Crocodilians - How nature crafted the perfect predator

 *This blog was originally posted on 26.11.2020 on the Palaeomanchester blog*


Extant Crocodilia consists of 24 recognised species of Crocodylidae (Crocodiles), Alligatoridae (Alligators and Caimans) and Gavialidae (Gharials and Tomistoma). Crocodilia first appeared in the late Cretaceous period, roughly 95 million years ago. Members of the Pseudosuchia clade, which all recorded Crocodilia are classed under, first appeared in the Early Triassic period, 250 million years ago. Pseudosuchia is part of the Archosauria clade, which is the same clade that contains Dinosaurs, Pterosaurs and Birds. This means that the closest living relative to crocodiles are in fact birds.

Simplified Phylogenetic tree of Archosauria, Steven M. Carr, Memorial University of Newfoundland, 2002.

The evolution of crocodilians within the Mesozoic era saw very noticeable changes. To get an idea of this, let’s look at Postosuchus, from the Late Triassic and compare it with the features of Deinosuchus, from the Late Cretaceous (roughly 130 million years apart). Looking at these two animals side by side, the differences become very clear. Postosuchus has an erect stance designed for obligate bipedalism, a short and slim snout, eyes and nostrils on the side of its head and a tail used for balance. The Deinosuchus, which shares many of the same features as modern crocodiles possesses features such as a sprawling stance, a long snout, eyes and nostrils on top of its head adapted to an aquatic lifestyle, and thick long tail, taking up half of its body length in order to use as a propeller to move through water.

Postosuchus skeleton reconstruction, Scott Hartman, 2018. Skeletaldrawing.com.
Deinosuchus skeleton reconstruction, Scott Hartman, 2018. Skeletaldrawing.com.

By looking at these it is clear to see how different both animals are, however if you compare the Deinosuchus to a modern Crocodilia such as the Saltwater Crocodile (Crocodylus porusus), there is very little morphological differences, despite the 73 million year age gap. Similarities between the two include a secondary bony palate, which allows the animal to breathe through its nostrils whilst underwater, a tail half the size of total body length and a long snout adapted for basking and catching prey. It is these similarities that show the design of modern Crocodilians has been around for roughly 100 million years with very little need to change. The main noticeable difference is the size, as Deinosuchus grew to roughly 10 metres in length, whilst the Saltwater Crocodile, being the largest extant crocodilian, can grow to 6 metres in length.

As the idea of evolution is for organisms to adapt in order to gain an advantage in their environment, these similarities in morphology could be interpreted in a way that shows crocodilians have evolved into a “perfect” form and have not needed to change much in millions of years. Excluding minor changes to better accommodate certain habitats, all modern crocodilians have the same fundamental body plan, as well as living in areas all over the world. This is shared with crocodilians from as long ago as the Cretaceous period.

Worldwide extant crocodilian distribution. (Green = land, Blue = Water). From data collected by Cogger, H.G & Zweifel, R.G. (1998).

Crocodilians have evidently maintained the niche of being dominant predators in whatever environment they inhabit. Whether they were sharing the landscape with Dinosaurs of the Mesozoic, Megafauna of the Pleistocene or large predatory Mammals of today, they have survived extinction events and maintained their role as an apex predator all over the world.

References
– Conley, K. and Shilton, C., 2018. Crocodilia. Pathology of Wildlife and Zoo Animals, pp.855-870.
– Dinopedia. n.d. Deinosuchus. [online] Available at: <https://dinopedia.fandom.com/wiki/Deinosuchus&gt;
– Dinopedia. n.d. Postosuchus. [online] Available at: <https://dinopedia.fandom.com/wiki/Postosuchus&gt;
– Mun.ca. 2002. Phylogeny of Archosauria. [online] Available at:<https://www.mun.ca/biology/scarr/Phylogeny_of_Archosauria.htm&gt;

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