10-5 Crocodiles

Animals that looked like crocodiles formed one of the two branches of the archosaurs. Because the Colorado Plateau region was a lowland with low-lying plains and swamps in the Triassic, crocodile-like animals lived there.   The ancestors of modern crocodiles evolved in the Early Cretaceous, and this group survived the End Cretaceous extinction. 

The Great Permian Extinction (250 Ma) at the end of the Permian Period and beginning of the Triassic Period eliminated 75% of all animal species and left many ecological niches open. Animals quickly evolved to fill those niches. The rate of DNA mutation had not changed, but the rate of physiological change was accelerated by the available niches into which the changed animals could occupy.  One of those niches was the crocodile niche, ambush hunter from the water. 

One of the survivors of the Great Permian extinction looked like a crocodile, Proterosuchus (Figure 10-9, Section 10-2). Proterosuchus was the ancestral archosauriformes. One of the groups that evolved within the archosauriformes was the phytosauria, which inhabited the Colorado Plateau region during the Triassic Period. They were a sister group to the Archosauria, from which the crocodiles (Pseudosuchia) and birds (Avemetatarsalia) evolved.

The phytosaurs occupied the crocodile niche in the Triassic although they were not the ancestors of today's crocodiles.  The strata of the Chinle Formation provide a record of the phytosaurs that lived in the Triassic swamps of the Colorado Plateau region. As with the phytosaurs, many animals in the history of the earth have looked like crocodiles. They were not the ancestors of crocodiles but engaged in a similar lifestyle (ambush hunting) and thus evolved to look like crocodiles. 

Petrified Forest National Park is the source of many phytosaur (not ancestors of crocodiles) and crocodylomorph (ancestors of crocodiles) fossils. During the Triassic, the region of Petrified Forest Natural Park was a lowland region near sea level with extensive swamps. The Chinle Formation at Petrified Forest was deposited during the period 228 to 208 Ma, which was the late Triassic. This video shows some of the amazing formations and tree fossils in Petrified Forest. You do not need to watch the entire video.

The following video describes the evolution of the Crurotarsi (see cladogram above), beginning with the phytosaurs at Petrified Forest National Monument. 

Several members (strata) of the Chinle Formation are exposed at Petrified Forest National Monument, covering the period between 228 – 208 Ma (Figure 10‑38). In addition to animals, they contain petrified trees from the lowland swamps that covered this region. The elevation is now 4,000 feet after uplift by the Laramide Orogeny. The lowest member of the Chinle Formation at Petrified Forest National Monument is the Mesa Redondo Member, which covers the period from 227 to 225 Ma.  Scientists in Figure 10‑37 are searching for evidence of fossils in Mesa Redondo strata. It looks like hard work.

Scientists track the evolution of the phytosaurs and other animals in the strata of the Chinle formation (Figure 10‑38). The phytosaur, Smilosuchus, is in the Mesa Redondo member of the formation. It had heavy armor, was long snouted and was an ambush hunter like modern crocodiles. They hid in the water near the shore and attacked animals that came to the water. As with proterosuchus, it had legs under the body. Protome and Pseudopalatus (Figure 10‑38) were other phytosaurs from the Chinle Formation.

The Crocodylamorpha appeared in the Late Triassic of the Colorado Plateau. These were the actual ancestors of modern crocodiles and are included in the Pseudosuchia. Hesperosuchus (Figure 10‑39) was a Late Triassic crocodylomorph from the Petrified Forest, Blue Mesa, and Mesa Redondo members of the Chinle Formation. Paleontologists think that it hunted for fish in shallow lakes. The fact that it had smaller forelimbs indicates that it may have been facultatively (partly) bipedal.

The Crocodylamorpha appeared in the Late Triassic of the Colorado Plateau. These were the actual ancestors of modern crocodiles and are included in the Pseudosuchia. Hesperosuchus (Figure 10‑39) was a Late Triassic crocodylomorph from the Petrified Forest, Blue Mesa, and Mesa Redondo members of the Chinle Formation.

The paracrocodylomorph, Postusuchus (Figure 10‑40), is in all of the members of the Chinle Formation, from the Mesa Redondo Member to the Owl Rock Member.

The Moenavi Formation overlies the Chinle Formation (Figure 10‑41). Protosuchus (first crocodile) was a Late Triassic - Early Jurassic crocodyliformes in the Moenave Formation (210 – 195 Ma) at Dinosaur Canyon. Although it may not look closer to crocodiles, it was a step closer to eventual evolution of modern crocodiles. Evolution sometimes takes a circuitous route.

The Kayenta Formation, which overlies the Moenave Formation (Figure 10‑33), has few crocodile fossils because dry land replaced the swamps that formed the Chinle and Moenave Formations, which ended the habitat for crocodiles on the Colorado Plateau. The evolution of crocodiles shifted to southern Gondwana, and the fossils of the next steps of evolution of crocodiles are in locations such as South Africa and Argentina, which were together in the continent of Gondwana. There were many species of crocodiles during the Jurassic and Cretaceous. The Notosuchia (Figure 10‑42) were dry land crocodylomorphs that lived from the mid Jurassic (167 Ma) to the Late Cretaceous (66 Ma). 

Goniopholis (Figure 10‑43) was a Neosuchia crocodyliform that lived in swamps from the Late Jurassic (155 Ma) to the Early Cretaceous (140 Ma).

The Eusuchia (true crocodiles) appeared in the Early Cretaceous (130 Ma). They survived the End Cretaceous extinction and became the ancestors of all modern crocodiles and alligators, which scientists classify as Eusuchia. Two crocodylians that survived the end Cretaceous extinction were Borealosuchus (Figure 10‑44) and Brachychampsa (Figure 10‑45).

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