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Ichthyosauromorpha
Ichthyosauromorpha
Ichthyosauromorpha
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Ichthyosauromorpha
Temporal range:
Early Triassic - Late Cretaceous, 251.3–90 Ma Inferred Permian origins[1]
Hupehsuchus nanchangensis
Ichthyosaurus somersetensis
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Neodiapsida
Clade: †Ichthyosauromorpha
Motani et al., 2014
Subgroups

The Ichthyosauromorpha are an extinct clade of Mesozoic marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia.

The node clade Ichthyosauromorpha was first defined by Ryosuke Motani et al. in 2014 as the group consisting of the last common ancestor of Ichthyosaurus communis and Hupehsuchus nanchangensis, and all its descendants. Their synapomorphies, unique derived traits, include: the presence of an anterior flange on the humerus and radius; the lower end of the ulna being as wide as or wider than the upper end, the forelimb being as long as or longer than the hindlimb, the hand having at least three quarters of the length of the upper arm and lower arm combined, the fibula extending behind the level of the thighbone, and the transverse process of the vertebral neural arch being reduced or absent.[2]

The Ichthyosauromorpha were previously thought to have likely originated in China during the upper Lower Triassic period, about 248 million years ago. However, a 2023 study recorded the fossils of a derived marine ichthyosauromorph (an ichthyopterygian) in earliest Triassic rocks of Spitsbergen, Norway, just 2 million years after the Permian–Triassic extinction event. The presence of such a derived ichthyosauromorph so early in the Triassic indicates that the ichthyosauromorphs (and possibly ichthyosauriformes, depending on divergence estimates) probably originated prior to the end of the Permian and were thus survivors of the mass extinction as opposed to ecological successors that evolved following it.[1]

One branch consists of the Hupehsuchia, and the other of the Ichthyosauriformes, of which Cartorhynchus was a basal member. Other ichthyosauriforms were the Ichthyopterygia, containing the Ichthyosauria and allies. The last ichthyosaurs probably became extinct in the middle Cretaceous.

Their relationships with other reptiles are unresolved, due to their highly derived morphology even in their earliest known representatives,[3] though they are usually considered to be diapsids.[4] Ichthyosaur researcher Michael Maisch has alternatively argued that the group (or at least Ichthyosauriformes, as he regards hupehsuchians as unrelated) may have its origins within the primitive, non-diapsid lizard-like reptile group Procolonophoidea.[5] A 2022 study on the early evolution of reptiles classified the Ichthyosauromorpha as basal archosauromorphs, forming a clade with the other marine reptile groups Thalattosauria and Sauropterygia as sister to the rest of the Archosauromorpha.[6] A 2023 study describing the Triassic marine reptile Prosaurosphargis found a similar placement, albeit instead placing the three marine reptile groups within an expanded Archelosauria.[7]

Cladogram after Simões et al. 2022:[6]

Taxonomy

[edit]

Phylogeny

[edit]

The internal phylogenetic structure of the Ichthyosauromorpha is shown by this cladogram:

Ichthyosauromorpha

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Ichthyosauromorpha

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from Grokipedia
Ichthyosauromorpha is an extinct of marine reptiles within Diapsida, defined as the last common ancestor of Hupehsuchus nanchangensis and Ichthyosaurus communis and all its descendants, encompassing the groups and . These reptiles were fully adapted to aquatic life, featuring streamlined bodies, paddle-like limbs, and other fish-like traits that facilitated efficient swimming in marine environments. The clade originated in the (Olenekian stage, approximately 251–247 million years ago), emerging rapidly in the aftermath of the end-Permian mass extinction, which created ecological opportunities for marine recolonization. Ichthyosauromorpha underwent an initial burst of diversification during the Spathian substage, with fossils indicating a across ancient seaways in regions now corresponding to , , and other sites. Hupehsuchians, the basal members, were typically small (under 1 meter long) with elongated necks, slender ribs, and hyperphalangic flippers, while included larger, more derived forms like Cartorhynchus and Sclerocormus, which exhibited short trunks, deep ribcages, and adaptations for suction-feeding. Over time, evolutionary rates within Ichthyosauromorpha decelerated significantly—by a factor of about six—following the early , reflecting a transition from rapid adaptation to more stable ecological niches. The group persisted through the , , and into the , with ichthyosaurs (the dominant subgroup) reaching sizes up to 25 meters and diverse feeding strategies, from nektonic predators to deep-diving specialists. However, ichthyosauromorphs experienced bottlenecks, including a potential early demise of some stem lineages by the , before the main ichthyosaur . Their fossil record, particularly abundant from Lower Triassic strata in , provides critical insights into the of aquatic adaptations among reptiles post-extinction.

