April 4, 2013 | 15
Long-time Tet Zoo readers will know of my various efforts to get through all the temnospondyl lineages. Alas, I just haven’t been able to finish this grand project due to my getting stuck somewhere round about dissorophoids (see below for links to previous Tet Zoo temnospondyl articles). In frustration, here’s a section from late in the series: it’s a very brief bit of text on the (often?/sometimes?) marine trematosauroids.
Trematosauroids are long-skulled (even gharial-like), Triassic temnospondyls known from India, Pakistan, Africa, Germany, Svalbard, Greenland, Nova Scotia, the USA and elsewhere. Long thought restricted to the Early Triassic, it was discovered during the 1980s that they survived into the Late Triassic: Hyperokynodon, the animal in question, is the largest trematosauroid currently known, with a skull estimated at 80 cm in length (Schoch et al. 2002). [Life reconstruction of Trematosaurus above by the excellent Dmitry Bogdanov.]
More recently, a single ilium from the Toutunhe Formation of the Junggar Basin, China, has been identified as that of a trematosauroid: this unit is Middle Jurassic in age. If the bone is correctly identified, it shows that “trematosaurs were probably not an extremely short-lived family, but were in fact one of the most remarkably long-lived families of temnospondyls” (Maisch et al. 2004, p. 582).
Early trematosauroids had a narrow, wedge-shaped and blunt-snouted skull, rather small orbits located near the edges of the skull margins, and an elongate skull table. While retaining most of these features, the particularly long-snouted lonchorhynchines were unusual in that some taxa lack the anterior palatal vacuities that are otherwise so typical of temnospondyls. The cladogram shown here (from Steyer 2002) shows the trematosauroid clade Trematosauridae consisting of a monophyletic Trematosaurinae and Lonchorhynchinae; a more recent analysis (Schoch 2006) failed to support monophyly of the former.
Aphaneramma, a trematosauroid sometimes illustrated in those books that feature prehistoric ‘amphibians’, is a member of the lonchorhynchine clade, as are Cosgriffius, Erythrobatrachus and Wantzosaurus. Juvenile and adults specimens of Wantzosaurus from Madagascar indicate that the snout grew under strong positive allometry in these animals, though even juveniles were proportionally long-snouted compared to other trematosauroids and other temnospondyls (Steyer 2002). Changes of this sort seem to have been usual for long-snouted trematosauroids.
Particularly big palatal tusks are present in some basal trematosauroids, and in the South African form Microposaurus the teardrop-shaped nostrils accommodated the big tusks of the lower jaw’s tip: a unique configuration (Damiani 2004). Lateral line canals show that trematosauroids were aquatic predators, but beyond that little is known of their biology. The long, shallow body shape and elongate snout of some lonchorhynchines indicates that these animals were laterally undulating, pelagic or semi-pelagic predators that caught small, fast-moving prey with rapid lateral swipes of their jaws.
However, Trematolestes from the Middle Triassic of Germany is relatively deep-bodied. [Adjacent photo of holotype by Ghedoghedo.] A preserved patch of skin indicates that Trematolestes had naked skin, but some other trematosauroids had osteoderms covering their neck regions at least. Trematolestes is further interesting in that the palate (including the palatal vacuities) is covered with polygonal ossicles that were themselves covered with irregularly spaced denticles. This is a feature of pre-Mesozoic temnospondyls and its presence in a trematosauroid is surprising: Schoch (2006) suggested that the ossicles “probably formed a ‘flexible palate’ that may have assisted in swallowing large prey items” (p. 40).
For previous Tet Zoo articles on temnospondyls, see…
Refs – -
Damani, R. 2004. Cranial anatomy and relationships of Microposaurus casei, a temnospondyl from the Middle Triassic of South Africa. Journal of Vertebrate Paleontology 24, 533-541.
Maisch, M. W., Matzke, A. T., & Sun, G. 2004. A relict trematosauroid (Amphibia: Temnospondyli) from the Middle Jurassic of the Junggar Basin (NW China). Naturwissenschaften 91, 589-593.
Schoch, R. R. 2006. A complete trematosaurid amphibian from the Middle Triassic of Germany. Journal of Vertebrate Paleontology 26, 29-43.
- ., Milner, A. R. & Hellrung, H. 2002. The last trematosaurid amphibian Hyperokynodon keuperinus revisited. Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 321, 1-9.
Steyer, S. J. 2002. The first articulated trematosaur ‘amphibian’ from the Lower Triassic of Madagascar: implications for the phylogeny of the group. Palaeontology 14, 771-793.