lines of vertebrals with noticeable ‘bumps’ just before their posterior edges in SMNS 17757), and it appears to affect every vertebral, not just the second – fourth vertebrals. Even though SMNS 17757 is the largest proterochersid specimen from Germany (measuring around 38.5 cm in length), there is evidence that the thickening of the bone underlying the vertebral scutes did not simply occur during ontogeny, because in slightly smaller Proterochersis robusta specimens from CSMM and SMNS the dorsal surface is almost uniform (with bone thickening greatest in the previously mentioned SMNS 17561; Fig. 10N ), and in even larger specimens of Proterochersis porebensis sp. nov. (ZPAL V.39/48 and ZPAL V.39/49) no changes in vertebral thickness are visible at all. The posterior process of the entoplastron in SMNS 17757 is similar to that of Proterochersis robusta (SMNS 12777) or Proterochersis porebensis sp. nov., but its caudal end has a different shape (it ends abruptly and is only slightly expanded distally, with much weaker lateral projections and posterior projections). Whether this is caused by superficial damage or is an accurate reflection of the morphology as it was in vivo it is hard to tell.

Joyce et al. (2013) correctly noted that the eighth cervical vertebra is free and not fused to the first thoracic vertebra, nor to the nuchal bone ( Fig. 10I, L ). This is different than is the case in Proterochersis porebensis sp. nov. specimen ZPAL V.39/48, in which the last cervical is fused both with the thoracic vertebral column and with the carapace ( Fig. 3E, F ). Although ZPAL V.39/49 and SMNS 16442 lack the last cervical vertebra, a broken lamina of bone is present where the vertebra contacted the carapace, indicating that it was fused. Proterochersis robusta specimens SMNS 12777 and SMNS 16603 do not have this vertebra preserved, as well, but its impression is visible as an empty cavity not separated from the thoracic vertebrae row, suggesting that it grew together with the first thoracic vertebra and probably contacted the carapace ( Fig. 10H, K ).

The configuration of the scapulocoracoid is also notably different. The dorsal process of the scapula is at its base turned slightly more towards the glenoid than in Proterochersis porebensis sp. nov. specimen ZPAL V.39/48, and the angle between the acromion and the coracoid ( Fig. 11 ) is noticeably larger than in ZPAL V.39/48 (about 150 ° versus 120 °). Such differences might have been caused by compaction, but this would distort fragile ridges of the acromion, and these are straight and seem to be undisturbed in both specimens, hence the organization of the pectoral girdle was probably the same in vivo as in fossils, and is not disfigured by diagenetic processes. A similarly large coracoid – acromion angle is present only in Proganochelys quenstedti , and other turtles seem to have about 120 ° or less. This condition is unfortunately unknown for Odontochelys semitestacea .

Other features of the vertebral column and the pelvic girdle were adequately described by Joyce et al. (2013), and are essentially the same as in Proterochersis spp. , but some corrections are needed. The thyroid foramina were rather smaller and more regular (more similar to their shape in Proterochersis porebensis sp. nov.) in life than was illustrated in Joyce et al. (2013), and their ridges are damaged in the specimen described. The sutures that led Joyce et al. (2013) to believe that the ilia contacted the carapace via the descending process are ambiguous. This purpoted descending process forms, in its dorsoposterior part, a caudally oriented process (even more clearly visible in some of the Proterochersis specimens), which is very similar to the posterior ilial process of Odontochelys semitestacea , Proganochelys quenstedti , and Palaeochersis talampayensis (but fully grown to the shell), and its relationship with the sacral ribs is identical. Therefore that part is best interpreted as the true point of contact between the pelvis and carapace. The extent of preparation does not allow for a clear view of the articulation shape, but the lateral process of the ilium similar to this of Proterochersis spp. is not visible.

The shape of the dorsal and anterior surface of the carapace, as well as the configuration of the scapulocoracoid and the last cervical vertebra indicate that this specimen is notably different from both Proterochersis species. Because of that, its assignment to a new genus, Keuperotesta , and a new species, Keuperotesta limendorsa , is proposed.

