The Feeding Habits of Mesosauridae

Abstract

Mesosauridae comprises the oldest known aquatic amniotes which lived in Gondwana during the Early Permian. Previous work in the Uruguayan mesosaur-bearing Mangrullo Formation suggested that mesosaurids lived in an inland water body, inferred as moderately hypersaline, with exceptional preservational conditions that justified describing these strata as a Fossil-Lagerstätte. Exquisitely preserved articulated mesosaur skeletons, including gastric content and associated coprolites, from the Brazilian Iratí Formation in the State of Goiás (central-western Brazil) indicate excellent conditions of preservation, extending the Konservat-Lagerstätte designation to both units in the Paraná Basin. The near-absence of more resistant fossil remains, like actinopterygian and temnospondyl bones, demonstrates the faunistic poverty of the mesosaur-bearing "salty sea." Our studies of the alimentary habits of mesosaurids through the use of stereoscopic microscopy, light and electronic microscopy, and X-ray diffractometry suggest that the diet of mesosaurids was predominantly composed of pygocephalomorph crustaceans (possibly not exceeding 20 mm in length). However, the presence of bones and bone fragments of small mesosaurs in the gastric content, cololites, coprolites, and possible regurgitalites may also indicate cannibalistic and/or scavenging habits. Cannibalism is relatively common among vertebrates, particularly during conditions of environmental stress, like food shortage. Likewise, the apparent abundance of pygocephalomorph crustacean fossils in the Iratí and Mangrullo Formations, outside and within the studied gastric, cololite, and coprolite contents, might have to do with environmental stress possibly caused by volcanic activity, in particular ash spread into the basin during the Early Permian. In this context, casual necrophagy on the dead bodies of small mesosaurs and large pygocephalomorphs might have been an alternative alimentary behavior adopted for survival in mesosaurs.

Was Mesosaurus a Fully Aquatic Reptile?

“…However, some of these features (though not pachyosteosclerosis) are also present in amphibious taxa. The presence of coprolites in close proximity to mesosaurid skeletons such as those we have recently described (Silva et al, 2017), which mostly appear not to have been expelled post mortem (see Hone et al, 2015), is rather uncommon because normally, it is difficult to identify the coprolite producer. Possible explanations for the numerous excrements associated to mesosaurid body remains include: (a), mesosaurids spent much time in confined areas in a transitional coastal environment and (b), they lived in dense populations (Dentzien-Dias et al, 2012).…”

“…Mesosaurids have been considered mainly aquatic, but derived from a terrestrial form that had subsequently become aquatic (Laurin and de Buffrénil, 2016) or to have derived directly from semi-aquatic ancestors (Romer, 1957). Mesosaurids fed on aquatic prey items and defecated in a transitional environment (possibly a lagoon or a hypersaline sea) in shallow water (Piñeiro et al, 2012a,c;Silva et al, 2017). Nares have migrated farther away from the tip of the rostrum and are located dorsally, eyes are large and the snout is long.…”

Conflicting evidence for the use of caudal autotomy in mesosaurs

“…Stratagraphically, the Irati Formation is divided into two distinct members, the lower Taquaral Member and the higher Assistência Member 25,26 Though relatively large chondrichthyans are present they are not found in the same strata as mesosaurs 27 , with mesosaurs being found in the upper Assistência Member 25 . The only taxa known from the Assistência Member are mesosaurs, paleonisciform fish, pygocephalomorph crustaceans, ostracods, brachiopods, foraminiferans, and sponges 24,25,27,28 . The potential lack of predation pressure in this aquatic environment suggests that mesosaurs may not have autotomized their tails, as this behaviour is primarily used in other taxa to escape predators 10,11,20,29 .…”

“…If this was the case, the ability to autotomize the tail would have been beneficial for younger mesosaurs, increasing their chances to avoid predation by older individuals. However, this hypothesis of juvenile cannibalism lacks convincing evidence 28 .…”

Comparative digestive physiology of archosaurs with notes on bird origins

“…Notwithstanding, it cannot be ruled out that theropods occasionally regurgitated large bones in form of a mostly loose material. Such behavior is encountered in some tetrapods, including basal amniotes (Silva et al 2017). Some clues about the physiology of early archosaurs are provided by the comparative physiology of birds and crocodiles (Table 1).…”