Metoposaurus: the flat-skulled temnospondyl of the Late Triassic

Metoposaurus was a large aquatic temnospondyl of the Late Triassic. Its broad, flat skull carried rows of small marginal teeth, larger palatal tusks and canals of the lateral-line system. Several European species are recognised, but older assignments of isolated bones once made the genus seem more widespread than the evidence supports. The distinctions between directly preserved anatomy and reconstructed behaviour place it in the ancient amphibian catalogue.

Quick facts

A name whose range once grew too large

Richard Lydekker introduced the name Metoposaurus in the nineteenth century. Later authors referred scattered metoposaurid bones from several regions to the genus, and its apparent range expanded. Modern revisions are more cautious. Better-supported European species include M. diagnosticus, M. krasiejowensis and M. algarvensis, described from Portugal in 2014.

North Africa also preserves metoposaurid temnospondyls, some with a similar broad skull. Family resemblance does not make every one of them a species of Metoposaurus. The genus must be diagnosed from a combination of skull characters and the quality of the associated material. This matters when comparing bone beds: a layer containing many metoposaurids does not necessarily contain the same species as a European locality.

A flat skull and a water-motion sensor

The skull was exceptionally broad and low. Its eye sockets sat well forward, unlike the more rearward orbits of Mastodonsaurus. Small conical teeth lined the jaw margins, and larger tusks stood on the palate. Together they could grip slippery prey, but they were not a chewing apparatus.

Grooves in the skull roof mark the canals of the lateral-line system. In water, such sensors detect local movement and pressure changes. Their bony traces are direct fossil evidence. A rapid strike from a resting position is a plausible hunting model, but no fossil records a complete attack sequence. Fish are reasonable prey, while small aquatic tetrapods and invertebrates remain possible additions rather than a confirmed list.

The animal had an elongated trunk, a strong tail and relatively small limbs. It could brace against the bottom or move in shallow water, but the skeleton does not suggest efficient long-distance travel on land. Another aquatic temnospondyl, Gerrothorax, was much smaller and had a different skull and branchial apparatus. A flattened outline alone does not guarantee the same mechanics or lifestyle.

Growth written in bone

Histological sections from the shoulder, thigh and other bones preserve changes in tissue deposition. Faster-growing zones alternate with lines that mark slower phases. Young animals grew more rapidly, then growth slowed as they approached adult size. These observations help reconstruct growth, but they are not a simple birthday counter. A line is not guaranteed to represent exactly one year, because environmental seasonality and interruptions can change the record.

The internal structure of skull bones also changed as animals matured. Researchers use such changes to distinguish age-related variation from features that separate species. This is especially important in a genus whose fossils were historically gathered under broad names. The Triassic Gerrothorax provides another comparison among aquatic temnospondyls, but it belongs to a different branch and is not a growth stage or close synonym of Metoposaurus.

What bone beds can and cannot tell us

Some Late Triassic sites contain the remains of many metoposaurids. One familiar explanation is drought: animals gathered in a shrinking pool and died there. That can happen, but it is not the only process capable of making a dense bone layer. Currents may sort and transport bones, and repeated episodes can accumulate remains over a longer interval.

Position, orientation, breakage, sorting and surrounding sediment help distinguish those possibilities. A mass of bones should not automatically be described as a herd that died together in one afternoon. Taphonomy records what happened after death as well as the setting in which the animals lived. Each bone bed needs its own geological context.

Reconstructing the animal

The broad skull, tooth rows, lateral-line canals and sampled bone tissues are directly preserved. Total length of roughly two to three metres depends on the associated skeleton and restored proportions. Soft skin, colour, the precise tail edge and the exact position of a hunting animal in a seasonal lake are artistic choices. The fossils show a substantial aquatic predator, but they do not reveal a single universal behaviour for every species or locality.

Within the catalogue of ancient amphibians, Metoposaurus can be compared with Mastodonsaurus without treating their similarly aquatic appearance as proof that they were close relatives.