From the Stefanello et al 2023 abstract:
“Triassic cynodonts from South America are key taxa in the investigation of the emergence of mammalian characters. One of the most iconic species from the Carnian is Prozostrodon brasiliensis, found in Late Triassic strata from southern Brazil.
The holotype of Prozostrodon (Fig 1) is missing the back half of the skull and mandibles. Those missing parts are present in the new specimen (Fig 1).
From the Stefanello et al 2023 abstract:
“This non-mammaliaform cynodont represents the earliest-diverging member of Prozostrodontia, a clade that encompasses Mammalia and their closest relatives.”
First time I’ve heard of this clade. It is valid according to the LRT.
According to Wikipedia
(Mammaliaformes page), “Mammaliaformes (“mammalian forms”) is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts.”
This is a bad definition.
It includes only one ingroup taxon (Mammalia is ok, but a genus or species would have been better). Thus it includes no last common ancestor and no way to determine the LCA. A cladogram by Luo and colleagues 2015 determined that Adelobasileus and Sinoconodon were co-last common ancestors, but the rest of the cladogram suffers from taxon exclusion and so recovers an odd tree topology with multituberculates nesting close to monotremes. Adelobasileus was considered the ‘oldest mammal’ by Lucas and Hunt 1990. In the LRT Adelobasileus nests with Therioherpeton prior to Mammalia.
After that detour, let’s get back to Prozostrodon.
Here in
the large reptile tree (LRT, 2212 taxa) the two Prozostrodon specimens (Fig 1) nest together between Thrinaxodon and Probainognathus despite the many subtle differences between the two (Fig 1).
From the Stefanello et al 2023 abstract:
“Previous descriptions of the skull of Pr. brasiliensis were based on specimens that did not preserve the posterior region, obscuring essential details of the basicranium. Here, we describe a new, complete, and exceptionally well-preserved skull of Pr. brasiliensis found in the same block as the holotype skeleton of the early predatory dinosaur Gnathovorax cabreirai and rhynchosaur specimens.
That’s an interesting menagerie. Gnathovorax is the basal-most dinosaur in the LRT.
“Anatomical data from this specimen provide novel insights into the initial radiation of prozostrodontian cynodonts and reveal a new endemic clade of South American cynodonts – Prozostrodontidae – on the stem lineage of mammals.”
Yes it does!
References
Lucas SG and Hunt 1990. The oldest mammal. New Mexico Journal of Science 30(1):41–49.
Luo, Z-X et al (4 co-authors) 2015. Mandibular and dental characteristics of Late Triassic mammaliaform Haramiyavia and their ramifications for basal mammal evolution. PNAS. 112(51): E7101-9.
Stefanello M et al (4 co-authors) 2023. A complete skull of a stem mammal from the Late Triassic of Brazil illuminates the early evolution of prozostrodontian cynodonts. Journal of Mammalian Evolution. DOI: 10.1007/s10914-022-09648-y
The Pterosaur Heresies 