Cteniogenys and the discrete success of Choristoderes

Throughout the ages, some body plans and lifestyles seem to be so efficient that they’ve been repeatedly selected by different groups of organisms. Extant crocoyles are a good example of a successfull design as aquatic ambush predators, inhabiting today’s most tropical freshwaters (with two species also consistently adapted to life at sea). Although other extinct groups of Crocodylomorphs evolved to occupy different niches, from marine predators, terrestrial carnivores and even terrestrial herbivores, crocodyles are mainly recognized as aquatic predators lurking in the shallows. In the past, other groups of reptiles adapted to this lifestyle, such as lesser known Phytosaurs and even more obscure, Choristoderes.

Choristoderes (Order Choristodera) were semi-aquatic diapsids with an extremely successful history, with fossils present from Jurassic to Miocenic faunas (over 100 million years of existence). Their affinities are still unresolved, initially thought as Lepidosauria (closer to extanc lizards and snakes) but being recently considered an early offshoot of stem Archosauromorpha (the group that includes crocodyles and dinosaurs). They all share a number of morphological traits adapted to a semi-aquatic existence (external nares at snout tip, heavily ossified gastralia, laterally compressed tail and limbs adapted for paddling) as well as some distinctive traits: back of the skull expanded with bony archs to support well developed bite muscles, thorax dorso-ventraly flattened, non overlapping very small scales and palatal+vomer+pterigoyd dentition to assist with holding and swallowing prey.

Four main morphotypes are recognized: 1) long snout lizard-like 2) short snouted lizard-like 3) long neck lizard-like and 4) gharial-like, all preying on small or juvenile fish. Interestingly, Choristoderes have always been  found along other aquatic repiles, such as crocodyles, but they seem to have avoided competion and used slightly non-overlapping niches.

choristodera

Choristodera morphotypes: Cteniogenys, Champsosaurus, Hyphalosaurus and Monjurosuchus (top to bottom).

Cteniogenys sp. is one of the oldest Choristoderes, found in mid to late Jurassic faunas of Western Europe (including the immensely rich Guimarota formation of Portugal) and late-Jurassic to Cretaceous faunas of North America (Morrison formation and Alberta). It was a small (25-50 cm long, probably under 500g of weight) lizard-like that likely lived by the water edge, capturing small fish and invertebrates. Extant chinese crocodyle lizards (Shinisaurus crocodilurus) might be a good lifestyle analogue.

Although small and discrete, the first choristoderes, such as Cteniogenys, defined the main adaptations that made the whole group extremely successfull for millenia to come. Certainly acting low key helped surviving crocodyle (or Crocodylomorph) infested waters.

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