Physical characteristics

Body plan

Ichthyosauromorphs exhibited a range of body plans adapted to aquatic environments, evolving from more lizard-like forms in basal taxa to highly streamlined, torpedo-shaped bodies in derived ichthyosauriforms that superficially resembled modern cetaceans. Basal members, such as Cartorhynchus lenticarpus, possessed a short, constricted snout, a relatively flexible trunk, and large forelimbs suggestive of amphibious capabilities, measuring approximately 40 cm in length. In contrast, advanced ichthyosauriforms developed an elongated snout, a rigid trunk with reduced vertebral flexibility, and a body profile optimized for sustained swimming, with the tail serving as the primary propulsor through a lunate fluke. Limb modifications were a key , transforming limbs into paddle-like flippers for and stability rather than . Fore- and hindlimbs featured hyperphalangy, with excess phalanges—up to 30 or more per digit in some —allowing for broader, more flexible paddles that enhanced maneuverability in water. These flippers, supported by elongated zeugopodial elements, contributed minimally to drag compared to the body but were crucial for control during turns. Preserved skin impressions reveal a smooth, scaleless in ichthyosauriforms, akin to that of leatherback turtles, which minimized hydrodynamic drag and facilitated efficient cruising. This supple, fibrous skin, often phosphatized in fossils, lacked scales and may have included layers for insulation, further reducing friction during movement. Size varied dramatically across the clade, from diminutive early forms like Cartorhynchus at around 40 cm to gigantic later ichthyosauriforms exceeding 20 meters, such as Shonisaurus sikanniensis, reflecting rapid evolutionary scaling for open-ocean niches. Sensory adaptations included notably large eye sockets, particularly in derived taxa, which supported eyes up to 26 cm in diameter for enhanced visual acuity and sensitivity in low-light underwater conditions, enabling effective predation at depth.

Skeletal features

The skull of ichthyosauromorphs is characterized by a long rostrum in early forms, often equipped with conical teeth adapted for grasping prey, though some hupehsuchians exhibit reduced or complete toothlessness, as seen in with its edentulous jaws resembling those of baleen whales. Large s surrounded by sclerotic rings are a consistent feature across the , providing for the eyes and indicating enhanced visual capabilities in aquatic environments; for instance, in Chaohusaurus brevifemoralis, the shows a large lateral and dorsal extent with approximately 16-18 sclerotic plates. The vertebral column in ichthyosauromorphs features a high number of presacral vertebrae, reaching up to 86 in some derived such as liangae, which contributes to an elongated body for streamlined swimming. Neural spines vary significantly: hupehsuchians like Eohupehsuchus display tall, bipartite neural spines in the dorsal region, supporting a more rigid , whereas such as Chaohusaurus exhibit low, broad neural spines that enhance flexibility. Rib structure in basal ichthyosauromorphs includes double-headed ribs, allowing articulation with both the vertebra and capitulum, a trait evident in forms like Chaohusaurus. Pachyosteosclerosis, involving thickened and densified bones, is prominent in early taxa for buoyancy regulation during aquatic transitions; this is particularly noted in the pachyostotic, interlocking ribs of hupehsuchians, which form a rigid "" around the trunk to limit flexibility and aid in stability. In contrast, show less pronounced interlocking but retain dense rib elements in basal species like Cartorhynchus. The pectoral and pelvic girdles are expanded to support limb modifications, with enlarged in the pectoral region and robust ilia in the pelvic girdle; for example, in Eohupehsuchus, the is smaller than the lunate , while Chaohusaurus features a that wraps ventrolaterally. Flipper hyperphalangy is a hallmark , with up to 10 or more rows of phalanges in the autopodia, enabling rigid, paddle-like propulsion; this is exemplified in where phalangeal counts can exceed standard pentadactyl patterns, as in Chaohusaurus with formulas like 1-3-3-3-1. Key differences between hupehsuchians and underscore their distinct adaptations within Ichthyosauromorpha: hupehsuchians possess elongated necks with 9-10 and interlocking forming a protective corset, as in , promoting rigidity for near-shore maneuvering, while have shorter necks (e.g., 5 in Cartorhynchus) and a more flexible suited to open-water cruising.

