Herbst and Hutchinson 2018
summed up current knowledge on the fully aquatic basal tetrapod with vestigial limbs, Crassigyrinus (Fig. 1): “The Carboniferous tetrapod Crassigyrinus scoticus is an enigmatic animal in terms of its morphology and its phylogenetic position. Crassigyrinus had extremely reduced forelimbs, and was aquatic, perhaps secondarily. Recent phylogenetic analyses tentatively place Crassigyrinus close to the whatcheeriids. Many Carboniferous tetrapods exhibit several characteristics associated with terrestrial locomotion, and much research has focused on how this novel locomotor mode evolved.”
“We used computed tomographic scanning to search for more data about the skeletal morphology of Crassigyrinus and discovered several elements previously hidden by the matrix. These new discoveries give us a better understanding of the anatomy of this aberrant animal and the morphological variation present in early tetrapods.”
Not an enigma in the LRT
In the large reptile tree (LRT, 1428 taxa) Crassigyrinus (Fig. 2) nests with Ventastega, and slightly more distantly with Whatcheeria and Pederpes. Herbst and Hutchinson do not mention Ventastega. Reversals in the pectoral girdle are at play here. This is yet another temnostylian taxon returning to a more aquatic lifestyle, adopting a more tadpole-like morphology into adulthood.
Crassigyrinus scoticus (Watson 1926, Clack 1998; 2m in length; Early Carboniferous, Viséan, 340 mya) has been described as taxonomically enigmatic (see below). Wikipedia has more on its history of discovery and taxonomy.
This aquatic tetrapod had tiny limbs and likely a long deep tail. The palate has been described as ‘very fish-like’. The vertebrae were not well ossified with no sign of posterior facets to unite them. The postfrontals contacted each other medially, separating the frontals from the parietals. The skull was relatively tall on this active predator with large teeth. The basioccipital is not developed into a formed occipital condyle, but then the neck is so short that the pectoral girdle starts beneath the lateral skull bones.
Ahlberg PE and Milner AR 1994. The origin and early diversification of tetrapods. Nature 368, 507-514.
Clack JA 1998. The Scottish Carboniferous tetrapod Crassigyrinus scoticus (Lydekker) – cranial anatomy and relationships. Transactions of the Royal Society of Edinburgh: Earth Sciences 88, 127-142.
Clack JA 2002. Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Herbst EC and Hutchinson JR 2018. New insights into the morphology of the Carboniferous tetrapod Crassigyrinus scoticus from computed tomography. Earth and Environmentsal Science Transactions of the Royal Society of Edinburgh 1–19.
Panchen AL 1985. On the amphibian Crassigyrinus scoticus Watson from the Carboniferous of Scotland. Philosophical Transactions of the Royal Society of London B 309: 505-568.
Panchen AL 1990. The pelvic girdle and hind limb of Crassigyrinus scoticus (Lydekker) from the Scottish Carboniferous and the origin of the tetrapod pelvic skeleton. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 81(1):
Panchen AL 1991. The early tetrapods: classification and the shapes of cladograms in: Origins of the Higher Groups of Tetrapods: Controversy and Consensus. Eds. Schultze HP and Trueb L. Comstock Publishing Associates, Cornell University Press, Ithaca and London.
Watson DMS 1926. Croonian Lecture – The evolution and origin of the Amphibia. Philosophical Transactions of the Royal Society B 214:189–257.