Mesosaurus, Gervais, 1865

The “pygal” caudal segment of Mesosaurus

The first seven to ten caudal vertebrae (observed intraspecific numerical variations may reflect sexual dimorphism; see Shine et al. 1999) constitute a specialized string characterized by the presence of ribs firmly attached to the short transverse processes, the lack of haemal arches (chevrons) and also the absence of “fracture/suture planes”. However, in mesosaurs the last vertebrae of the pygal segment can carry an haemal arch.

This configuration follows the previously documented trait of basal stegocephalians, in which all presacral vertebrae, as well as the first caudals, bear ribs ( Romer 1947).

The ribs of the first five caudal vertebrae are long, stout and bent posteriorly, as is also observed for instance in captorhinids ( Heaton & Reisz 1980; Berman & Reisz 1986), diadectids ( Berman & Henrici 2003) and seymouriamophs ( Berman et al. 1987). From the sixth to the ninth or tenth vertebrae, the ribs abruptly decrease in size with the last three either bent anteriorly or pointing laterally ( Fig. 1 View FIG ). These anteriormost nine to ten caudal vertebrae may be considered as the “pygal-like” segment, which in extant squamates possessing intravertebral autotomy, is associated with the insertion of the caudofemoralis longus muscle, and thus autotomy cannot occur in vertebrae of this segment which is typically characterized by the absence of fracture planes ( Ritzman et al. 2012).

Due to the presence of long and firmly fused ribs, a condition which is morphologically similar to that observed in the posterior dorsal vertebrae ( Fig. 1 View FIG ), the ribs of the first ten caudal vertebrae of mesosaurs are often preserved parallel to the stratification, and thus, the vertebrae are mostly exposed in ventral or dorsal view, or in a frontal or antero/posterolateral view on rare occasions. This makes it challenging to confirm the existence of potential “fracture planes” (from now on referred to as “fracture/suture planes”) within these anteriomost caudal vertebrae. However, some fortuitous isolated vertebrae recognized as part of the “pygal segment” because they bear short transverse processes carrying a rib, show that there are no central fractures in these vertebrae at the midcentrum area. The presence of haemal arches in the last vertebrae of the “pygal-like” segment can also be confirmed by examining these isolated vertebrae ( Fig. 2C, D View FIG ).

The postpygal caudal segment of Mesosaurus

The putative “fracture/suture planes” described byMacDougall et al. (2020) as evidence for autotomy in mesosaurs are present in vertebrae placed posterior to the last vertebra of the pygal segment. This vertebral segment will be known as the postpygal string and is characterized by vertebrae with diminutive transverse processes in the cranialmost segments (or none whatsoever, farther caudally), and are predominantly preserved in lateral view ( Fig. 3 View FIG ).

The mesosaurid “postpygal” segment is formed by at least 56 vertebrae, some of which articulate with short ribs, if at all, but this condition is gradually lost in caudal direction. Most of them also bear haemal arches (chevrons); and only the last five or six vertebrae near to the tip of the tail are simplified and lack neural spines or haemal arches ( Fig. 3B, C View FIG ).

This distribution indicates that Mesosaurus possessed a long tail consisting of more than 60 vertebrae, a length similar to what has been suggested for Captorhinus Cope, 1895 and all the basalmost stegocephalians ( Romer 1947). Moreover, based on the described general pattern for mesosaur caudal vertebrae, it is possible to identify the relative position of isolated caudal vertebrae in the tail, and also determine if it is part of the “pygal-like” or the “post-pygal” series through the orientation of the transverse processes and ribs and by the presence/absence of haemal arches ( Fig. 3 View FIG ).