Acresuchus

''Acresuchus'' ("Acre crocodile") is an extinct monospecific genus of medium-sized caiman from the Late Miocene of western Brazil and Venezuela. The genus contains a single species, ''Acresuchus pachytemporalis''. ''Acresuchus'' is a close relative of the giant caiman ''Purussaurus.''

Description

The hypodigm of ''Acresuchus'' consists of the holotype, UFAC-2507, a nearly complete skull, and several referred specimens, most found in Niteroi, located in the Solimões Formation in Acre. ''Acresuchus'' takes its name from the state it was found in (Acre), while the species name ''pachytemporalis'' refers to the extreme thickening of the squamosal bones in the holotype. Another specimen, MCNUSB-PB-02FU-RS43, a skull fragment, was found in the middle member of the Urumaco Formation in Venezuela.

According to the describing authors, ''Acresuchus'' is a "caimanine with a medium-sized body, teeth with smooth (non-serrated) carinae, orbits large in comparison with other caimanines, having roughly straight margins that are larger than the infratemporal fenestrae, circular external naris, posterior margin of the skull table transversely straight to slightly concave, posterolateral margin of squamosal upturned throughout the entire dorsal lateral margin with a dorsoventral expansion in the posterior portion of the eminence," with the last trait being a unique trait within the subfamily Caimaninae.

The holotype of ''Acresuchus'' has an estimated skull length of 51.5 cm (20.3 in), smaller than all three species of ''Purussaurus'', which suggests that ''Acresuchus'' may have represented a transitional form between the traditional caiman body plan and that of ''Purussaurus''. ''Acresuchus'' lacked an enlarged external nostril, which in ''Purussaurus'' is hypothesized to reduce stress when biting, thus creating a more powerful bite. However, ''Acresuchus'' has small depressions in the posterior portions of the maxillae, similar to the deep excavations found in ''Purussaurus''. These depressions may increase with body size in the ''Acresuchus''-''Purussaurus'' clade.

''Acresuchus'' shares traits with ''Purussaurus'' that may have been adaptations to large body size, such as large supratemporal fenestrae. These provided quicker capture of prey and better thermoregulation.

A 2022 study estimated the body size of ''Acresuchus,'' ''Purussaurus'', and another extinct caimanine, ''Mourasuchus'' using skull width and the dorsocranial length of the skull, determining that dorsocranial length is affected by changes in the skull throughout growth. Therefore, the authors used their estimates derived from skull width, estimating the length and mass of ''Acresuchus'' as and , respectively, using ordinary least squares regression, while the estimates made utilizing a phylogenetic least squares regression recovered estimates of and . The authors of the study acknowledged the lower results of the phylogenetic approach, stating that it may be due to a weaker correlation between skull width and body size compared to OLS.

Classification

The describing authors of ''Acresuchus'' entered the data gleaned from its specimens into a phylogenetic analysis to test its affinities. The analysis found it to be a member of Caimaninae closely related to ''Purussaurus'', which was confirmed by subsequent studies. The result of the initial analysis is shown in the cladogram below.

Paleoecology

''Acresuchus'' shares many features with the living black caiman, including size, which suggests it would have a similar diet. If this assumption is correct, juvenile ''Acresuchus'' would have preyed on small invertebrates, with adults feeding on fish and small to medium-sized mammals. The teeth at the back of the skull in ''Acresuchus'' are blunt, indicating that it may have also eaten hard-shelled animals.

References

{{Taxonbar, from=Q96336249

Jacarea
Prehistoric pseudosuchian genera
Miocene crocodylomorphs
Miocene reptiles of South America
Monotypic prehistoric reptile genera