July 3, 2013 | 43
A small group of long-snouted swimming reptiles from the Permian of Brazil, Uruguay, South Africa and Namibia – the mesosaurs – represent the oldest amniote group known to have taken to life in the marine realm. Do not confuse them with mosasaurs, a group of large to gigantic swimming lizards from the Cretaceous, usually considered close relatives of monitors and gila monsters.
Mesosaurs were small, with total lengths of 30-100 cm. The presence of these small swimming reptiles on both sides of the Atlantic was famously used to help support the existence of continental drift, since their distribution seemed to show that Africa and South America had once been in contact (Du Toit 1927).
A long, deep tail, elongate feet (evidence of webbing is preserved in some specimens (Rossmann & Maisch 1999)), thickened, usually dense-boned ribs, numerous fine, needle-like teeth set within elongate jaws and retracted nares indicate that mesosaurs were aquatic predators of small, krill-like crustaceans or similar prey. They probably used the long, flexible neck to sweep their jaws from side to side (Modesto 2006).
It has been implicated on a few occasions that mesosaurs may have been suspension-feeders but in fact the tooth spacing seen in these reptiles does not support this interpretation. On this subject, Collin & Janis (1997) wrote “[interpretation of mesosaurs as suspension-feeders] depends on an erroneous interpretation of the mandibular teeth as small marginal upper teeth, and a more detailed examination of mesosaur functional morphology suggests that they probably captured individual prey selectively rather than processing large volumes of water nonselectively as to true suspension feeders (S. Modesto, personal communication)” (p. 453).
Signore et al. (2002) reported new details on the skull morphology and possible lifestyle of the mesosaur Stereosternum. They noted that snout shape in this taxon recalled that of the extant crocodylians Gavialis and Tomistoma and they therefore proposed that, like these crocodylians, Stereosternum was piscivorous. They also noted that a juvenile Stereosternum had a shorter snout and limbs less suited for aquatic propulsion than adults and therefore suggested that juveniles may have been less aquatic than adults.
Within recent decades, just three mesosaur taxa have conventionally been recognised: Mesosaurus tenuidens Gervais, 1865, Stereosternum tumidum Cope, 1885 and Brazilosaurus sanpauloensis Shikama & Ozaki, 1966. However, several others have been named over the years. M. pleurogaster Seeley, 1892 has generally been regarded as a synonym of M. tenuidens but still requires evaluation (Modesto 1996). Noteosaurus africanus Broom, 1913 – named for an articulated hindlimb, pelvis and partial tail from the Dwyka Formation of South Africa – was argued by Modesto (1996) to be indistinguishable from either Mesosaurus or Stereosternum and hence a nomen dubium while Ditrochosaurus capensis Gürich, 1889 from Namibia – at times referred to as Mesosaurus capensis – was regarded by Rossmann & Maisch (1999) as another junior synonym of M. tenuidens. It seems that Stereosternum and Mesosaurus are sister-taxa.
Mesosaurs have usually been regarded as anapsid – that is, as lacking bony openings in the postorbital or temporal region of the skull. However, a lower temporal fenestra is present in Mesosaurus at least where it’s bordered by the jugal, quadratojugal and squamosal bones (Piñeiro et al. 2012a). Since this is similar to the condition present in synapsids it might show that lower temporal fenestration is primitive for Amniota (the more popular view is that the anapsid condition is primitive for Amniota). It’s also possible, however, that fenestration appeared and disappeared several times independently within this section of the cladogram. Brazilosaurus appears to lack temporal fenestration (Rossmann 2002).
Mesosaurs have usually been imagined as animals of coastal habitats but sedimentological evidence from Uruguay shows that at least some populations of Mesosaurus inhabited hypersaline, lagoon-like habitats where the only apparent prey items were pygocephalomorph crustaceans (Piñeiro et al. 2012b). A need to discharge excess salt may explain the presence of the so-called narial obturator foramen, a unique opening located just posterior to the naris. An oval region on the palate, adjacent to the choana, has also been identified as a possible salt gland. Small, spike-like teeth are present across part of the palate. Possible direct evidence for diet comes from a Brazilosaurus specimen that has two large, ovoid structures preserved within the stomach region (Rossmann 2002), though exactly what they are is unknown. Brazilosaurus has far shorter, stouter teeth than Mesosaurus and was evidently doing something very different.