Evolutionary history

Origins and early evolution

The Ichthyosauromorpha emerged in the aftermath of the end-Permian mass extinction approximately 252 million years ago, with the earliest known fossils dating to the stage of the , around 248–247 million years ago. This rapid post-extinction recovery allowed these reptiles to colonize marine environments, filling predatory niches left vacant by the collapse of pre-extinction marine ecosystems. The clade's origins are closely tied to terrestrial s, with basal forms exhibiting primitive skeletal features suggestive of an amphibious lifestyle during the initial transition to fully aquatic habitats. Fossil evidence for the geographic origins of Ichthyosauromorpha is concentrated in , particularly in the Lower strata of and provinces, where the Luoping Biota and related deposits preserve key basal taxa. Notable early members include the hupehsuchians, such as Hupehsuchus and Parahupehsuchus, which represent the basalmost ichthyosauromorphs and form the to more derived ; these enigmatic reptiles, characterized by elongated neural spines and polydactylous flippers, are exclusively known from this region, supporting a South Chinese cradle for the clade. Recent discoveries, including a new hupehsuchian species Chaohusaurus zhangjiawanensis from Province (, ~248 million years ago), further highlight ecological diversity among basal forms in shallow-marine settings. Early , like Utatsusaurus hataii from the Lower of (approximately 245–250 million years ago), further illustrate this initial radiation, with Utatsusaurus regarded as one of the most primitive ichthyopterygians based on its generalized skull and limb structure. Additional discoveries, such as Cartorhynchus lenticarpus and Sclerocormus parviceps from Province (both ~248 million years old), highlight the diversity of basal forms, with Cartorhynchus (~1.6 meters long) showing suction-feeding adaptations and Sclerocormus (~1.6 meters) exhibiting a flexible atypical of later ichthyosaurs. The early evolution of Ichthyosauromorpha featured a pronounced burst in body size and morphological disparity, enabling quick adaptation to open-ocean predation. Within roughly 3 million years of their first appearance, some lineages achieved lengths exceeding 17 meters, as seen in Cymbospondylus youngorum from the of (~246 million years ago), outpacing the size observed in later marine tetrapods like cetaceans. This rapid growth is evidenced by the Fossil Hill Fauna's size spectrum, from small basal forms to giants, reflecting exploitation of abundant pelagic prey such as and ammonoids in recovering seas. High rates characterized this phase, with ichthyosauromorph diversity doubling to at least 15 species within the Spathian substage (~1 million years), driven by ecological opportunities in post-extinction oceans and culminating in an early peak of evolutionary rates before a mid- bottleneck. Fossils from the , including vertebral and humeral remains (~249 million years old) comparable to Utatsusaurus (2.6 meters) and Cymbospondylus (5 meters), indicate that this radiation extended beyond early on, correlating with ammonoid zones shared across the Panthalassic Ocean.

