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

Abstract

Brasilodon quadrangularis (Cynodontia, Probainognathia) is an iconic non-mammaliaform cynodont from the Late Triassic of Brazil ( Riograndia Assemblage Zone, Candelária Sequence), being considered as the sister taxon of Mammaliaformes. Although its phylogenetic position is very important, several aspects of its postcranial anatomy remain unclear or unstudied. Here, we present a detailed description of the postcranial elements referred to Brasilodon , including previously mentioned specimens and new ones, which add relevant information about its postcranial morphology and provide a new insight into the anatomical transition between advanced non-mammaliaform cynodonts and early mammaliaforms. Functional and ecological implications are also investigated, based on the postcranial morphology and muscular reconstructions. The postcranium of Brasilodon differs from most non-mammaliaform cynodonts and presents similarities with tritylodontids, early mammaliaforms and extant therians, such as a ventrally oriented scapular glenoid facet, a distinct and ossified greater humeral tubercle, lack of ectepicondylar foramen, olecranon process, hemispherical humeral and femoral heads and a prominent intertrochanteric crest. The humeral torsion, the length of the deltopectoral crest, the large bicipital groove and the well-developed lesser tubercle, indicate that the forelimb of Brasilodon was hold in a semi-sprawling position, with well-developed adductor muscles to maintain the body off the ground. The short femoral neck and the strong medial projection of the femoral head indicate the femur was held in a more erect posture than in basal non-mammaliaform cynodonts. The anterodorsally projected iliac blade with reduced postacetabular process, reduction of the anterior part of the pubis, medially located lesser trochanter indicate a basically mammalian pattern of pelvic musculature, able to swing the femur in a nearly parasagittal plane.

Incomplete convergence of gliding mammal skeletons*

“…6) diverged from a common ancestor over 200 million years ago (Fig. 1; Huttenlocker et al 2018), and thus are expected to have skeletal differences owing to this considerable phylogenetic separation (e.g., Luo 2015;Guignard et al 2019). This may help account for the lack of overlap in morphospace of extant taxa and fossil haramiyidans (Figs.…”

A new specimen provides insights into the anatomy of Irajatherium hernandezi, a poorly known probainognathian cynodont from the Late Triassic of southern Brazil

“…The deltopectoral crest is well‐developed (Figure 6a), and, as in the holotype, it is oriented approximately 90° with the axis that passes through the lesser trochanter (Martinelli et al, 2005). As in most non‐mammaliaform cynodonts, including R. guaibensis , the deltopectoral crest is well‐developed (Gaetano et al, 2017, 2018; Guignard et al, 2018, 2019a, 2019b; Jenkins Jr, 1971; Liu et al, 2017; Martinelli et al, 2005). Distally to the lesser trochanter, there is a tuberosity on the medial surface of the humerus that may correspond to the attachment area of the M. teres major .…”

“…Located in the municipality of Faxinal do Soturno, State of Rio Grande do Sul, this site has revealed an astonishing quantity and diversity of Norian vertebrates (Bonaparte, Schultz, & Martinelli, 2010; Schultz et al, 2020). After its discovery at the end of the 1990s, this site yielded hundreds of tetrapod remains that stand out for their exceptional preservation, including several cranial and postcranial materials of non‐mammaliaform cynodonts (Bonaparte et al, 2001, 2003, 2005; Bonaparte, Schultz, & Martinelli, 2010; Guignard et al, 2019a, 2019b; Martinelli et al, 2005; Martinelli & Soares, 2016; Oliveira et al, 2011; Soares et al, 2011), one partial skeleton of the saurischian dinosaur Guaibasaurus candelariensis (Agnolín & Martinelli, 2012; Bonaparte et al, 2007), remains of a putative pterosaur (Bonaparte, Schultz, & Soares, 2010; but see Soares et al, 2013; Dalla Vecchia, 2014), skulls and postcranial elements of procolophonoids (Cisneros & Schultz, 2003), sphenodontians (Arantes et al, 2009; Bonaparte & Sues, 2006; Ferigolo, 2000; Hsiou et al, 2015; Romo de Vivar, Martinelli, Hsiou, & Soares, 2020; Romo de Vivar & Soares, 2015), and skull remains of non‐rhynchocephalian lepidosauromorphs (Bonaparte, Schultz, & Martinelli, 2010; Romo de Vivar, Martinelli, Fonseca, & Soares, 2020). In addition, fish remains, insects, conchostracans, invertebrate ichnofossils, dinosaur footprints, and gymnosperms have been recovered at the site (Barboni & Dutra, 2013; Rohn et al, 2014; Silva et al, 2012), although from overlaying levels.…”

The therian sternum at the lateral somitic frontier: Evolution of a composite structure

“…Nonmammalian cynodonts displayed a range of locomotor styles, with some taxa approaching the parasagittal limb orientation of mammals (Kemp 2005, Guignard et al. 2019). This sternal pattern is also found in living monotremes. Pattern 4: No interclavicle is visible.…”