Chirotherium sp.

Chirotherium is actually known only as an ichnogenus, as we only have evidence from its footprints. All we have gauged about its morphology and ecology comes from its footprint morphology and other features. These look, by coincidence, remarkably like the hands of apes and bears, with the outermost toe having evolved to extend out to the side like a thumb, although probably only functioning to provide a firmer grip in mud, hence the name 'Cheirotherium' - 'Hand-beast'. In many cases, however, only the three largest foreward-facing toes are seen. We gauge that these land-based crocodile-like reptiles were probaby from the Pseudosuchia clade, likely the Prestosuchidae or Rauisuchidae groupings.

Chirotherium belongs to the group Archosauria, which includes a diverse array of reptiles such as crocodiles, birds, and their extinct relatives, including the dinosaurs. Within Archosauria, Chirotherium is thought to be related to early archosauriforms, a group of archosaurian reptiles that predates the split between the lineage leading to modern crocodiles and the one leading to dinosaurs and birds. These early archosaurs were often crocodile-like, though many evolved to be more terrestrial, with longer legs and a more upright stance.

Chirotherium would have occupied a terrestrial habitat with a diet and ecological role that reflected its size and locomotive capabilities. It was a relatively large predator or scavenger, potentially competing with other large reptiles of its time.

The evolutionary history of archosaurs is marked by significant diversification during the Triassic period. Early archosaurs like Chirotherium were precursors to the more specialised forms that would later evolve into the diverse groups seen in the Jurassic and Cretaceous periods, including the dinosaurs and modern crocodilians. The split between the archosaurian lineages leading to dinosaurs and crocodiles marks a key evolutionary divergence, with Chirotherium representing an early branch of this complex evolutionary tree.

Reptiles, as traditionally defined, are a group of tetrapods that typically have ectothermic (cold-blooded) metabolisms and develop through amniotic eggs. However, modern taxonomy, based on genetic and paleontological evidence, considers reptiles a paraphyletic group. This is because birds (class Aves), which evolved from dinosaurs, are more closely related to crocodilians than to other reptiles. Therefore, birds are often included within the reptilian group in many cladistic systems, redefining Reptilia as a monophyletic clade that includes both birds and reptiles. The exact definition of this clade varies among scientists, with some preferring the term Sauropsida to encompass all amniotes more closely related to reptiles than to mammals.

The earliest proto-reptiles appeared in the Carboniferous period, evolving from reptiliomorph tetrapods that were increasingly adapted to life on land. In addition to modern reptiles, there were many now-extinct groups, some of which disappeared during mass extinction events, such as the Cretaceous–Paleogene extinction. This event wiped out pterosaurs, plesiosaurs, and all non-avian dinosaurs, along with many species of crocodilians and squamates like mosasaurs. Reptiles are tetrapod vertebrates, meaning they either have four limbs or descended from ancestors that did. Unlike amphibians, reptiles do not undergo an aquatic larval stage. Most are oviparous (egg-laying), but some squamates are viviparous, with embryos developing inside the mother, nourished by a placenta rather than enclosed in eggshells.

As amniotes, reptile eggs are protected by membranes, enabling reproduction on dry land. Some viviparous reptiles have placentas analogous to those of mammals and provide initial care to their young. The earliest amniotes, including stem-reptiles, were relatively small and inconspicuous compared to larger tetrapods like Cochleosaurus. However, during the Carboniferous Rainforest Collapse, primitive tetrapods were devastated, while stem-reptiles thrived in the drier conditions. Unlike amphibians, which required water for reproduction, early reptiles could lay their shelled eggs on land, giving them an advantage in the new environment. As a result, amniotes rapidly diversified, adopting new feeding strategies, including herbivory and carnivory, and outcompeting primitive tetrapods.

The dominance of reptiles in terrestrial ecosystems set the stage for the Mesozoic era, also known as the 'Age of Reptiles.' A 2021 study of reptile diversity during the Carboniferous and Permian periods revealed a higher level of diversity than previously thought, comparable to or even exceeding that of synapsids. This period has been proposed as the 'First Age of Reptiles.'; The Permian–Triassic extinction event, the most significant mass extinction in Earth's history, caused a prolonged die-off, eliminating most of the earlier parareptile and synapsid megafauna. True reptiles, particularly archosauromorphs, survived and thrived, characterized by elongated hind legs and an upright posture. These early archosaurs eventually gave rise to the dinosaurs and pterosaurs, as well as the ancestors of modern crocodiles.

During the Triassic period, archosaurs became the dominant group, evolving into the dinosaurs and smaller theropods, which later gave rise to birds. The sister group to archosaurs is Lepidosauromorpha, which includes lizards, tuataras, and their fossil relatives. Mosasaurs, one of the major groups of Mesozoic marine reptiles, belong to this clade. Other marine reptiles, such as ichthyosaurs and sauropterygians, evolved during the early Triassic, but their exact phylogenetic placement remains debated. Some researchers link them to lepidosauromorphs, while others associate them with archosauromorphs or classify them as diapsids outside these two groups.

The close of the Cretaceous period saw the extinction of many Mesozoic reptilian megafauna during the Cretaceous–Paleogene extinction event. Large marine reptiles, except for sea turtles, perished, and only semi-aquatic crocodiles and the lizard-like choristoderes, which later became extinct in the Miocene, survived. Of the dinosaurs, only small beaked birds survived this mass extinction, which marked the end of the Mesozoic and the rise of the Cenozoic era. In the aftermath, mammals and birds rapidly diversified, filling the ecological niches left by extinct reptiles. While reptile diversification slowed, they remained key components of the megafauna, especially in the form of large tortoises. Though mammals and birds came to dominate most terrestrial ecosystems, reptiles continued to play significant ecological roles.