- Also called:
- pseudosuchian or crocodylotarsan
- Related Topics:
- crocodile
- phytosaur
- rauisuchid
- aetosaur
- Crocodylomorpha
crurotarsan
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What are crurotarsans?
Crurotarsans, or "cross-ankles," are a group of archosaurs (ruling reptiles) more closely related to modern crocodiles than to modern birds. The name refers to the arrangement of bones in their ankles, which allows for two distinct walking postures. Crurotarsans comprise one of the two major clades of archosaurs. The other is Avemetatarsalia, which includes dinosaurs, pterosaurs, and modern birds.
During the Triassic Period, crurotarsans flourished, filling similar ecological roles to dinosaurs. While most crurotarsan lineages are extinct, crocodilians still survive today. Paleontologists believe that volcanic eruptions and climate change caused the demise of most crurotarsans, which opened ecological niches for dinosaurs and allowed for their diversification.
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When did crurotarsans live on Earth?
Crurotarsans, a group of archosaurs more closely related to crocodiles than birds, thrived during the Triassic Period, from 251 million to 200 million years ago. They shared ecological niches with dinosaurs, becoming the dominant vertebrate group of the time. Most crurotarsan lineages went extinct during the end-Triassic extinction event, linked to volcanic eruptions and climatic changes. However, the crocodylomorphs, including modern crocodiles, are a surviving lineage.
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What animals today are related to crurotarsans?
Crurotarsans are an archosaur group, also known as "ruling reptiles," that are more closely related to modern crocodiles than to modern birds. Crocodilians, which include crocodiles, alligators, caimans, and gharials, are the only living crurotarsans today. These semi-aquatic predators have retained much of their form since the Triassic period.
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What are some key features of crurotarsan reptiles?
Crurotarsans, or “cross-ankles,” are archosaurs more closely related to modern crocodiles than to birds. The name refers to the arrangement of bones in their ankles, which facilitates two distinct walking postures. They comprise one of the two major clades of archosaurs. The peg-in-socket connection between the astragalus and calcaneum allows for ankle rotation, enabling both a "high walk" and a sprawling stance.
Crurotarsans include phytosaurs, Crocodylomorpha, aetosaurs, ornithosuchians, and rauisuchids. They shared ecological niches with dinosaurs and were the dominant vertebrate group during the Triassic Period. Most crurotarsan lineages went extinct during the end-Triassic extinction event.
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How are crurotarsans different from other ancient reptiles like dinosaurs?
Crurotarsans, belonging to the clade Crurotarsi, are archosaurs more closely related to modern crocodiles than to modern birds. The name "cross-ankles" refers to the unique peg-in-socket arrangement of their ankle bones, specifically between the astragalus and calcaneum, which facilitates ankle rotation and distinct walking postures.
This ankle structure allows crurotarsans to adopt both a "high walk," where the body is elevated when the legs are brought inward, and a sprawling stance, where the body is lowered when the legs are spread apart. In contrast, other archosaurs, including dinosaurs, have ankle bones that allow foot movement in only one direction, limiting them to a more erect posture.
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Why are crurotarsans important for understanding the evolution of modern crocodiles and their relatives?
Crurotarsans are important for understanding the evolution of modern crocodiles and their relatives because they are more closely related to modern crocodiles than they are to modern birds. The clade Crurotarsi contains the Crocodylomorpha lineage, which includes modern crocodiles and their direct ancestors.
The name Crurotarsi, meaning "cross-ankles," comes from the arrangement of bones in their ankles that allows for two distinct walking postures, a sprawling stance and a "high walk." This adaptation is a distinguishing feature from other archosaurs.
During the Triassic Period, crurotarsans were the dominant vertebrate group, sharing many of the same ecological niches as the dinosaurs.
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crurotarsan, (clade Crurotarsi), any member of clade Crurotarsi, the group of archosaurs, or “ruling reptiles,” more closely related to modern crocodiles than modern birds. Although the group flourished during the Triassic Period (251 million to 200 million years ago) and most lineages have become extinct, some representatives, the crocodilians, still survive. Crurotarsi means “cross-ankles” in Latin, a name derived from the arrangement of the bones in their ankles that allows for two distinct walking postures.
The crurotarsans make up one of the two major clades of archosaurs. (The members of clade Avemetatarsalia, which include dinosaurs, pterosaurs, and modern birds, make up the second group.) Crurotarsans are distinguished from other archosaurs by the arrangement of the bones in their ankles; the connection between the astragalus (the talus, or medial proximal ankle bone) and the calcaneum (the lateral proximal ankle bone) in crurotarsans is a peg-in-socket, which allows for ankle rotation. This adaptation enables the animal to display a “high walk” (which elevates the body when the animal’s legs are brought inward toward the midline of the body) and a sprawling stance (which lowers the body when the legs are spread far apart from one another). In contrast, the ankle bones in other archosaurs allow the foot to bend only one way, which limited these animals to a more-erect posture.
The crurotarsans comprise several distinct lineages. Some paleontologists argue that the phytosaurs (“plant reptiles”), also known as parasuchians, were the first lineage to evolve; they were semiaquatic and possessed extended snouts similar to those found in modern gavials. Phytosaurs were so named because petrified mud filled in the grooves and leveled the teeth of the first fossil specimen. Since flattened teeth are indicative of a lifestyle that relies on crushing and grinding plant material, phytosaurs were thought to be herbivorous until researchers discovered and removed the petrified mud.
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The clade also contains the Crocodylomorpha, which is the only lineage made up of living species, which includes modern crocodiles and their direct ancestors. Some of the largest extinct members of this group grew to more than three times the size of modern forms. Deinosuchus riograndensis, a species that lived between about 82 million and 75 million years ago, during the Cretaceous Period, grew up to 9.1 to 10.7 metres (30 to 35 feet) long and weighed about 3.6 tonnes (about 8,000 pounds). Sarcosuchus imperator, which lived about 110 million years ago, was even larger, growing to 12.2 metres (40 feet) long and weighing about 7.2 tonnes (about 15,900 pounds).
Other lineages include the aetosaurs (“eagle lizards”), a group of herbivorous quadrupedal species that lived during the Late Triassic (229 million to 200 million years ago), the ornithosuchians (“bird crocodiles”), the rauisuchids (“Rau’s crocodiles”), and several single, ungrouped taxa. There is much disagreement concerning the evolutionary position of the ornithosuchids within the Crurotarsi, because they appear to possess both crocodilian and dinosaur-like features that paleontologists have not fully sorted out. In addition, there is much debate over whether the rauisuchians constituted an actual monophyletic group—that is, one that can be traced to a single common ancestor—or whether they should be subdivided further.
Paleontological research suggests that the crurotarsans shared many of the same ecological niches as the dinosaurs and were the dominant vertebrate group during the Triassic. Although most crurotarsans were quadrupedal, studies note that some forms, such as Effigia okeeffeae, were bipedal. (E. okeeffeae evolved some 80 million years before the first bipedal dinosaurs appeared.) Most crurotarsan lineages died out during the end-Triassic extinction event, and their demise has been attributed to a combination of volcanic eruptions and climatic changes. Furthermore, many paleontologists claim that the loss of large numbers of crurotarsan species at the end of the Triassic opened up ecological niches for the dinosaurs that provided for their rapid diversification and subsequent dominance.