Euryapsida and kin - the reptilian invasion of the seas

John Merck


First complete Plesiosaurus recovered by the Annings, 1829 from Wikipedia
Eventually we will move to the Archosauromorph side of the reptilian tree, but first, a problem.....

We risk neglecting a reptilian radiation that was both:

The marine reptiles of the Mesozoic, including: These are a problem because although they are well-studied and known, their derived condition makes them very difficult to place on the amniote tree.


The euryapsid condition in Neusticosaurus
Traditionally, these creatures (and a few others) were termed "euryapsid" (broad-arched) based on their possession of: We now view this as a derived form of the diapsid condition. In the Linnean system, critters with the euryapsid condition formed the group Euryapsida. Is it monophyletic? Stay tuned.

First, we describe the well-known groups whom everyone agrees to be monophyletic:

Eosauropterygia:

Eosauropterygia: (Early Triassic - terminal Cretaceous) Plesiosaurs, nothosaurs, and relatives. These encompass an outstanding size range from the 20 cm. Triassic pachypleurosaurs to plesiosaurs the size of sperm-whales. Their general characteristics:

Beneath the skin, these creatures are characterized by:
Eosauropterygian diversity:


Dactylosaurus from Palaeos
Pachypleurosauria: (Early - Middle Triassic) Small (20 - 100 cm) near-shore creatures of the Triassic Tethys Sea. Inhabited shallow marine environments and shoreline. Ballast provided by pachyostotic ribs and vertebrae. Skulls characterized by:

Could they walk on land? Probably. In many we see sexual dimorphism in which one gender has robust forelimbs and other has slender ones. Could this distinguish females coming ashore to lay eggs? Maybe, however close relatives practice ovovivipary - retaining eggs until hatching then "giving live birth". This is done by some squamates and we have fossils of small nothosaurs with large embryos (Cheng et al, 2004).


The nothosaur Ceresiosaurus by D. Bogdanov from Wikipedia
Nothosauria: (Middle - Late Triassic) Small to large predators, inhabiting near-shore waters and shorelines. Their skulls display adaptations to the capture of large prey, captured by movement of the entire head on the end of the neck (head hunter). As in other predators of this sort:

Derived features include:



The stem pistosauroid Pistosaurus from Palaeos

Pistosauroidea: (Early Triassic - terminal Cretaceous)

Pistosauroids, including the classic plesiosaurs were the only Eosauropterygians to invade the open ocean, but they did it in stages: Earliest known members of the plesiosaur lineage were nothosaur-like in general form, but by the Late Triassic, forms like Pistosaurus were finally invading the open oceans.

Osteological trends: Among pistosauroids, we see:



Synopsis of Benson et al. 2012 hypothesis
Plesiosauria: (Early Jurassic - terminal Cretaceous) The classic eosauropterygians of the mid Mesozoic. These experienced their primary radiation at the beginning of the Jurassic, and represent the second amniote radiation of the open oceans. Characterized by: Thus, fully dependent on flippers for locomotion.


Meyerasaurus from plesiosauria.com
Synapomorphies include (Rieppel, 1997):


Rhomaleosaurus cramptoni by D. Bogdanov from Wikipedia
The popular image of plesiosaurs is of the derived long-necked, small-headed forms, however the ancestral state of body proportions is more like in the stem pistosauroid Pistosaurus, with a moderately long head on a moderately long neck (E.G. Rhomaleosaurus - right). From this beginning, plesiosaurs diversified into forms that were:

Plesiosaur diversity:

Rhomaleosauridae: (Jurassic) Relatively unspecialized with robust skulls. Synapomorphies include:

Neoplesiosauria: (Jurassic - Cretaceous) All other plesiosaurs, including Pliosauridae and Plesiosauroidea. These encompass short-necked and long-necked forms respectively:

Pliosauridae: (Jurassic - Cretaceous) Short-necked forms with large heads, including the largest plesiosaurs, with skulls up to 3 m. (E.G. Pliosaurus kevani)



Plesiosaurus dolichodirus from Palaeos
Plesiosauroidea: (Jurassic - Cretaceous) Long-necked forms with small heads. Among them, Plesiosaurus itself is relatively primitive. Osteological trends include:

Plesiosauroid include superlatives such as:



Placodus gigas by Dan Varner from oceansofkansas.com

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. 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.


Placodus gigas
Placodus is among the more basal (and less weird) placodonts. Distinctive features include:


Placochelys placodonta from Qilong
More derived placodonts form the Cyamodontoidea Many developed extensive dermal armor reminiscent of that of turtles. At least one member, Henodus chelyops has been recovered from lagoonal/estuarine environments. How it fed is enigmatic, but it does not seem to have been crushing hard shelled prey as the others did. Grazing and suspension feeding have been proposed.



Ichthyosaurus communis by Julius Csotonyi

Ichthyopterygia:

(Early Triassic - beginning of Late Cretaceous)

Included many ecologically shark or dolphin-like pelagic predators. Initially near-shore creatures, ichthyosaurs were the first marine reptiles to invade the open oceans, doing so during the Late Triassic.

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


Motani 1999 basically wrote the book on ichthyosaur plylogeny (synopsis at right). Ichthyosaurs represent a pectinate tree with early, morphologically primitive members at the base and derived versions at the crown. Some landmarks:


Utatsusaurus hataii from Motani et al. 1998
Utatsusaurus hataii (Early Triassic) Among the first ichthyosaurs and phylogenetically most primitive. Roughly the size of an otter, it nevertheless was unable to come onto land, and shows the general synapomorphies of Ichthyopterygia, including:


Chaohusaurus from Wikipedia
Chaohusaurus: (Early Triassic) reflects the synapomorphies of Eoichthyosauria including: Although as the image indicates, it is noteworthy for giving live birth, according to Motani et al. 2014.