NEW TAXA AND INTRASPECIFIC VARIABILITY AND

Figures

Figure 3. Proterochersis porebensis sp. nov.: A, B, carapace of holotype (ZPAL V.39/48) in dorsal view; C, D, plastron of holotype (ZPAL V.39/48) in ventral view; E, F, anterior part of carapace and vertebral column of ZPAL V.39/72 in ventral view, with the fused last cervical vertebra (note the first thoracic rib parallel with the second, and forming a costal); G, H, caudal notch of the holotype (ZPAL V.39/48) in posterior view; I–K, comparison of the proximal heads of right femur of Proterochersis porebensis sp. nov. (I, ZPAL V.39/48), Proganochelys quenstedti (J, SMNS 16980), and Palaeochersis talampayensis (K, PULR 68, left-mirrored for easier comparison, based on Sterli et al., 2007) in dorsal view. Scale bars: A–D, 30 cm; I–K, 5 cm; E–H, not drawn to scale.

Figure 9. Keuperotesta limendorsa gen. et sp. nov., SMNS 17757, shell: A, B, dorsal view; C, D, ventral view. Dark grey is matrix; hatched is damaged or broken bone. Note the unbroken, free dorsal surface of the eight cervical vertebra neural spine. Scale bar: 20 cm.

Figure 10. A–F, caudal notch of Proterochersis robusta in posterior view, SMNS 17561 (A and D), unnumbered CSMM (B and E), and SMNS 16603 (C and F, note that despite the lack of the central part of the last vertebral, the outline of the caudal notch is visible thanks to coarser, differently coloured sediment); G, J, posterior part of the right bridge of the holotype of Proterochersis robusta (SMNS 12777) in ventral view, revealing an outline of the damaged last inframarginal; H, K, anterior part of the holotype of Proterochersis robusta (SMNS 12777) in dorsal view, with an empty cavity left by the last cervical vertebra (note that it was not displaced, and nor can any discontinuities with the thoracic vertebral column be seen, indicating fusion as described in Proterochersis porebensis sp. nov.); I, L, anterior part of carapace and vertebral column of the holotype of Keuperotesta limendorsa gen. et sp. nov. (SMNS 17757) in ventral view, with the two last cervical vertebra free and relocated; M, the holotype of Keuperotesta limendorsa gen. et sp. nov. (SMNS 17757) in lateral left view (note the distinct broadened posterior parts of vertebrals II–IV and nearly straight profile of the first vertebral); N, Proterochersis robusta (SMNS 17561) in lateral left view (note the rounded profile and smooth changes of the vertebral elevation). Dark grey is matrix; hatched is damaged or broken bone. Not drawn to scale.

Figure 11. Majority rule (50%) consensus cladogram of the relationships of Proterochersidae with other Late Triassic and Early Jurassic turtles, with stratigraphic ranges and ventral outline of left scapulocoracoids (not drawn to scale) of the Triassic taxa presented. Jacknife frequency difference values are given above and bootstrap frequency difference values are given below the branches (both 10 000 replicates). The maximum temporal ranges of Proterochersidae are shown in black, with maximum possible overlap of the German and Polish taxa shown in dark grey (see text for discussion). The approximate angles between the coracoid and the acromion were measured as the angle between the extrapolated tangential to the lateral rim of the coracoid and to the most anterolateral part of the acromion, in order to avoid errors arising from the incompleteness and curvature of the acromion. A, Odontochelys semitestacea, coracoid of IVPP V 15653 (based on Li et al., 2008); B, Proterochersis porebensis sp. nov., ZPAL V.39/48; C, Keuperotesta limendorsa gen. et sp. nov., SMNS 17757 (right-mirrored for easier comparison); D, Proganochelys quenstedti, SMNS 16980 (subadult); E, Proganochelys quenstedti, SMNS 51600 (mould of original; adult); F, Palaeochersis talampayensis, PULR 68 (right-mirrored for easier comparison, based on Sterli et al., 2007). The angle for (F) is not given because there was no possibility of examination the specimen in person, but appears to be relatively low.