Diversification and extinction

Following their origins in the Early Triassic, Ichthyosauromorpha underwent a significant radiation during the Triassic-Jurassic transition, achieving peak taxic diversity in the with numerous coexisting species that collectively represent over 100 described ichthyosaur species across the clade's history. Recent finds, such as the new baracromian ichthyosaur Gadusaurus aqualigneus from (~193 million years ago), underscore the breadth of Early Jurassic diversity in the . This expansion involved niche partitioning, where forms adapted for surface swimming coexisted with deeper-diving specialists, as evidenced by variations in eye size and that facilitated different strategies in marine ecosystems. Evolutionary rates were initially high during the , enabling rapid morphological diversification and occupation of vacated ecological roles post-Permo- extinction, but these rates slowed markedly by the Jurassic, leading to a stabilization in disparity. Osteohistological analysis of large enigmatic bones from () deposits in confirms the presence of giant ichthyosaurs up to 25–30 meters long, indicating sustained evolution of large body sizes into the . In the , ichthyosaur diversity declined sharply, reducing the to a few specialized platypterygiid forms such as , which persisted as robust, large-bodied predators in coastal and epicontinental seas. The last known records date to the early stage, approximately 94 million years ago, after which no definitive fossils appear. This terminal decline was not linked to the end-Cretaceous event but rather to a two-phase in the , eliminating major subgroups like Ophthalmosaurinae and much of the remaining ecological breadth. Proposed causes include intensified competition from emerging fishes and certain groups, which diversified rapidly and occupied overlapping predatory niches, alongside possible niche saturation from prior evolutionary stasis. Environmental factors, such as oceanic anoxic events around the Cenomanian-Turonian boundary, likely exacerbated the crisis through volatile sea temperatures, fluctuating sea levels, and reduced oxygen levels that stressed communities. Contributing to vulnerability was the clade's slow evolutionary rates in the , limiting adaptability to these changes compared to more versatile competitors. Ichthyosauromorpha achieved a global distribution across the Tethys Sea, proto-Atlantic, and Pacific margins, with fossils documenting their presence in both hemispheres from coastal to open-ocean settings. Key sites include the of Holzmaden, , yielding exceptionally preserved specimens; assemblages from , ; and prolific Lower Triassic localities in , such as Province. Evolutionary trends featured increasing body size disparity, with forms reaching giant proportions faster than in later marine tetrapods, alongside heightened specialization in locomotion and predation that peaked in the before contracting in the .

Classification and phylogeny

Definition and taxonomy

Ichthyosauromorpha is an extinct clade of marine reptiles defined stem-based as the last common ancestor of Hupehsuchus nanchangensis and Ichthyosaurus communis, and all of its descendants. This definition, proposed by Motani et al. in 2015, formalized the grouping of early aquatic diapsids that bridge terrestrial reptiles and fully marine ichthyosaurs, originating around 248 million years ago in . The name Ichthyosauromorpha was initially suggested by Motani in 1999 for the stem-based equivalent encompassing hupehsuchians as relatives of ichthyopterygians, though without the phylogenetic evidence available later. The clade comprises two primary subgroups: and . includes basal aquatic diapsids from the of , characterized by corset-like rib cages formed by expanded, overlapping dorsal ribs and that create a rigid, tubular body structure for buoyancy and protection. encompasses more derived forms leading to proper, with early members like Cartorhynchus lenticarpus showing amphibious traits such as flexible flippers. Recent discoveries, such as the large-bodied Baisesaurus robustus from the Luolou Formation in 2022, indicate additional basal relatives potentially outside these core subgroups but within Ichthyosauromorpha. Historically, ichthyosaurs were classified in isolation as , with hupehsuchians treated as enigmatic diapsids lacking clear affinities until 2014–2015, when fossils linking the groups were described. Prior to this, marine reptiles were not recognized as a cohesive stem group, and ichthyosaurs were viewed separately from other secondarily aquatic clades. Synapomorphies uniting Ichthyosauromorpha include neural arches lacking transverse processes, proximally slender and non-pachyostotic dorsal ribs, and median gastral elements with long, sharp anterior processes. Aquatic adaptations such as incipient flipper limbs and pachyosteosclerosis (bone thickening for ) emerged within the , particularly in derived members. Nomenclaturally, Ichthyosauromorpha differs from the node-based , which excludes stem taxa like hupehsuchians and focuses on the crown group of true ichthyosaurs. Recent studies incorporating new Chinese fossils, including a 2023 hupehsuchian exhibiting advanced in the manus, have reinforced the clade's and refined its boundaries without altering the core definition. A 2025 study described Lentamanusuchus hubeiensis, a hupehsuchian with widely spaced autopodia, providing new insights into early limb adaptations.

