A late (Middle Triassic) survivor of a Viséan radiation: Bystrowiella

Most of the taxa
in the large reptile tree (LRT, 1023 taxa) are bunches of leaves on bushy branches, chronicling the slow but steady march of evolution and radiation. In a few cases, like SphenodonDidelphis and Monodelphis, body parts are relatively unchanged over tens to hundreds of millions of years. That seems to be the case once again with Bystrowiella schumanni (Fig. 1), a taxon that nests with Viséan (340 mya) radiation taxa, but appears 130 million years later in the Middle Triassic (208 mya), In other words, this taxon had a long ghost-lineage.

Figure 1. Bystrowiella materials. Noteworthy are the lack of antorbtial fenestra, lack of an intertemporal, great size of the femur relative to the humerus and pectoral girdle and the possibility that disassociated armor might have belonged to this taxon, convergent with chroniosuchians.

Figure 1. Bystrowiella materials. Noteworthy are the lack of antorbtial fenestra, lack of an intertemporal, great size of the femur relative to the humerus and pectoral girdle and the possibility that disassociated armor might have belonged to this taxon, convergent with chroniosuchians (inset). Scale bars indicate a larger humerus than was drawn with the scapula graphic.

Bystrowiella schumanni (Fig. 1, Middle Triassic) was considered a bystrowianid chroniosuchid by Witzmann and Schoch 2017 despite lacking the hallmark antorbital fenestra found on traditional chroniosuchids and having the medial premaxillary teeth larger than the lateral ones, along with a long list of other distinct traits. They reported, “In sum, the postcranial skeleton of Bystrowiella is much more amniote-like than that of chroniosuchids, and one might expect this morphology in a rather terrestrial animal.”
Chroniosuchids are otherwise known from the Early Permian to Late Triassic.

By contrast
the large reptile tree (LRT, 1023 taxa) nest chroniosuchids near the base of the new Archosauromorpha branch of the Reptilia (= Amniota), not as a basal  And it nests Bystrowiella as a late surviving member of the Viséan radiation that gave us reptiles, derived from basal Seymouriamorpha close to the origin of the Leponspondyli, but distinct from the lineage. They reported, “The most conspicuous character that is shared by chroniosuchians, Gephyrostegus and higher stem amniotes is the T-shaped interclavicle, and this character distinguishes chroniosuchians from embolomeres.”

Not sure about those osteoderms
They were found separate from the skull, but match back of the skull depressions. If they do belong to Bystrowiella, then they evolved by convergence with chroniosuchids over 130 million years.

Both analyses
nest Bystrowiella near Silvanerpeton, a stem or basal amniote from the Viséan. The Witzmann and Schoch tree nests other chroniosuchids there, too, though probably due to taxon exclusion.

Outgroups in the LRT include
the basal seymouriamorph Kotlassia and the basal seymouriamorph leposponyl, Utegenia. Despite its late appearance in the fossil record, phylogenetically that puts Bystrowiella at the very base of the clade that includes all reptiles (= amniotes), which makes it a VERY interesting taxon.

References
Witzmann F and Schoch RR 2017. Skull and postcranium of the bystrowianid Bystrowiella schumanni from the Middle Triassic of Germany, and the position of chroniosuchians within Tetrapoda. Journal of Systematic Palaeontology 29 pp.

http://dx.doi.org/10.1080/14772019.2017.1336579

 

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