Juvenile articulated Eusaurosphargis discovered

Scheyer et al. 2017
bring us a new largely articulated juvenile Eusaurosphargis specimen (PIMUZ A/III 4380; Figs. 1–3) very similar to the adult disarticulated specimen described by Nosotti and Rieppel 2003 (BES SC 390; Middle Triassic, ~240 mya, ~20 cm snout to vent length). Scheyer et al. had trouble nesting Eusaurosphargis correctly as a derived thalattosaur largely due to taxon exclusion (see below).

Figure 1. Adult and juvenile Eusaurosphargis specimens to scale.

Figure 1. Adult and juvenile Eusaurosphargis specimens to scale. The adult was disarticulated.

The old ‘adult’ specimen
was considered more closely related to Helveticosaurus (Fig. 4) than to placodonts. Here both Eusaurosphargis and Helveticosaurus nest within the Thalattosauriformes close to armored Vancleavea. Here Eusaurosphargis does not nest close to turtle-like Sinosaurophargis. The adult skeleton is completely disarticulated. That makes reconstruction particularly difficult. Thus the order of the traced vertebrae in dorsal view (Fig. 1) is largely guesswork. Likewise, the skull included some guesswork helped by phylogenetic bracketing.

Figure 2. The in situ juvenile specimen of Eusaurosphargis, the original tracing and DGS tracing of dorsal ribs (blue) and sternal ribs (green).

Figure 2. The in situ juvenile specimen of Eusaurosphargis, the original tracing and DGS tracing of dorsal vertebrae and elongated transverse processes (blue) and dorsal ribs (green). The specimen was exposed from below, but preserved right side up, hence the slight disarticulation of dorsal elements and the skull in marine sediments. CT scans indicate the buried sacral ribs were longer than traced here. 

The new ‘juvenile’ specimen
has a disarticulated skull, but most of the elements appear to be present, though some were originally unidentified and the squamosal, now a jugal, was misidentified.

Figure 3. Eusaurosphargis juvenile skull, pectoral and pelvic girdles reconstructed.

Figure 3. Eusaurosphargis juvenile skull, pectoral and pelvic girdles reconstructed. GIF animation second frame shows two views of the in situ skull. The juvenile includes articulated extremities. Boxed elements are the purported squamosals, here identified as jugals. Scheyert et al. did not attempt a skull reconstruction.

Scheyer et al. report
the armor and other elements “support an essentially terrestrial lifestyle for Eusaurosphargis and and that within the marine reptile ‘superclade’ E. dalsassoi potentially is the sister taxon of Sauropterygia.” Neither are supported by the large reptile tree (LRT 1027 taxa), which resurrected the clade, Enaliosauria for Scheyer’s ‘superclade.’

The elongated dorsal transverse processes
and osteoderms are convergent with those in placodonts and sinosaurosphargids.

The jugals are much larger than the squamosals
as in Helveticosaurus and Vancleavea. For reasons unknown, Scheyer et al. erroneously compared these elements with those of the more distantly related Askeptosaurus, which ALSO has tiny squamosals, like most, if not all, thalattosaurs.

Phylogenetic analysis
The Scheyer et al. inclusion set excludes so many pertinent taxa that it nests turtles with archosaurs and lepidosaurs. It also nests Eusaurosphargis close to placodonts. Correctly it nests Eusaurosphargis close to Helveticosaurus and Thalattosauriformes. Vancleavea was not included. It is clear that Scheyer et al. have no idea how the major taxa are actually arranged as documented in the LRT for the last seven years. They also employed suprageneric taxa. There’s no reason for such unprofessional  guessing to continue in professional studies.

Figure 4. Helveticosaurus had cheek teeth that look like baleen strainers and long fangs anteriorly. It was also much larger than Eusaurosphargis but was coeval. Vancleavea is shown to scale and to the same length.

