Enigmatic humerus from the mid-Permian of South Africa bridges the anatomical gap between “pelycosaurs” and therapsids

The fossil record of appendicular muscle evolution in Synapsida on the line to mammals: Part I—Forelimb

“…As inferred by the current analyses, major changes in forelimb musculature associated with the origin of therapsids included a shift in the insertion of key shoulder muscles on the proximal humerus, concomitant with a reorganization of proximal humeral structure more generally (characters 2, 7, 11, 18; see also Bishop et al, 2023), a shift in the insertion of the BICB on the radius (character 32), a less direct connection of the TRIC to the metacoracoid (character 37), and a stronger relationship between the FDL origin and ulnar shaft (character 50). Major changes occurring within nonmammalian cynodonts principally revolve around the transformation of deltoid and supracoracoid musculature, affecting the structure of the scapula (characters 5, 6, 21, 22) and humerus (characters 7, 23–25), in addition to the appearance of a novel muscle, the TMA (character 4).…”

“…Extensive first‐hand observation confirms that this scar is widely distributed throughout therapsids and cynodonts, occupying a consistent position on the humerus (Figures 7n,o and 10h–ao). The straightforward transformation from a distal position in “pelycosaurs” to a proximal position in therapsids, and the occurrence of a similar scar in an intermediate‐form humerus (Bishop et al, 2023), supports homology between the scarring in both groups. Thus, scarring can be traced continuously from stem amniotes through to theriimorphs, supporting the hypothesis of homology between the SHA and TMI.…”

“…The apex of the deltoid process is typically continued distally for a short distance by a sharp ridge that may also be scarred (Figure 9e,f). With the transition to therapsids, this region of the humerus is structurally simplified to form a single blade‐like expansion (Bishop et al, 2023; state 0); it more closely approximates the structure in extant nonmammals and may be more reasonably termed a “deltopectoral crest.”…”

“…Furthermore, there is no significant correlation between the inferred number of state changes at a given node and the proportion of missing data pertinent to that node (Figure 24c; p = 0.402, determined using ordinary least squares in PAST 3.01; Hammer et al, 2001), lending credence to the recovered pattern of change. The large anatomical shift associated with the origin of therapsids is not unexpected, given the >30 Ma ghost lineage separating the oldest known well‐supported members of Therapsida and their immediate sister group, the sphenacodontid “pelycosaurs” (Angielczyk & Kammerer, 2018; Bishop et al, 2023; Sidor & Hopson, 1998). This reiterates that a critical chapter of synapsid anatomical evolution remains to be understood.…”

Adaptive landscapes unveil the complex evolutionary path to mammalian forelimb function and posture

“…Specifically, Hipposaurus overlaps with reptiles in morphospace and optimizes humeral torsion, length and 'swing' speed (Figures 1 and 3), indicating a reptile-like sprawling posture and kinematics 19 . Although this combination suggests adaptations for fast limb movements, it may not be reflective of the ancestral therapsid condition, due to a persistent ghost-lineage following their divergence from pelycosaurs 52 . Later theriodont therapsids, the gorgonopsians and therocephalians, experienced similar but less extreme shifts on the transitional landscape, sitting on the outskirts of the reptile and therian regions of morphospace (Figures 4, 5, Supplementary Figures 6, 7).…”