Mesozoic Rulers of Land and Sea

Archosauromorpha: (Permian - Quaternary) For our purposes: All organisms more closely related to archosaurs than to lepidosaurs. A total-group definition.

The node-based group on which Archosauromorpha is anchored is Archosauria, (Triassic - Quaternary) the last common ancestor of crocodilians and birds and all of its descendants - a crown group. As a total-group, Archosauromorpha has no formal synapomorphies.
Nevertheless, known archosauromorphs display evolutionary trends that make them recognizable.

Thus, ancestrally archosauromorphs had comparatively long and stiff necks (although this could be reversed secondarily.)

Basal stem archosauromorphs:

Unlike the primitive relatives of lepidosaurs, whose fossil record is sparse, the basal (i.e. non-archosaurian) archosauromorphs were common and ecologically diverse during the Permian and Triassic. Highlights include:

Prolacerta broomi, from Online Biolog Library - SUNY Orange
Protorosauria: (Late Permian - Late Triassic): Small to moderately large saurians close to the ancestry of archosaurs.

Hyperodapedon huxleyi by D. Bogdanov from Wikipedia
Rhynchosauria. (Triassic)

Early to Late Triassic ambulatory scissors for processing tough plant material.

Choristodera: (Triassic - Neogene)

Champsosaurus by Polygone Studio
Originally only known to paleontologists as large long-snouted and medium-snouted forms from the Late Cretaceous and Early Paleogene. These were superficially convergent on crocodilians but, when considered in detail, extremely different as well. Eventually, more basal forms were identified going as far back as the late Triassic from fragmentary specimens of non-descript smaller reptiles already housed in museums.

Euparkeria capensis model by Charlie McGrady Studio
Archosauriform - grade archosauromorphs: (Late Permian - Triassic)

Crown-group Archosauria is nested inside Archosauriformes, a clade containing both Archosauria and critters that would, in the in the pre-Phylogenetic Systematics days, have been called archosaurs based on general similarity, but today are seen to lack some synapomorphy of Archosauria.

These outliers include:

Skull of Euparkeria capensis
Synapomorphies of Archosauriformes: Beyond this, we see some general tendencies: We will take these trends up again when we discuss crown-group Archosauria.

But first, a problem.....


The euryapsids encompass a great range of morphological diversity in three major groups:

The pachypleurosaur Keichousaurus from
Eosauropterygia: (Middle Triassic - end of Cretaceous) The plesiosaurs of your childhood prehistoric animal books are best known examples, but they encompass more diversity, including an outstanding size range from the 30 cm. Triassic pachypleurosaurid Keichousaurus (right) to creatures the size of sperm-whales. Their general characteristics:

The nothosaur Ceresiosaurus by D. Bogdanov from Wikipedia

Their greatest range of morphological disparity was in the Late Triassic among the small pachypleurosaurs and medium to large nothosaurs.

The plesiosaur Seeleysaurus by D. Bogdanov from Wikipedia

Plesiosauria: (Triassic - end of Cretaceous)

Plesiosaurs were the only Eosauropterygians to invade the open ocean. By the beginning of the Jurassic, forms like Plesiosaurus showed the major plesiosaurian derived features:

Thus, fully dependent on flippers for locomotion.

Plesiosaurs were successful and diverse for the rest of the Mesozoic. Although ancestrally long-necked, short-necked, large-headed forms (often called "pliosaurs") evolved at least twice (in Jurassic and Cretaceous repsectively). Ecologically these were top predators. indeed, the Jurassic group contained animals like Liopleurodon the size of a sperm whale.

Other plesiosaurs took the long-necked morph to extremes, for example Elasmosaurus. Two possible functions:

Placodus gigas by Dan Varner from
Placodontia: (Middle - Late Triassic) including Placodus pictured here was restricted to shallow marine environments of the middle and late Triassic. They probably swam clumsily or walked on the bottom in the manner of a snapping turtle. In contrast to their locomotor apparatus, their skulls were intensely modified for withstanding terrific biting forces.

Placodus is among the less weird placodonts. Many developed extensive dermal armor reminiscent of that of turtles.

Cyamodus skull from BIO356 lab - University of Toronto.
The dorsal view at right shows the great size of the muscles mass that closed the jaw. The palatal view shows the transformation of the teeth (marginal and palatal) into a pavement of button-like teeth with which they presumably crushed hard-bodied invertebrates. Take my word for the fact that their skulls were greatly reinforced to transmit the biting forces involved.

Ichthyosaurus communis by Julius Csotonyi
Ichthyosauria: (Early Triassic - beginning of Late Cretaceous)

Included many ecologically shark or dolphin-like pelagic predators. In fact, ichthyosaurs were the first marine reptiles to invade the open oceans.

Ichthyosaurs appear in the fossil record highly modified for marine life, with:

Utatsusaurus hataii from Nature
The first ichthyosaurs including the Early Triassic Utatsusaurus (right) were rather different from the familiar forms in being:

By the Late Triassic, there were whale-sized ichthyosaurs like Cymbospondylus.

by Arturo de Manuel
Ichthyosauria: (Early Triassic - beginning of Late Cretaceous)

The early Jurassic was the peak of ichthyosaur diversity and the time in which they assumed their familiar form. This is also the interval in which we have soft-tissue impressions that reveal the dorsal and caudal fins of these animals. (Note: a few of these seem completely genuine but many were "improved" by 19th century preparators.) Most preyed on small fish and cephalopods, but some were orca-sized macropredators.

