Euryapsida and kin - the reptilian invasion of the seas

John Merck

Links to cladogram and phylogram.


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)

Min ∇ (Pachypleurosauria, Nothosauria, Plesiosauria)

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. Recent analyses either recover these as a paraphyletic grade-group of basal eosauropterygians (Shang et al., 2020, Wang et al., 2022) or as a monophyletic group (Xu et al., 2023).

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. Robust morph could be:

We have fossils of the pachypleurosaur Keichousaurus with large embryos (Cheng et al, 2004), suggesting live birth of young.


The nothosaur Ceresiosaurus by D. Bogdanov from Wikipedia
"Nothosauria": (Middle - Late Triassic) Monophyletic or paraphyletic? See Wang et al., 2022. 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:

This year's nothosaurian poster-child: Brevicaudosaurus jiyangshanensis with its improbably short fat tail.


M'mmm! Mud! Paludidraco multidentatus by N. Tamura does its suspension/deposit-feeding thing
But note! Some "nothosaurs" like Simosaurus and Paludidraco may have dabbled in suspension feeding.

The stem pistosauroid Pistosaurus, ~ 3 m. from Palaeos

Pistosauroidea: (Early Triassic - terminal Cretaceous)

Max ∇ (Plesiosauria ~ Nothosauria, Pachypleurosauria)

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 Middle Triassic, forms like Pistosaurus were finally invading the open oceans.

Osteological trends: Among pistosauroids, we see:



Synopsis of Benson et al. 2012 hypothesis
Plesiosauria: (Latest Triassic - terminal Cretaceous)

Max ∇ (Plesiosaurus ~ Augustasaurus)

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. Did they employ rowing or subaqueous flight? Krahl, et al., 2022 supports the latter with the longitudinal twisting of flippers as a crucial element, based one finite element analysis. Pourfarzan et al., 2022 concur, and proceed model plesiosaur taxa based on hydrodynamic models that show that agility is inversely correlated with flipper aspect ratio (length vs width).


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)

Min ∇ (Plesiosaurus, Pliosaurus)

All other plesiosaurs, including Pliosauridae and Plesiosauroidea. These encompass short-necked and long-necked forms respectively:

Pliosauridae: (Jurassic - Cretaceous)

Max ∇ (Pliosaurus ~ Plesiosaurus)

Short-necked forms with large heads, including the largest plesiosaurs, with skulls up to 3 m. (E.G. Pliosaurus kevani). Note, though, that the earliest known plesiosaur - Rhaeticosaurus martens from the Late Triassic:



Plesiosaurus dolichodirus from Palaeos
Plesiosauroidea: (Jurassic - Cretaceous)

Max ∇ (Plesiosaurus ~ Pliosaurus)

Long-necked forms with small heads. Among them, Plesiosaurus itself is relatively primitive. Osteological trends include:

Plesiosauroid include superlatives such as:

Plesiosaur Ecology: Long-necked plesiosauroids seem to have exploited a hybrid predation strategy combining elements of pursuit and ambush predation, however there were variations. O'Keefe et al., 2017 describe the elasmosaurid Morturneria saymourensis as a suspension or deposit feeder, on the basis of its dentition.

Criteria of bone histology similar to those applied to terrestrial sauropsids, suggest that plesiosaurs may have had elevated body temperature and metabolic rate:



Placodus gigas by Dan Varner from oceansofkansas.com

Placodontia:

(Middle - Late Triassic)

Placodontiformes: Max ∇ (Paraplacodus ~ Psephoderma)
Placodontia: Min ∇ (Paraplacodus, Psephoderma)

Includes 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.

