A partial skeleton of Chiniquodon (Cynodontia, Chiniquodontidae) from the Brazilian Middle Triassic

Abstract

-In this paper, we describe new postcranial remains of Chiniquodon cf. C. theotonicus, a chiniquodontid cynodont from the Therapsid Cenozone, from the Santa Maria Formation, Middle Triassic of Southern Brazil. In the described specimen are preserved almost all presacral vertebrae, the sacral vertebrae, an incomplete pelvic girdle, the left femur, and two metapodials. Some of these bones show slight differences relative to those already described for C. theotonicus, especially in the femur and in the pelvic girdle. Since the species can actually include the materials attributed to the genera Probelesodon (except from P. sanjuanensis) and Belesodon, however, these differences may represent normal ontogenetic variation in the species rather than being of taxonomically diagnostic value.Key words: Triassic, Santa Maria Formation, Brazil, Chiniquodontidae, Chiniquodon, postcranial skeleton.RESUMO -Neste trabalho são descritos novos elementos pós-cranianos de Chiniquodon cf. C. theotonicus, um cinodonte chiniquodontídeo da Cenozona de Therapsida, da Formação Santa Maria, Triássico Médio do sul do Brasil. No espécime descrito estão preservadas quase todas as vértebras pré-sacrais, as vértebras sacrais, a cintura pélvica incompleta, o fêmur esquerdo e dois metapodiais. Alguns destes ossos apresentam pequenas diferenças em relação àqueles previamente descritos para C. theotonicus, especialmente no fêmur e na cintura pélvica. Uma vez que esta espécie pode, na realidade, incluir os materiais atribuídos aos gêneros Probelesodon (exceto de P. sanjuanensis) e Belesodon, estas diferenças podem representar variações ontogenéticas normais dentro de uma espécie e, assim, não ter grande valor taxonômico. Palavras-chave:Triássico, Formação Santa Maria, Brasil, Chiniquodontidae, Chiniquodon, esqueleto pós-craniano.

The postcranial anatomy of Brasilodon quadrangularis and the acquisition of mammaliaform traits among non-mammaliaform cynodonts

“…The centra are flat on the anterior side and concave on the posterior side (platycoelous), as seen in some tritylodontids such as Tritylodon and Bienotheroides [19] and in the early mammaliaform Morganucodon [32]. Centra are deeply amphicoelous in most non-mammaliaform cynodonts (e.g., [12, 17–18, 38, 79–80]). The neural arches are poorly preserved, almost all vertebral processes are broken.…”

“…Anapophyses occur in all basal epicynodonts (e.g., Galesaurus , Thrinaxodon ) and several cynognathians (e.g., Cynognathus , Diademodon , Cricodon , Andescynodon , Luangwa , Protuberum ; [17, 38]). They are not observed in the axial skeleton of Procynosuchus [14], several traversodontids (e.g., Menadon , Massetognathus , Exaeretodon ; [12, 81–82] and probainognathians [16, 80, 83]. The pre- and postzygapophyses are eroded but postzygapophyses appear to have a nearly horizontal orientation and are placed in a relatively high position on the neural arch (Fig 2A).…”

“…All preserved ribs lack the expanded costal plates or processes. Costal plates are represented in all basal epicynodonts in which the postcranium is known [13, 84] and most cynognathians (e.g., Cynognathus , Diademodon , Luangwa , Andescynodon , Menadon , Protuberum , Pascualgnathus , and Scalenodon ; [17, 38, 80, 82]). They were lost in several traversodontids (e.g., Exaeretodon , Boreogomphodon ; [80, 85]) and all probainognathians (e.g., [18, 43, 83]).…”

“…quadrangularis , corresponding to a small prominence, as in Prozostrodon [43] and tritylodontids [16, 104], or completely absent as in Therioherpeton and the early mammaliaforms (e.g., Eozostrodon , Megazostrodon , Erythrotherium ; [32]). In other non-mammaliaform cynodonts, the postacetabular region of the blade is well-developed [12–13, 38, 80, 82, 86–87, 95, 105]. …”

“…It lies anteriorly rather than dorsally to the acetabulum, leaving the dorsal region of the acetabulum open. This condition is present in Aleodon [80], traversodontids [12, 82, 86], Prozostrodon [43], and tritylodontids [104]. By contrast, in the basal cynodonts Procynosuchus [14], Galesaurus , and Cynognathus , the iliac buttress has a dorsal position.…”