Figure 4. Helveticosaurus had cheek teeth that look like baleen strainers and long fangs anteriorly. It was also much larger than Eusaurosphargis but was coeval. Vancleavea is shown to scale and to the same length.

‘Homologies’ reported by Scheyer et al.:
“PIMUZ A/III 4380 shares with Palatodonta bleekeri (and placodonts such as Paraplacodus broilii and Placodus gigas) the deep skull shape and wide snout with large external nares, as well as the double tooth row in the upper jaw (on the maxillae and palatines) and a single row in the lower jaw.” These traits are likewise found by homology in Helveticosaurus and Vancleavea where known. Scheyer et al. feel the freedom to make these comparisons to placodonts because their incorrect (based on massive taxon exclusion) cladogram nests Eusaurosphargis close to placodonts. This is the authority of the LRT and its large gamut, specimen-based taxon list at work. When Scheyer et al. have a comparable taxon list, then we can discuss differences in scoring, if they arise.

Terrestrial?
Scheyer et al. report, “Given the large number of pachypleurosaurs of similar size range, among a plethora of thousands of other fossils, we corroborate the previous idea that E. dalsassoi had a terrestrial habitat preference.” This makes no sense. In the LRT pachypleurosaurs arise from marine taxa and give rise to marine taxa. Thus, based on phylogenetic bracketing. pachypleurosaurs are marine (or at least aquatic), too,

Scheyer et al. report, “the short and proximally dorso-ventrally wide tail would be similarly inefficient in providing propulsion.” You don’t have to get around fast in order to be aquatic. The flattened turtle-like appearance of several saurosphargids and placodonts have similar short-comings. And look at Helveticosaurus, the acknowledged sister (Fig. 4).

Scheyer et al. report,The stylopodial elements (humerus and femur) are tubular, moderately thin-walled bones with large marrow cavities” typical of terrestrial, not marine diapsids. They do not report similar tests on the universally accepted sister taxon, Helveticosaurus (and Vancleavea), but the proximal limb elements look similar from the outside (Fig. 4).

The All-Aquatic Superclade of Chen et al. 2014
does not include mosasaurs, but does include a few representatives of most other marine clades (but not nearly the number of taxa as in the LRT). While this is confirmation of the results first reported here in 2011, the topology of the Chen et al. cladogram has serious problems all based on taxon exclusion. Such problems are minimized in the LRT based on its large gamut where there is no need to ‘delete problematic characters’ in order to achieve a result that makes sense. 

A larger gamut phylogenetic analysis
nests Helveticosaurus, Vancleavea and Eusaurosphargis within the Thalattosauriformes, despite over 1020 opportunities to nest elsewhere. Their disparate morphologies hint at further transitional and unusual morphologies to come.

Congeneric? Yes. Conspecific? No.
The smaller Eusaurosphargis nests with the larger one in the LRT. So they could be congeneric. However, comparing the reconstructions of the two shows several differences in the skull bones that preclude the two from being conspecific. Such juvenile/adult conspecific relationships in fossils found years apart and miles apart are, by their very nature, very rare, but they do occur.

References
Chen X-H, Motani R, Cheng L, Jiang D-Y and Rieppel 2014. The Enigmatic Marine Reptile Nanchangosaurus from the Lower Triassic of Hubei, China and the Phylogenetic Affinities of Hupehsuchia. PlosOne online.
Nosotti S and Rieppel O 2003. Eusaurosphargis dalsassoi n.gen. n.sp., a new, unusual diapsid reptile from the Middle Triassic of Besano (Lombardy, N Italy). Memories of the Italian Society of Natural Science and the Museum of Natural History in Milan, XXXI (II).
Scheyer T et al. (5 other authors) 2017. A new, exceptionally preserved juvenile specimen of Eusaurosphargis dalsassoi (Diapsida) and implications for Mesozoic marine diapsid phylogeny. Nature.com/scientific reports online.

wiki/Eusaurosphargis

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