Metriorhynchus

Metriorhynchus is a Jurassic marine crocodylomorph whose meaning changed after a modern taxonomic revision. Older books placed many short- and long-snouted species in the genus, often illustrating it with M. superciliosus. The 2020 revision restricted Metriorhynchus to its type species, M. geoffroyii, and transferred M. superciliosus to the new genus Thalattosuchus. The profile belongs in the catalogue of ancient crocodylomorphs.

Marine adaptations such as flipper-like limbs, a tail fin and reduced armour are well documented across metriorhynchids. They are family-level evidence and should not be mistaken for a complete skeleton of the more narrowly defined Metriorhynchus.

Quick facts

Why the name needed a revision

In the nineteenth century, Georges Cuvier, Étienne Geoffroy Saint-Hilaire and Hermann von Meyer studied scattered skulls from Normandy and other parts of western Europe. The original type series associated with M. geoffroyii proved to combine remains from more than one metriorhynchid. When the name was introduced, a single, unequivocal holotype had not been fixed.

Modern researchers reconstructed the specimen history, selected a lectotype and redescribed the type species. Phylogenetic analyses had already indicated that the old, broad Metriorhynchus did not form one evolutionary branch. Restricting the name therefore removed a historical catch-all rather than merging nearly all marine crocodile relatives into one genus.

Old museum labels and illustrations may still use the earlier classification. The familiar species M. superciliosus is now Thalattosuchus superciliosus, so the name on an image must be checked against the species and the age of the source.

A crocodile relative adapted to the sea

Metriorhynchids lost the heavy external osteoderms carried by many other crocodylomorphs. Their forelimbs and hind limbs became flattened flippers, and the end of the tail bent down to support the lower edge of a two-lobed swimming fin. The tail supplied much of the thrust while the limbs helped with steering and stability.

This body plan differs sharply from that of a living crocodile moving between a river and its banks. It indicates a strongly marine way of life and very limited ability to support the body on land. No direct eggs or pregnant Metriorhynchus specimen is known; live birth is a strong functional hypothesis for the family, not an observed fact for this genus.

The transformation to a marine body also occurred in ichthyosaurs, but they were not crocodile relatives and followed a separate evolutionary history.

Evidence for salt-excreting glands

Marine animals must regulate the salts they take in. Skulls of metriorhynchids preserve cavities and natural casts associated with enlarged nasal glands, along with passages interpreted as routes for secretion. Comparative work across thalattosuchians supports the gradual development of a salt-excretion system.

For fossils historically called Metriorhynchus, internal depressions in front of the eye have been used to infer salt glands. Better natural casts are known in other members of the family. Osmoregulation is well supported for the broader lineage, but soft gland tissue should not be drawn in detail from a single external opening.

Skull, teeth and food

Metriorhynchids had elongated jaws and conical teeth, but their feeding styles were not all identical. Some had relatively slender snouts suited to seizing smaller prey; others evolved deep skulls and cutting teeth. The limited material of M. geoffroyii makes its precise menu harder to reconstruct than that of a nearly complete animal.

Fish and cephalopods are plausible prey for a medium-sized marine metriorhynchid. To establish a specific diet, researchers would need evidence such as stomach contents, feeding traces or wear on securely attributed teeth. The larger Dakosaurus had a more robust skull and serrated teeth, so it should not stand in as a model for every relative. Metriorhynchids shared Jurassic seas with plesiosaurs and many bony fishes, but co-occurrence alone is not evidence that one hunted another.

Size, age and geography

Older estimates of about three metres often used species that belonged to the former, much broader genus, including forms now assigned to Thalattosuchus. The fragmentary type material of M. geoffroyii does not support one precise total length. A medium-sized metriorhynchid is a reasonable broad description, but the dimensions of other genera should not be transferred to it.

The securely defined type species is associated with Late Jurassic deposits in western Europe, especially the Kimmeridgian. Wider Middle to Late Jurassic ranges in older summaries reflect the earlier genus concept. The family itself was more geographically extensive than the restricted type species.

Reading a reconstruction today

Flippers, a tail fin and an streamlined marine body are supported by comparison across metriorhynchid fossils. The head in a current reconstruction should match M. geoffroyii, not the skull of Thalattosuchus superciliosus. Colour, skin pattern and a particular hunting scene are not preserved.

The story of Metriorhynchus is therefore also a lesson in how scientific names change. An older reconstruction may reflect the evidence available at the time, but a modern label requires checking which species it depicts under the revised classification.