Phylogenetic relationships

Ichthyosauromorpha represents a of reptiles defined as the last common ancestor of nanchangensis and communis, and all its descendants, encompassing and as sister groups. Within , forms part of the broader stem-based Ichthyosauromorpha, a lineage exhibiting euryapsid-like cranial morphology but recognized as an independent aquatic radiation distinct from other major marine groups such as . This separation highlights parallel evolutionary trajectories among Early Triassic , with Ichthyosauromorpha specializing in fully pelagic niches while explored more coastal and durophagous adaptations. Phylogenetic analyses have consistently placed as the sister taxon to within Ichthyosauromorpha, based on shared features such as elongated neural spines and polydactylous limbs in basal members. The matrix of Motani et al. (2015), incorporating 41 taxa and 128 characters, supports this topology with moderate Bremer support (values of 2–3), positioning basal ichthyosauromorphs like Cartorhynchus lenticarpus as transitional forms bridging hupehsuchians and more derived ichthyosaurs. Alternative hypotheses linking Ichthyosauromorpha to archosauromorphs, such as through purported affinities with prolacertiforms, have been debated but largely refuted by subsequent datasets favoring a basal placement outside . As a stem-based , Ichthyosauromorpha includes pre-ichthyosaurian "experimental" forms as well as the derived crown-group . These stem ichthyosauromorphs, such as those from the Spathian stage of the , document an early burst of morphological disparity, with aberrant body plans indicating rapid adaptation to marine environments shortly after the end-Permian extinction. Controversies persist regarding the inclusion of certain taxa, such as Wumengosaurus delicatomus, which some analyses position as a sister to Ichthyosauromorpha due to shared aquatic traits like elongated bodies, while others classify it within Pachypleurosauria based on vertebral morphology. Similarly, Thaisaurus chonglakmani is debated as a basal ichthyosauromorph or an early ichthyosauriform, with its incomplete preservation complicating resolution in phylogenetic matrices. Recent 2020s studies, including the description of Baisesaurus robustus from the Luolou Formation, have refined basal topologies using high-resolution osteological data, strengthening the sister-group relationship between Hupehsuchia and Ichthyosauriformes while excluding problematic taxa. Ichthyosauromorpha originated from terrestrial basal akin to younginiforms (e.g., Youngina capensis), as evidenced by plesiomorphic traits like amphiuma-like skulls in earliest forms, marking one of multiple independent aquatic transitions among alongside those of sauropterygians and thalattosaurs. Outgroup comparisons in broad diapsid matrices, using taxa such as kansensis and Hovasaurus boulei, underscore this basal position, with Ichthyosauromorpha diverging prior to the split between Lepidosauromorpha and .

