Piñeiro et al. 2021
recognize only the genus Mesosaurus (Figs. 1–4) in the Paraná (Brazil) and Karoo (South Africa) basins. The authors suggest that coeval genera, Sterosternum and Brazilosaurus (Figs. 1–4), are “nomina dubia taking into account that the autapomorphies that supported these taxa cannot be confirmed to be absent in Mesosaurus.”
No phylogenetic analysis was presented.
Rather the authors made a trait-by-trait report and several ratio comparisons across many specimens. So they went deep into charts and statistics.
I’ve seen statistical support studies fail. Bennett’s studies on Pteranodon come to mind in which he found gender differences where none appear in a cladogram.
Only 23 of 305 specimens preserved a skull
in association with a complete cervical series. Note the holotype of Stereosternum (Fig. 4) is one of the headless specimens.
Prior to this paper
in the large reptile tree (LRT, 1941+ taxa) Brazilosaurus sanpaukebsus (Shikama and Ozaki 1966; Piñeiro et al. 2021; Permian, Brazil) nested between Sterosternum and Mesosaurus with complete resolution. The holotypes were not tested then. They are being added now, starting with Mesosaurus.
Mesosaurus tenuidens (Gervais 1864-66; MNHN 1865-77; Early Permian ~290 mya, up to 100 cm in length), was long considered a basal reptile related to those that never had temporal openings. Not so. Here the temporal opening are secondarily closed off.
Stereosternum tumidum (Cope 1885; Early Permian ~290 mya, 30 cm in length) Here both temporal openings are present.
Mesosaurus teeth appear to be much longer than those of related mesosaurs.
Piñeiro et al. report, “Modesto (1996, 1999) suggested that the marginal teeth in specimens assigned to Mesosaurus are longer than those in specimens assigned to Stereosternum
because he found that the largest teeth in Mesosaurus are equivalent to the length of five tooth positions, whereas in Stereosternum, the longest teeth would occupy the length of only three sockets. However, these comparative measures are to be taken carefully, considering that there is a great variability in the distance among tooth sockets in mesosaurs, including an important degree of deformation that occurs during fossilization processes.”
The authors wrote:
“identification of new specimens using the available diagnostic characters are arbitrary and influenced by high subjectivity.”
A phylogenetic analysis is essential. Reconstructions (Fig. 2) are helpful. As in many genera (e.g. Triceratops, Pteranodon, Rhamphorhynchus), it is possible that no two individual specimens are alike.
The authors still believe (without testing) the invalid myth that mesosaurs were basal amniotes. They are not. In the large reptile tree (LRT, 1941+ taxa) mesosaurs nest with thalattosaurs and ichthyosaurs, derived from pachypleurosaurs (Fig. 4) and basal aquatic diapsids.
The authors wrote:
“After the detailed revision of the type specimens of the three currently accepted mesosaur taxa, for which we include here good-quality photographs, and considering the lack of statistical support for the most applied putative diagnostic features such as the different ratio found when comparing skull and cervical region lengths and the low or higher intensity of pachyosteosclerosis observed in dorsal ribs, which can be controlled by taphonomic and ecological conditions, we recognize Mesosaurus as the only mesosaurid taxon in the Paraná and Karoo basins, probably including dwarf individuals.”
IMHO the authors should have analyzed just these three holotype mesosaur specimens (with proper outgroups) to determine similarities and differences, rather than inundate themselves with 305 specimens juveniles, adults, partials, etc. Thereafter, once morphological patterns had emerged (or not) then expand the study to other specimens to see which genus each new one was closest to, dropping them in one at a time, as done here in the LRT. A phylogenetic analysis is usually essential, but usually only at the generic level and above. DGS reconstructions also help to organize fossils.
Here’s a possible, perhaps probable, problem:
If you study 305 specimens of three closely related genera, the chance that you are going to see a transitional spectrum of taxa is much greater. The differences will be harder to tell, as Piñeiro et al. 2021 demonstrate. The differences blur and blend… as they should! As an example, Modesto 1999 attributed the BHM 999 specimen (Fig. 5) to Stererosternum even though it has a Mesosaurus-like cranium. Start with the holotypes. Then work from there.
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Modesto SP 1999. Observations on the structure of the Early Permian reptile Stereosternum tumidum Cope. Palaeontol. Afr. 35, 7–19.
Piñeiro G, Ferigolo J, Mones A and Demarco PN 2021. Mesosaur taxonomy reappraisal: Are Stereosternum and Brazilosaurus valid taxa? Revista Brasileira de Paleontologia 24(3):205–235. A Journal of the Brazilian Society of Paleontology.
Shikama T and Ozaki H 1966. On a Reptilian Skeleton from the Palaeozoic Formation of San Paulo, Brazil. Transactions and Proceedings of the Palaeontological Society of Japan. New Series. 64: 351–358.