Congenital Malformations of the Vertebral Column in Ancient Amphibians

Abstract

Temnospondyls, the largest group of Palaeozoic and Mesozoic amphibians, primitively possess rhachitomous vertebrae with multipartite centra (consisting of one horse-shoe-shaped inter- and paired pleurocentra). In a group of temnospondyls, the stereospondyls, the intercentra became pronounced and disc-like, whereas the pleurocentra were reduced. We report the presence of congenital vertebral malformations (hemi, wedge and block vertebrae) in Permian and Triassic temnospondyls, showing that defects of formation and segmentation in the tetrapod vertebral column represent a fundamental failure of somitogenesis that can be followed throughout tetrapod evolution. This is irrespective of the type of affected vertebra, that is, rhachitomous or stereospondylous, and all components of the vertebra can be involved (intercentrum, pleurocentrum and neural arch), either together or independently on their own. This is the oldest known occurrence of wedge vertebra and congenital block vertebra described in fossil tetrapods. The frequency of vertebral congenital malformations in amphibians appears unchanged from the Holocene.

Ichthyosaurian palaeopathology: evidence of injury and disease in fossil ‘fish lizards’

“…Congenital malformations (e.g. defects in somitogenesis: Witzmann et al (2014)) and dental pathologies (e.g. Kear, 2001) have also been recorded but are not the focus of our review.…”

Pathology in a Permian parareptile: congenital malformation of sacral vertebrae

“…Defects of formation – the absence of a structural element – are more common in the fossil record. Any region of the sclerotomal precursors of vertebral elements can be affected, and can occur unilaterally or bilaterally, such as in hemi or wedge vertebra (Kaplan et al ., ; Witzmann et al ., , ). As we reported above, the presence of well‐formed neural arch structures of S2‐4 indicate the dorsal and central sclerotome formed and regionalized properly.…”

“…Pareiasaurs were one of a few early amniote lineages to evolve large body size in the Paleozoic. As sacral vertebrae are the main bony contact between the appendicular and axial skeleton, fusion in this region has been considered adaptive for mechanical strength to support increased mass, or for terrestrial locomotion (LeBlanc & Reisz, ; Witzmann et al ., ). Evidence of experimentation with non‐sprawling locomotory adaptations has previously been documented in pareiasaurs (Turner et al ., ).…”

“…In contrast, congenital malformations of fossil vertebrae are less common, but have the potential to be informative for understanding the evolution of vertebral development (Tanke & Rothschild, ; Zhang, ; Müller et al ., ; Pierce et al ., ; Fleming et al ., ). While the earliest occurrences of vertebral pathology have been documented in early Permian fossil amphibians (Witzmann, ; Witzmann et al ., ), corresponding pathologies have not been previously reported in a basal amniote or parareptile.…”

Mini‐series: palaeopathology – a fresh look at ancient diseases in the fossil record

“…Turner & Sidor (2017) explore congenital fusion of three vertebrae (block vertebra) in the sacral region of a pareisaur, a Late Permian basal amniote, by failure of segmentation early in embryogenesis. Congenital vertebral malformations are very rare in the fossil record and have mostly been described in basal tetrapods (Witzmann et al, 2014). One implication of this might be that the mechanism of vertebral segmentation was different and more variable in basal tetrapods than in extant tetrapods.…”