The plesiosaur Pliosaurus kelvani from Sciency Thoughts
Origins: The term euryapsid - "broad-arch" was coined long before the cladistic age to refer to creatures with: There was no presumption of their being monophyletic.

In Merck's humble opinion....

For all their disparity of form, Euryapsida is monophyletic and a basal member of Archosauromorpha. Euryapsids are recognized, in part, by:

But this opinion is not universally held. Others regard Euryapsida as:

Edennasaurus, a thalattosaur.
It's not like euryapsids are poorly known. They were among the first fossil vertebrates to be scientifically described in the early 19th century, and are known from a great diversity of specimens. Why are these particular marine reptiles so intractible? Two major sources of trouble.

Long branch attraction: Derived members of the group are very dissimilar to one another and filled with evolutionary derivations. This problem is resolved, in part, by the recognition of the close relationships between euryapsids and Thalattosaurs, a minor Triassic group of marine reptiles.

Unclear homology: When it comes to assessing the identity of skeletal elements, euryapsids, particularly ichthyosaurs, are very daunting. Consider two examples:

Extinction: The creatures described on this page arose in the Early Triassic and diversified throughout the period, however the Triassic ended with a significant extinction event. The precise cause is not known, although the Central Atlantic Magmatic Province (CAMP), a first-class flood basalt associated with the opening of the Atlantic Ocean dates from that time. Of all the creatures described in this lecture, only these survived:

The survivors belonged to Archosauria:

Postosuchus kirkpatricki by Kahless28
Archosauria: (Triassic - Recent) The most recent common ancestor of crocodilians and birds and all of its descendants. Members of this group dominated the top of the terrestrial food chain from the Middle Triassic to the end of the Cretaceous and remain an important part of terrestrial biota. Both the crocodilian and bird stems are full of charismatic and interesting fossil critters.

They have a long list of technical synapomorphies, of which this course will only sample a few biologically significant ones.

As with Mammalia, it is impossible to know whether the soft tissue synapomorphies listed here don't actually diagnose larger groups. Still, when we combine them with features of Archosauriformes (which includes very archosaur-like critters) we see two major biological themes:

  • Food processing: In lepidosaurs, we saw an evolutionary trend toward the ability to swallow larger and larger items. Animals that did this had to cope with a surface volume ratio problem: It took much longer to digest food. Thus, we got the proverbial python resting for a week after each meal. How did this happen?

    Terrestrisuchus by Pristichampsus
    An animal that was able to digest food faster, and to unlock the energy of digested food faster. Such an animal was capable of more energetic activity, and in the Early Triassic we begin to see archosaurs with adaptations such as longer limbs or limbs designed for an upright posture suggesting critters that could move around for longer periods of time. (The croc-relative Terrestrisuchus at right is an extreme example.) Did this happen by chance or was the stage set by some other evolutionary development?

  • Contrast of lepidosaur and archosaur locomotion is revealing:

    Eastern fence lizard Sceloporus undulatus

  • This simple development set the stage for the evolution of archosaurs into animals that could dedicate their lungs to breathing in a way that other saurians couldn't. Moreover, it set the stage for the evolution of new locomotor strategies including:

    Who are the archosaurs? There are two major node-based groups.

    As their names indicate, synapomorphies of these groups have to do with features of their ankles and feet. Interesting because these features predispose members of both groups to evolve toward erect posture. At right: Crurotarsan (left) and avemetatarsalian (right) ankle schematics highlighting two proximal tarsals, the astragalus (A) and calcaneum (C).

    In the case of Crurotarsi the calcaneum (heel) takes the form of a lever for the rotation of the foot on the shin. That action works most efficiently when the stance is more nearly erect.

    Crurotarsi took over land ecosystems during the Middle and Late Triassic only to suffer greatly in the Late Triassic extinction event, with only the lineage that gave rise to Crocodylia surviving.

    Crurotarsan superlatives:

    Ornithosuchus by D Bogdanov from Wikipedia
    Ornithosuchidae: (Late Triassic)

    Big predators.

    Typothorax by ahless28
    Aetosauria: (Late Triassic)

    Fasolasuchus by Kahless28
    "Rauisuchian-grade" crurotarsans: (Middle - Late Triassic) A paraphyletic group of top terrestrial predators of the Late Triassic.

    Extinction The terminal Triassic really was the end of an era. For the last 30 million years, rauisuchians, aetosaurs, and various non-archosaurian archosauromorphs had ruled the roost, (accompanied by occasional small mammal-like cynodonts, dinosaurs, pterosaurs, and the first turtles.) Then came a significant extinction event. The precise cause is not known, although the Central Atlantic Magmatic Province (CAMP), a first-class flood basalt associated with the opening of the Atlantic Ocean dates from that time.

    From Crurotarsi, only Crocodylomorpha survived:

    Crocodylomorpha: (Late Triassic - Recent) Animals more closely related to Crocodylia than to any rauisuchian-grade crurotarsan. Although living members are large freshwater predators, the earliest forms were small, long-legged, and terrestrial like Pseudhesperosuchus (Late Triassic).