Acresuchus

Quick facts

Acresuchus pachytemporalis was a large caiman from the Late Miocene wetlands of western Amazonia. It was not a miniature copy of Purussaurus: the skull combined its own temporal bosses, moderate snout and varied tooth row.

The holotype UFAC-2507 from the Solimões Formation includes a nearly complete skull with both halves of the lower jaw. At 51.5 centimetres long, it provides a sound anatomical basis even though the front was broken after burial.

Raised bones behind the eyes

Additional skull roofs show the same unusual feature. The rear outer margins of the squamosals turn upwards and thicken into paired bosses. These are bony elevations, not preserved keratin horns.

A display function has been suggested by comparison with living crocodilians that expose raised skull contours during signalling. Fossils do not record sex, colour or behaviour, so the proposal remains comparative.

Not another twelve-metre giant

The skull is substantially smaller than the 80–140 centimetre crania assigned to species of Purussaurus. A complete skeleton is unknown, and the original description did not produce one exact body length. Calling Acresuchus a twelve-metre animal would therefore transfer a relative's dimensions without evidence.

Large orbits, a moderately broad snout and teeth with smooth cutting edges fit a generalist predator. Some rear teeth were rounder and could crush resistant objects. Turtles, molluscs or crustaceans are possibilities, not a preserved menu.

Relationship with Purussaurus

The original phylogenetic analysis placed Acresuchus as the sister taxon of Purussaurus. Their shared large supratemporal openings concern jaw musculature, but the nostril had not reached the giant proportions of the later genus. This mosaic illuminates a branch in which extreme size evolved without proving a direct ancestor-descendant chain.

A diverse western Amazonian system

The Solimões and Urumaco formations record rivers, lakes, swamps and floodplains. Acresuchus shared the broader system with several caimans adapted to different feeding roles. Resource partitioning is plausible, but direct interactions are not preserved.

The Neogene Period guide explains the changing drainage systems before the modern Amazon took shape. The catalogue compares this crown caiman with much older non-crocodylian branches.

What the skull can and cannot measure

A well-preserved cranium records the shape of the snout, orbit and jaw-muscle chamber more reliably than it records total body length. Crocodilian head-to-body proportions vary with lineage and growth, so extending one modern ratio to Acresuchus would create a precise-looking but model-dependent answer.

The several skull roofs are valuable because the paired bosses recur in more than one specimen. Repetition makes accidental damage less likely, but it still does not reveal whether the feature differed between sexes or changed during maturity. Any courtship display shown in art is therefore an explicit behavioural hypothesis.

Evidence, inference and reconstruction