At the other end, the stem placodont, Palatodonta bleekeri (Middle Triassic), lacks crushing teeth altogether but shares procumbent incisors with placodus. (Neenan et al., 2013)



Ichthyosaurus communis by Julius Csotonyi

Ichthyopterygia:

(Early Triassic - Early Late Cretaceous)

  • Ichthyopterygia: Min ∇ (Chaohusaurus, Utatsusaurus, Parvipelvia)

    Included many ecologically shark or dolphin-like pelagic predators. Ichthyosaurs and their close relatives were the first marine reptiles, appearing in the Early Triassic. Initially near-shore creatures, Late Triassic ichthyosaurs became the first marine reptiles to invade the open oceans. Remarkably, Kear et al., 2022 report derived ichthyosaur remains from only 2 my after the End Permian Extinction Event and conclude that ichthyosaurs must have been present in the Late Permian. This extraordinary claim requires verification.

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


    Motani 1999 basically wrote the book on ichthyosaur phylogeny although it has been refined subsequently (synopsis at right of Bindelini et al., 2021). Ichthyosaurs represent a pectinate tree with early, morphologically primitive members at the base and derived versions at the crown. Some landmarks:


    Chaohusaurus from Wikipedia
    Chaohusaurus: (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:: Although as the image indicates, it is noteworthy for giving live birth, according to Motani et al. 2014. In this, it resembles derived ichthyosaurs of the Jurassic, suggesting that this is also an ancestral feature for the group.


    Utatsusaurus hataii from Motani et al. 1998
    Grippioidea (Early Triassic) Represented by the well-known Utatsusaurus hataii shows the general synapomorphies of Ichthyopterygia, including:

    Ichthyopterygian irritant: The interpretation of the temporal region as having a large three-dimensional supratemporal occupying the posterior border of the supra temporal fenestra, and posterolateral corner of the skull, and a small plate-like squamosal on the side of the skull is universally accepted by ichthyosaur specialists. This creates the odd situation where:

    Your instructor is skeptical, but bows to the weight of scholarly opinion.

    Ecology of basal ichthyopterygians: Despite their long snouts, Chaohusaurus and Utatsusaurus don't seem to have been simple "snout-hunters," as their posterior teeth were bulbous, as if for crushing hard prey. Did they, perhaps, probe into soft sediment for shelled prey then crush it in the backs of their jaws?

    Ichthyosauria: (Middle Triassic - Early Late Cretaceous)

  • Ichthyosauria: ∇ (Cymbospondylus, Parvipelvia)

    Phylogenetically defined as the last common ancestor of Cymbospondylus and Jurassic ichthyosaurs and all descendants. The synapomorphies:


    Cymbospondylus from Motani 1999
    Cymbospondylidae: (Middle - Late Triassic) Medium to large animals. From such beginnings we get, by the Late Triassic, s whale-sized ichthyosaurs like Cymbospondylus. This was, nevertheless, an elongate eel-shaped creature. In Cymbospondylidae, we finally see ichthyopterygians with uniformly sharp pointed teeth.

    The image of Cymbospondylus at right allows us to track a trend in ichthyosaur evolution: The nasals become increasingly large, overlapping adjacent elements like the frontals. This parallels other long snouted marine amniotes, who use overlap of adjacent dermal elements to strengthen the region between the orbits - a zone of weakness. Parallel developments include:


    Hueneosauria: (Middle Triassic - Early Late Cretaceous)

    Min ∇ (Mixosaurus, Parvipelvia)

    Named for early 20th century ichthyosaur luminary Friedrich von Huene.


    Mixosaurus cornelianus from Renesto et al., 2020.
    Mixosauridae: (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. Some superlatives:


    Shonisaurus poplars by Ron Garrett from Oceans of Kansas. The dorsal fin is speculative.
    "Shastasaurs": (Late Triassic). The Late Triassic saw the radiation of ichthyosaurs that clearly invaded the open ocean realm - the first amniotes to do so unambiguously. This "shastasaurid" grade included the largest marine reptiles on record and possibly the largest animals ever. (See Shonisaurus sikanniensis from Nicholls and Manabe, 2004.)

    Parvipelvia: (Early Jurassic - Early Late Cretaceous).