Paleobiology

Locomotion and ecology

Ichthyosauromorphs exhibited diverse swimming mechanics adapted to their aquatic environments, with basal forms relying on undulatory tail for locomotion. Species such as Cartorhynchus lenticarpus from the possessed elongated, flexible bodies that facilitated eel-like undulation, allowing maneuverability in shallow coastal waters. In contrast, derived evolved a more advanced thunniform swimming style, characterized by an oscillating caudal fluke that generated thrust through rapid lateral movements. Hydrodynamic models and analyses of vertebral flexibility indicate that these derived forms could achieve efficient cruising speeds comparable to those of modern tunas for their size, enabling efficient long-distance travel in open oceans. Their streamlined body plans and limb-derived flippers further minimized drag during , enhancing overall hydrodynamic . Ecologically, ichthyosauromorphs functioned as versatile predators across coastal and pelagic habitats throughout the . evidence, including coprolites containing scales, hooks, and shell fragments, reveals a diet dominated by , squid-like cephalopods, and ammonites, reflecting opportunistic feeding strategies suited to abundant marine prey. Certain taxa, such as ophthalmosaurid ichthyosaurs, adapted to deeper-water niches, as evidenced by their disproportionately large eyes—up to 23 cm in —which provided enhanced vision in low-light conditions, supporting predation on vertically migrating prey. These adaptations underscore their role in structuring marine food webs, from nearshore ambush hunting in basal forms to sustained pursuits in open seas by more specialized lineages. Recent analyses of vertebral microstructure suggest the development of endothermy-like physiology supporting sustained pelagic lifestyles. Buoyancy control in ichthyosauromorphs was primarily managed through skeletal modifications, including pachyosteosclerosis, which involved increased compactness and volume to increase overall body . This condition allowed for , counteracting the positive from air-filled lungs and enabling sustained submergence without excessive energy expenditure on active trim adjustment. The ribcage structure, featuring elongated and flexible , further supported this by permitting expansion for air intake during periodic surfacing, as inferred from articulated skeletons showing respiratory adaptations to marine existence. Later derived forms trended toward reduced for improved agility, but pachyosteosclerosis remained prominent in basal ichthyosauromorphs inhabiting shallower environments. As apex predators in marine ecosystems, ichthyosauromorphs occupied top trophic levels, preying on large-bodied vertebrates including other marine reptiles and fishes. The Thalattoarchon saurophagis, exceeding 8 m in length with robust dentition for dismembering prey, exemplifies this role, dominating faunas shortly after the Permian-Triassic extinction. Contemporaneous competition with thalattosaurs and early sauropterygians likely influenced niche partitioning, as these groups co-occurred in nearshore assemblages but diverged in prey preferences and habitat use, contributing to the rapid assembly of complex trophic networks. Stable isotope analyses of and bioapatite provide of fully aquatic lifestyles among ichthyosauromorphs, with δ¹⁸O values consistent with equilibration to marine devoid of freshwater influences (typically 17–22‰ in phosphate oxygen). These isotopic signatures indicate habitation in warm-water habitats with seawater temperatures of 20–30°C, consistent with their distribution in tropical to subtropical paleoenvironments during the and . Such data corroborate a commitment to pelagic existence, with minimal reliance on coastal or terrestrial resources.

Reproduction

Ichthyosauromorphs are known to have been viviparous, giving live birth rather than laying eggs, based on exceptional preservation of gravid females containing embryos or neonates within the . The earliest evidence comes from the basal ichthyosauriform Chaohusaurus from the Nanlinghu Formation in Province, , where a specimen preserves three embryos positioned as if during birth, with one inside the , one partially exiting the , and one outside the mother. These embryos, measuring about 18 cm in length (roughly 18% of adult size), exhibit advanced developmental features such as partial of the and isodont dentition, indicating prolonged internal gestation rather than early-stage eggs. In more derived , birth orientation shifted to tail-first in many cases, likely an adaptation to minimize drowning risk during delivery in aquatic environments, as seen in genera like Stenopterygius and Mixosaurus. Litter sizes in varied but typically ranged from 2 to 10 offspring, with embryos developing to advanced stages within the mother before birth. For instance, Jurassic specimens of from the preserve up to eight embryos within the of the adult, each about 7 cm long and showing well-formed vertebrae, forefins, and ribs consistent with late . Smaller litters of 1–2 are recorded in basal forms like , while examples such as contain fewer, physically diminutive embryos that fill the body cavity, underscoring internal development without of external egg-laying. No direct fossils of reproductive organs have been preserved, limiting insights into fertilization or duration. Sexual dimorphism appears subtle and rare in the fossil record, potentially expressed in differences in fin shape or skeletal robusticity rather than pronounced size disparities. Studies of Stenopterygius suggest variation in humeral and forelimb proportions, with one morphotype showing more robust limbs possibly linked to sex, though stratigraphic data are often lacking to confirm this. Such traits may have influenced swimming efficiency or mate selection, but evidence remains indirect without associated gravid specimens. The adoption of in ichthyosauromorphs occurred early in the , around 248 million years ago, predating the evolution of live birth in cetaceans by over 190 million years and contrasting sharply with the egg-laying strategies of basal reptiles. This reproductive mode likely originated in terrestrial ancestors before full aquatic adaptation, as indicated by head-first birth orientation in basal taxa like Chaohusaurus, a pattern inherited from land-dwelling amniotes. Key specimens from sites like Anhui Province continue to illuminate these early evolutionary transitions, highlighting as a critical for .

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