Alligatorellus

Quick facts

Alligatorellus was a tiny neosuchian from the islands of Late Jurassic western Europe. Nearly complete articulated skeletons from France and Germany preserve a triangular head, large eyes, slender limbs, long tail and two rows of bony plates along the back.

Despite its name, it was not a miniature species of living alligator. It belonged to Atoposauridae, an early crocodyliform branch. Two species are recognised: French A. beaumonti and German A. bavaricus.

The French slabs

Paul Gervais described two small skeletons from the lithographic limestones of Cerin in 1871. The name-bearing specimen MNHN 15639 preserves an almost complete skull and spine, most limbs, ribs and a continuous dorsal shield on slab and counterslab.

A second skeleton, MNHN 15638, is also nearly complete but compressed in another orientation and missing parts of the tail and left forelimb. Their articulation lets researchers compare limb proportions, vertebral sequence and armour position within single individuals.

A separate Bavarian species

Peter Wellnhofer initially treated Bavarian fossils as a geographic subspecies. Reassessment in 2014 elevated A. bavaricus to species rank. Its holotype from the early Tithonian Solnhofen limestones is an articulated skeleton.

Closed skull and vertebral sutures show that this individual was mature. Proportionally larger orbits, folds behind the nostrils, small slit-like openings in the nasals, skull ridges and limb ratios distinguish it from the French form. These differences cannot all be dismissed as the effects of youth.

A partial skeleton from Kelheim has been called Alligatorellus sp. Its preservation and differences do not justify adding another named species.

Head and teeth

Seen from above, the skull formed a short sharp triangle with large orbits. Shallow pits covered the roof, while the snout was smoother. The frontal bone remained broad between the eyes and the rear skull margin enclosed the occipital region from above.

Teeth were small, pointed and comparatively uniform. Longitudinal grooves occurred on the crowns, but cutting serrations and an enlarged fifth maxillary tooth were absent. This contrasts with the more varied dentition of Theriosuchus, once grouped in the same family.

Such teeth could catch insects, aquatic invertebrates or very small vertebrates. No reliable stomach contents choose among them. Tooth shape supports small prey, not a single exact diet.

Body proportions and armour

The tail exceeded half the presacral body length. Slender limbs suggest a mobile animal capable of supporting itself on land. Species differed in relative lengths of the upper and lower limb bones, which assists identification.

Paired rows of sculptured osteoderms extended from the neck along the back and tail. Each plate carried pits and a long ridge. Ridge position differs between the species, and the French form retained extra small tail plates. Two rows of oval plates also protected the underside of the tail.

The armour could provide protection and stiffen the trunk, but its exact mechanical contribution has not been measured. It should not be reconstructed as the massive shield of a large modern crocodilian.

Islands, lagoons and burial

Cerin and Solnhofen lay among islands and shallow lagoons. Fine carbonate mud, weak bottom-water disturbance and rapid burial preserved articulated bodies. A skeleton in lagoon limestone does not prove permanent life in salt water; a terrestrial carcass could be washed from shore.

Slender legs fit movement on land, while the long tail could assist swimming. No trackway is tied directly to a skeleton, so the proportion of time spent in water is unknown.

Species, growth and relationships

Alligatorellus, Alligatorium and Atoposaurus were once proposed as growth stages of one animal. Skull, limb and osteoderm comparisons do not form the expected developmental sequence. Morphometric work also separates them.

A 2016 revision restricted Atoposauridae to these western European genera and placed them among basal neosuchians. Other analyses recover broader arrangements, making family limits less stable than the diagnostic anatomy of Alligatorellus itself.

The familiar 42–55 centimetre range describes known skeletons, not a measured biological maximum. Flattening, curved spines and missing tail tips prevent false precision.

What an illustration can show

A small complete silhouette, triangular head, large eyes, slender legs and paired armour rows are directly constrained. Skin colour, soft-tissue pattern, exact posture and chosen prey are reconstructed.

The Jurassic Period guide explains the island setting, while the catalogue contrasts this tiny neosuchian with large river predators.

How the skeletons are compared

Articulated fossils allow measurements to be kept within one individual. Researchers can compare the humerus with the forearm, the femur with the lower leg, or the tail with the presacral spine without combining animals of unknown size. This is especially useful for a half-metre crocodyliform whose isolated bones could easily be mistaken for juveniles of a larger form.

Maturity is assessed from several observations rather than body length alone. Closed sutures in the skull and vertebral column of the Bavarian holotype show that it had reached a mature state despite retaining a small body and large-looking eyes. Its differences from the French species therefore cannot all be explained as juvenile proportions.

Preservation and anatomical distortion

The fine limestone slabs preserve completeness but flatten three-dimensional structures. A skull pressed from above can appear wider, and a spine curved after death changes a direct nose-to-tail measurement. Some tail tips and limb ends are incomplete. Published size ranges consequently describe the preserved individuals and reasonable corrections, not an exact upper limit for every adult.

Slab and counterslab also distribute bones between opposing surfaces. Details visible on one half may be moulds on the other, so descriptions integrate both parts. The continuity of the armour rows is much firmer evidence than the exact relief of every individual plate.

Evidence, inference and reconstruction