The phylogenetic position of mesosaurs has proved difficult to resolve: recent proposals place them within Reptilia and near the origin of Parareptilia (and, variously, as the sister-group to remaining parareptiles, or the sister-group to the whole of Parareptilia). There’s no good evidence linking them with the superficially similar ichthyosaurs, hupehsuchians or thalattosaurs: members of all of those groups possess skull bone characters nesting them deep within Diapsida, and mesosaurs emphatically lack such characters.
We have very little direct data on mesosaur palaeobiology, though one particularly exciting discovery was made very recently and got the international news coverage that it deserved. I’m referring to the discovery of mesosaur babies. One of these (discovered in the Brazilian Iratí Formation) was found inside the body of an adult and hence should be referred to as an embryo; many others (26 in total, all from the Mangrullo Formation of Uruguay) were found outside the bodies of adults, but often preserved in close association with them (Piñeiro et al. 2012c). These might be embryos that became aborted or otherwise disassociated with their mother’s bodies post-mortem, or they might be new babies that died in close association with their parents. There’s no evidence for cannibalism on the juveniles, and a statistical test showed that the association between the babies and adults is not random (Piñeiro et al. 2012c).
These finds show that mesosaurs retained a low number of babies (one or two) in the body for an extended period of time; however, it’s not possible to say from these finds whether live babies or shelled eggs were produced (Piñeiro et al. 2012c). If eggs were laid, we don’t know whether they were laid on land, or in shallow water. The latter possibility exists because there are extant pleurodire turtles that do it. But if the juveniles found alongside the adults in the Mangrullo Formation represent free-swimming juveniles rather than embryos, do we have evidence for some sort of parental care? Such a possibility is certainly consistent with the low number of offspring seemingly produced by these animals; at the moment, however, saying anything further would be speculation.
And… with mesosaurs out of the way, all I have to do now is get through all the other fossil marine reptile groups. Don’t hold your breath.
For other Tet Zoo articles relevant to some of the subjects covered here, see…
Refs – -
Collin, R. & Janis, C. M. 1997. Morphological constraints on tetrapod feeding mechanisms: why were there no suspension-feeding marine reptiles? In Callaway, J. & Massare, J. (eds) Ancient Marine Reptiles. Academic Press (London), pp. 451-466.
Du Toit, A. L. 1927. A geological comparison of South America with South Africa. Carnegie Institute Publications, Washington 381, 1-158.
Modesto, S. P. 1996. Noteosaurus africanus Broom is a nomen dubium. Journal of Vertebrate Paleontology 16, 172-174.
- . 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146, 345-368.
Piñeiro, G., Ramos, A., Goso, C., Scarabino, F. & Laurin, M. 2012b. Unusual environmental conditions preserve a Permian mesosaur-bearing Konservat-Lagerstätte from Uruguay. Acta Palaeontologica Polonica 57, 299-318.
- ., Ferigolo, J., Ramos, A. & Laurin, M. 2012a. Cranial morphology of the Early Permian mesosaurid Mesosaurus tenuidens and the evolution of the lower temporal fenestration reassessed. Comptes Rendus Palevol 11, 379-391.
- ., Ferigolo, J., Meneghel, M. & Laurin, M. 2012c. The oldest known amniotic embryos suggest viviparity in mesosaurs. Historical Biology 24, 620-630.
Rossmann, T. 2002. Studien an Mesosauriern (Amniota inc. sed., Mesosauridae): 3. Neue Aspekte zur Anatmie, Erhaltung und Paläoökologie aufgrund der Exemplare im Paläontologischen Institut der Universität Zurich. Neues Jahrbuch fur Geologie und Paläontologie, Abhandlungen 224, 197-221.
- . & Maisch, M. W. 1999. Das Mesosaurier-Material in der Bayerischen Staatsammlung für Paläontologie und Historische Geologie: Überischt und neue Erkenntnisse. Mitteilungen der Bayerischen Staatssammlung für Paläontologie und Historische Geologie 39, 69-83.
Signore, M., Raya, P. & Barbera, C. 2002. New specimens of Stereosternum tumidum (Mesosauria) and notes on its palaeoecology. In Norman, D. & Upchurch, P. (eds) SVPCA 50, Cambridge 2002, Abstract Volume. University of Cambridge, p. 38.