Cymbospondylus from Motani 1999
Cymbospondylus: (Late Triassic) The above were medium-sized animals. From such beginnings we get, by the Late Triassic, s whale-sized ichthyosaurs like Cymbospondylus. This was, nevertheless, an elongate eel-shaped creature. Motani 1999 defines Ichthyosauria as the last common ancestor of Cympospondylus and more derived ichthyosaurs. The synapomorphies:


Mixosaurus cornelian's from Ichthyosaurs - a Day in the Life. The dorsal fin is speculative.
Mixosaurus: (Middle Triassic) small ichthyosaurs representing the first appearance of a more compact body form and distinctly larger forelimbs. A strange autapomorphy: the anterior terrace of the supratemporal fenestra extends far anteriorly past the orbits.


Shonisaurus poplars by Ron Garrett from Oceans of Kansas. The dorsal fin is speculative.
Euichthyosauria: (Late Triassic - beginning of Late Cretaceous). The Late Triassic saw the radiation of ichthyosaurs that were derived in the possession of: These included the the "shastasaurid" grade some of whose members included the largest marine reptiles on record (See Shonisaurus sikanniensis from Nicholls and Manabe, 2004.)

Parvipelvia: (Early Jurassic - beginning of Late Cretaceous). More primitive ichthyosaurs were extinguished by the Triassic/Jurassic extinction event, however ichthyosaurs roared back in the early Jurassic in the form of Parviplevia, the ichthyosaurs with reduced pelves and hindlimbs. This is also the time of the rise of plesiosaurs. Plesiosauria and Parvipelvia have in common their invasion of the open oceans. 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.


Ichthyosaurus from Motani 1999
Parvipelvian trends:


From Ryosuke Motani's Ichthyosaur Page
Limb elements: With eosauropterygians, the homology of limb elements, even in highly derived taxa, is reasonably clear. The only issue is hyperphalangy, the their tendency to add phalanges to each finger. With ichthyosaurs, however, the identity of limb elements is difficult to assess, in part, because of hyperdactyly, their tendency to add extra fingers, as well. No surprise that a volume on homology would use an ichthyosaur paddle in its cover design.




Edennasaurus acutirostris.

Thalattosauria

(Middle - Late Triassic) A final group of well-known marine saurians. Thalattosaurs were medium - medium-large animals capable of movement on land or in water. Edennasaurus (right), a more basal form is roughly the size and proportions of the Early Permian Mesosaurus (remember?) Ancestrally, however, thalattosaurs were neither head-hunters nor snout-hunters but a little of each. The thalattosaurian radiation encompassed larger predators like Askeptosaurus and shell-crushing forms like Clarazia


Thalattosaurus alexandrae from Wikipedia
Thalattosaurian synapomrphies: In the last two characters, along with their nearly detached quadrates, they are reminiscent of squamates. Convergence or synapomorphy?

Indeed, what do these animals have to do with one another phylogenetically? Only one answer emerged clearly from the early days of cladistics:


Cryptocleidus oxoniensis (compare with Placodus)

Sauropterygia

(Early Triassic to Late Cretaceous) Storrs, 1991 clearly found Placodontia and Eosauropterygia to be sister taxa. Their synapomorphies include:

Beyond this no clear consensus exists regarding their positions. Some suggestions:

  • Sauropterygia
  • Ichthyopterygia

    The last decade has seen movement toward resolution through the addition of new taxa to the analyses:


    Hupehsuchus nanchangensis from Wikipedia

    Hupehsuchia

    (Early Triassic) A small group from the Early Triassic of China, broadly reminiscent of ichthyosaurs in general proportions. The first described was Hupehsuchus, impishly employed by Carroll and Dong 1991 to demonstrate the perils of cladistic analysis. Hupehsuchians are characterized by:


    Largocephalosaurus polycarpon from Li et al., 2014

    Saurosphargidae

    (Middle Triassic) Burst onto the scene as well-described taxa only in the last decade. Characteristics include: The last item, of course, is a potential synapomorphy with Sauropterygia.


    Wumengosaurus delicatomandibularis from Wu et al., 2011

    Wumengosaurus

    (Early Triassic) Originaly described as a largish pachypleurosaurid, this animal now emerges from phylogenetic analyses as a basal stem eosauropterygian or just outside of Eosauropterygia. It's conspicuous difference from other eosauropterygians is its long pointed snout.


    Cartorhynchus lenticarpus from Tetrapod Zoology

    Cartorhynchus

    And the latest addition from Motani et al., 2015, Cartorhynchus, a basal ichthyosaur with: Unlike proper ichthyopterygians or hupehsuchians, Cartorhynchus could come out of the water, and was apparently specialized for suction feeding.

    The real fun: Motani and colleagues phylogenetically analyzed representatives of the above taxa to obtain the result at right, finding an aquatic clade containing all of them. When characters deemed potentially convergent because they related to aquatic specializations were removed, this large clade broke into two distinct clades:

    None of this is crazy. It has been noted previously that although they differ in many ways (E.G. in Ichthyopterygia the supratemporals are very large), various of these groups display similarities:

    And that is the state of the art. Note that this large clade is closer to Archosauromorpha. (What does that mean, actually?)

    Additional reading: