Anarosaurus: a lightly built Triassic pachypleurosaur

Anarosaurus was a small pachypleurosaur from Middle Triassic deposits of Germany and the Netherlands. Two species are recognised: the poorly known A. pumilio and the better sampled A. heterodontus from Winterswijk. Its long trunk and limbs remained more distinct than the rigid flippers of later marine reptiles. Bone microstructure also sets it apart: Anarosaurus retained a relatively open marrow cavity instead of the strong skeletal bone mass increase common in several aquatic relatives. Those details make the genus a useful comparison in the marine reptile catalogue.

Quick facts

Two names with unequal fossil records

Wilhelm Dames introduced Anarosaurus pumilio in 1890 from a small, incomplete articulated skeleton with a skull. The specimen was once held in Göttingen but was lost or destroyed during the Second World War. Descriptions and drawings preserve the basis of the name, yet researchers cannot now re-examine the bones or apply modern imaging to them. This makes the type species unusually difficult to compare directly with newly collected material.

A. heterodontus was named from a right dentary collected near Freiburg. Later finds from the Lower Muschelkalk at Winterswijk in the Netherlands supplied skulls, partial skeletons and separate bones. These fossils made it possible to study the skull and postcranial skeleton in much greater detail. They also show why a single isolated jaw does not capture the full variation present along the tooth row.

Both species are assigned to Pachypleurosauria, a group of small-bodied sauropterygians common in Triassic seas. Their exact relationships within that radiation have been revised as additional skull and limb characters were compared. Taxonomic placement should therefore be separated from the more secure observation that the Winterswijk fossils belong to a small aquatic reptile with a long trunk and four limbs.

Teeth and feeding limits

The teeth of A. heterodontus differ in shape and size at different positions in the jaws. That heterodonty is a direct anatomical feature, not proof of a specialised diet by itself. Narrow pointed crowns are compatible with taking small animal prey, but no stomach contents identify a particular fish or invertebrate as routine food.

The skull is relatively small and elongated. As in other pachypleurosaurs, the jaw joint and tooth arrangement constrain how the animal could close its mouth, but the precise force and feeding motions cannot be read from one isolated tooth. A fossil assemblage containing fish and invertebrates describes the wider habitat; it does not establish which items an individual Anarosaurus ate.

A coastal animal with an unusual skeleton

Winterswijk fossils come from shallow-water deposits of the Germanic Basin. Sediment, associated marine fossils and the local stratigraphy support a nearshore setting with changing water conditions. Bone beds and disarticulated remains can result from transport, weathering or disturbance, so they are not a direct record of a single living community in one moment.

The limbs retained separate digits and were not transformed into the continuous, streamlined paddles seen in later plesiosaurs. The body plan supports swimming, but it also lacked some of the extreme specialisations of fully pelagic reptiles. Limited movement onto exposed ground is anatomically more plausible than it would be for a later plesiosaur; no trackway or other direct evidence shows Anarosaurus walking ashore.

What growth marks can and cannot tell us

Thin sections of long bones preserve vascular canals and successive growth layers. In the sampled A. heterodontus, tissue organisation indicates relatively rapid deposition compared with several Alpine pachypleurosaurs. Growth models combine those marks with measurements and assumptions about how the skeleton changed during life. They estimate individual trajectories; they do not provide a universal age or exact adult length for every member of the genus.

Microanatomy also records a large medullary cavity and comparatively low bone compactness. Several close aquatic relatives show more extensive infilling of the marrow region, which increased skeletal mass and may have helped control buoyancy. Anarosaurus lacked that degree of osteosclerosis in the bones studied. This supports a less specialised aquatic condition, not a terrestrial lifestyle: the geological setting and the rest of the skeleton still point to a coastal swimmer.

Its record is consequently strongest where fossils preserve actual bones, teeth and histological structure. Exact colour, daily routine, reproductive behaviour and the amount of time spent on shore remain unknown. The distinction between direct anatomy and ecological inference is essential when reconstructing this small Triassic reptile.

Compare this animal with related fossils in the marine reptile catalogue.