    Min ∇ (Temnodontosaurus, Ichthyosaurus)

    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.)

    Of special interest:



    Ichthyosaurus from Motani 1999
    Parvipelvian trends:

    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.

    Thalattosauriformes/Thalattosauria

    (Middle - Late Triassic)

    Thalattosauriformes: Min ∇ (Thalattosaurus, Edennosaurus, Askeptosaurus)
    Thalattosauria: Max ∇ (Nectosaurus, Herscheleria ~ Edennosaurus, Askeptosaurus)(Nicholls, 1999)

    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 durophagous forms like Thalattosaurus.


    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:

    Recent Arrivals:



    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. Recent analyses place Saurosphargidae as sister taxon to Sauropterygia (Wang et al., 2022) or as a basal member (Wolniewicz et al., 2022).

    Middle Triassic saurosphargids tend to have flattened and laterally expanded trunk profiles in dorsal view, however the Early Triassic saurosphargiform Pomolispondylus baini is elongate. (Cheng et al., 2022.)


    Atopodentatus unicus

    Atopodentatus

    (Early Triassic) New from Cheng at al., 2014, a 3 m. grazing herbivore (?):


    Hupehsuchus nanchangensis from Wikipedia

    Hupehsuchia

    (Early Triassic) A small group from the Early Triassic of China, a mash-up of ichthyosaurs in general proportions, eosauropterygians in stiffening of the trunk, and pladoconts in dermal armor. The first described was Hupehsuchus, impishly employed by Carroll and Dong, 1991 to demonstrate the perils of cladistic analysis. Hupehsuchians are characterized by:


    Cartorhynchus lenticarpus from Tetrapod Zoology

    Cartorhynchus

    From Motani et al., 2015, Cartorhynchus, a basal Early Triassic ichthyosauriform from southern China with: Unlike proper ichthyopterygians or hupehsuchians, Cartorhynchus could come out of the water, and was apparently specialized for durophagous feeding.


    Sclerocormus cf. parviceps from Qiao et al., 2022

    Sclerocormus

    From the Early Triassic of Southern China: an ichthysauriform with: Sclerocormus forms a link between basal ichthyosauriforms and the enigmatic later Triassic durophagous Omphalosauridae. Recent analyses show Sclerocormus plus later Omphalosauridae as sister taxon to Cartorhynchus. (Qiao et al., 2022) Their obvious synapomorphy: The nasals extend anteriorly to the tip of the snout.


    Euryapsid phylogeny from Wang et al., 2022
    The real fun: Starting with Motani et al., 2015. phylogenetically analyzed representatives of the above taxa to obtain the result at right, finding an aquatic clade containing all of them (Corresponding to the traditional "Euryapsida." Important taxon names defined in the literature: 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: The Motani et al., 2015 pattern has been largely maintained by later work. In one sense, this result is fragile and in another it is robust:


    Basal convergence from Wang et al., 2022

    The Archaeopteryx of euryapsids

    What we would most love to see is a creature close to the ancestry of Euryapsida when it was just beginning to transition to an aquatic life-style - the Archaeopteryx of euryapsids. We don't have it, but can infer that it lived in the Late Permian. But we are getting close. Wang et al., 2022 describe Hanosaurus hupehensis, the sister taxon of sauropterygians and saurosphargids and basal member of Sauropterygiformes. The morphological gap between this long, slender, short-limbed creature, basal ichthyopterygians, and thalattosaurs is not so great.

    And there is hope: The Nanzhang-Yuan'an marine reptile fauna of Hubei Province, China records conditions 4-5 my after the End Permian Extinction Event, and has recently been the source of many interesting taxa (Cartorhynchus, Sclerocormus, Pomolispondylus, Atopodentatus, Hanosaurus). God willing, it has more yet to offer. And if its stratigraphy can be solidified, Svalbard may offer key insights. Watch this space.

    Additional reading: