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.
- Overlapping cervical ribs.
- Relatively long cervical vertebrae.
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
- The earliest, like Protorosaurus were fully terrestrial and ecologically similar to living varanid lizards. This comparison invites speculation that protorosaurians, like modern varanid squamates, probably used gular pumping of the hyoid skeleton to ventilate the lungs more efficiently.
Hyperodapedon huxleyi by D. Bogdanov from Wikipedia
Early to Late Triassic ambulatory scissors for processing tough plant material.
Choristodera: (Triassic - Neogene)
Champsosaurus by Polygone Studio
Euparkeria capensis model by Charlie McGrady Studio
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
- Antorbital fenestra
- Mandibular fenestra
- Increased size
- Fore and hindlimbs that assume a more nearly upright posture than in other diapsids.
But first, a problem.....
The euryapsids encompass a great range of morphological diversity in three major groups:
- Ichthyosauria - eel - swordfish shaped marine predators.
- Eosauropterygia - plesiosaurs and their kin
- Placodontia - non-charismatic bottom-walkers that could crush anything
The pachypleurosaur Keichousaurus from Plesiosauria.com
- Marine predators
- Limb-propelled swimmers
- Typically with long necks
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.
- Shallow/near-shore marine.
- In many we see sexual dimorphism in which one gender has robust forelimbs and other has slender ones.
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:
- limbs fully transformed into flippers (no functional elbow or wrist)
- compact torsos incapable of side to side undulation
- small, short head.
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:
- Allowed biting parts to approach prey closely.
- Facilitated feeding off of the bottom.
Placodus gigas by Dan Varner from oceansofkansas.com
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.
Ichthyosaurus communis by Julius Csotonyi
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:
- limbs modified as flippers
- a distinct tail bend supporting a caudal fin
- greatly enlarged eyes
- no obvious means of coming ashore to lay eggs. For decades we have known that in derived forms, ichthyosaurs clearly gave live birth. Recently it has come to light that the very primitive ichthyosaur Chaohusaurus also did so.
Utatsusaurus hataii from Nature
- Eel-like in shape.
- With sharp teeth in front of snout and blunt teeth in back - possible shellfish probers?
By the Late Triassic, there were whale-sized ichthyosaurs like Cymbospondylus.
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
- A supratemporal fenestra
- A broad cheek arch separating it from a very shallow infratemporal embayment.
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:
- The shape of temporal openings. The infratemporal fenestra is, in most cases, reduced to a shallow embayment, while the bar separating it from the supratemporal fenestra is very wide. Here it is in:
- The bones of the palate form a floor that partially or completely conceals the braincase. Here it is in:
- The ichthyosaur Omphalosaurus
- The placodont Cyamodus
- The eosauropterygian Simosaurus
- Compare with Sphenodon, which shows the ancestral condition.
- Monophyletic but closer to Lepidosaurs
Edennasaurus, a thalattosaur.
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:
- Limb elements: With eosauropterygians, the homology of limb elements, even in highly derived taxa, is reasonably clear.
But consider their tendency to add phalanges to each finger. Which plesiosaurian phalanx is homologous to the second phalanx of the second digit in a terrestrial saurian? Is the question even meaningful. Maybe the true homology lies in the process that generates phalanges.
With ichthyosaurs, however, the identity of even the limb elements is difficult to assess, in part, because ichthyosaurs add extra fingers, as well as extra phalanges. No surprise that a volume on homology would use an ichthyosaur paddle in its cover design.
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:
- Crown-group Archosauria
- Derived members of Ichthyosauria
The survivors belonged to Archosauria:
Postosuchus kirkpatricki by Kahless28
They have a long list of technical synapomorphies, of which this course will only sample a few biologically significant ones.
- Serrated teeth.
- Four chambered heart.
- Muscular diaphragm separating the thoracic and abdominal cavity.
- Lungs with unidirectional air flow.
- Nest building and parental care of young.
- Cranial pneumaticity: The region of the braincase and middle ear is invested with complex air-filled channels and cavities. These may function in hearing to assess direction of sound.
- The ability to bite chunks out of food effectively
- an accelerated metabolism.
- Archosaurs display the opposite trend, the ability to unlock nutrients quickly by reducing food to small bites before swallowing. (Though NOT chewing)
- This development seems to have coincided with an improved ability to get oxygen to the tissues
- A four chambered heart (review) is essential for this, but not sufficient. Getting blood to the lungs does little good if the lungs aren't receiving much air.
- Diaphragm actively pumps air into and out of lungs, working synergistically with ribs.
- Since 2010 we have known that flow through lungs, long known in Birds are actually a synapomorphy of Archosauria.
Terrestrisuchus by Pristichampsus
Eastern fence lizard Sceloporus undulatus
- Among lepidosaurs, the side to side undulation of the torso adds greatly to the propulsive forrce of the limbs. Significantly, the muscles that effect that undulation attach to the ribs. What else are ribs used for? Breathing. Now the headache- For breathing, the ribs must expand and contract symmetrically on both sides. For locomation, they must alternate sides. The result is that the faster a lepidosaur runs, the less it can breathe. Not surprisingly, living squamates are masters of anaerobic metabolism. (Monitors with their gular pumping notwithstanding.)
High walk by Vicky Baldwin from Wikipedia
- Among archosaurs, the muscles that undulate the torso attach not to the ribs, but to lateral projections of the vertebrae. Additionally, archosaurs hold their torsos straight when they move, relying on the limbs for propulsion.
- the high walk (right) of crocodilians in which the limbs are positioned nearly beneath the body (similar to non-mamalian therapsids).
- running gaits like galloping and bounding.
Who are the archosaurs? There are two major node-based groups.
- Crurotarsi: Includes the archosaurs more closely related to crocs than to birds.
- Avemetatarsalia: Includes the archosaurs more closely related to birds than to crocs. Includes living birds and extinct dinosaurs and pterosaurs.
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.
Ornithosuchus by D Bogdanov from Wikipedia
Typothorax by ahless28
- Archosaurian herbivores with small heads and toothless upturned snouts.
- Most early archosaurs had two rows of bony scutes (sales with bony cores) down their back. In aetosaurs, these were expanded into extensive armor, with additional rows along the sides and belly. In some genera, the side rows bore impressive spikes.
Fasolasuchus by Kahless28
- 2.5 - 7 m. long.
- Members achieved fully erect stance and striding walk by angling the hip socket downward. (Compare to the method used by dinosaurs and mammals.
- Indeed, Chirotherium, a common Late Triassic ichnotaxon (trackway form), is attributed to "rauisuchians." Whoever made it was clearly standing perfectly erect.
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: