MP9: Chapter 9: Evolution of the Semi-aquatic Lifestyle in Archosaurs – Evidence from the Tetrapod Footprint Record

From Ichnology: Papers From Ichnia III
by H. Klein and S.G. Lucas.

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This chapter is from GAC's Miscellaneous Publication 9: "Ichnology: Papers from Ichnia III" editted by Dr. Duncan McIlroy. To purchase the entire book on disc, please see the Miscellaneous Publications section of our bookstore at http://www.gac.ca/publications/view_pub.php?id=262.

Abstract:
The semi-aquatic lifestyle in archosaurs is documented by skeletal remains of specialized ?proterosuchids, phytosaurians, poposauroids, crocodylomorphs and avemetatarsalians from the Late Permian–Early Triassic to the present. Correspondingly, tetrapod footprints of the ichnogenera Apatopus (phytosaurs), Batrachopus and Crocodylopodus (Crocodylomorpha), as well as those of modern crocodylians, preserve adaptive features that provide evidence of a semi-aquatic lifestyle and preceding terrestrial habitus. Ichnofossils and body fossils have similar stratigraphic ranges: Apatopus/semi-aquatic phytosaurs (Late Triassic); Batrachopus/terrestrial crocodylomorphs (Late Triassic–Early Jurassic); footprints/skeletons of semi-aquatic crocodylomorphs (Jurassic–Recent). The earliest phase of phytosaurian evolution, including a presumed terrestrial stage in the Early–Middle Triassic, is unknown. Morphologically and temporally, the chirotheriid ichnogenus Synaptichnium from the Early–Middle Triassic, matches hypothetical footprints of terrestrial phytosaurians. Features shared by Synaptichnium and the phytosaur ichnotaxon Apatopus are: 1) elongate pes imprints, digits increasing in length from I to IV, digit IV longest, 2) extended, antero-laterally directed digit V, 3) relatively large manus, digit III longest and 4) relative position/orientation of imprints. Differences are the compact metatarsal-phalangeal axis and thick phalangeal pads in Synaptichnium due to cursorial habits, whereas Apatopus has a crocodilian appearance. Furthermore, the scarcity of Apatopus vs. abundance of swim traces is considered to reflect an aquatic environment and poor preservation potential. The footprint record thus provides important evidence of the evolution of terrestrial to semi-aquatic lifestyles in more than one group of Mesozoic archosauromorphs.

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