Evolutionary history Ichthyosaur

1 evolutionary history

1.1 origin
1.2 affinity hupehsuchia
1.3 ichthyopterygia
1.4 later triassic forms
1.5 jurassic
1.6 cretaceous

evolutionary history
origin

grippia longirostris triassic of spitsbergen well-adapted aquatic lifestyle.

the origin of ichthyosaurs contentious. clear transitional forms land-dwelling vertebrate groups have not yet been found, earliest known species of ichthyosaur lineage being aquatic. in 1937, friedrich von huene hypothesised ichthyosaurs not reptiles, instead represented lineage separately developed amphibians. today, notion has been discarded , consensus exists ichthyosaurs amniota , descended terrestrial egg-laying tetrapoda during late permian or earliest triassic. however, has proven difficult establish position within amniote evolutionary tree. there several conflicting hypotheses on subject. in second half of twentieth century, had been usual assume ichthyosaurs anapsida, seen branch of primitive reptiles. explain appearance of ichthyosaurs in fossil record, , lack of clear affinities other reptile groups, anapsids supposed little specialised. hypothesis has become unpopular being inherently vague because anapsida unnatural paraphyletic group. modern exact quantitative cladistic analyses consistently indicate ichthyosaurs diapsida. studies showed basal, or low, position in diapsid tree. more analyses result in being neodiapsida, derived diapsid subgroup. of these, majority show them basal lepidosauromorpha; minority show them basal archosauromorpha.

until 1980s, close relationship assumed between ichthyosauria , sauropterygia, marine reptile group, within overarching euryapsida. today, however, rejected, euryapsida being seen unnatural polyphyletic assemblage of reptiles happen share adaptations swimming lifestyle. exception study in 1997 john merck, showed monophyletic archosauromorph euryapsids.

affinity hupehsuchia

hupehsuchus

since 1959, second enigmatic group of ancient sea reptiles known, hupehsuchia. ichthyopterygia, hupehsuchia have pointed snout , show polydactyly, possession of more 5 fingers or toes. limbs more resemble of land animals, making them appear transitional form between these , ichthyosaurs. initially, possibility largely neglected because hupehsuchia have fundamentally different form of propulsion, extremely stiffened trunk. similarities explained case of convergent evolution. furthermore, descent of hupehsuchia no less obscure, meaning possible close relationship hardly clarify general evolutionary position of ichthyosaurs.

in 2014, cartorhynchus announced, small species short snout, large flippers, , stiff trunk. lifestyle might have been amphibious. motani found more basal ichthyopterygia , named encompassing clade ichthyosauriformes. latter group combined hupesuchia ichthyosauromorpha. ichthyosauromorphs found diapsids.

the proposed relationships shown cladogram:

early ichthyopterygia

the earliest ichthyosaurs known , early-middle (olenekian , anisian) triassic strata of canada, china, japan, , spitsbergen in norway, being 246 million years old. these first forms included genera chaohusaurus, grippia, , utatsusaurus. diversity suggests earlier origin. more resembled finned lizards fishes or dolphins later, more familiar species similar. bodies elongated , used anguilliform locomotion, swimming undulations of entire trunk. land animals, pectoral girdles , pelves robustly built, , vertebrae still possessed usual interlocking processes support body against force of gravity. however, rather advanced in having limbs had been transformed flippers. warm-blooded , viviparous.

these proto-ichthyosaurs had such distinctive build compared ichthyosaurs proper motani excluded them ichthyosauria , placed them in basal position in larger clade, ichthyopterygia. however, solution not adopted researchers.

later triassic forms

mixosaurus

the basal forms gave rise ichthyosaurs in narrow sense sometime around boundary between triassic , middle triassic; earliest ichthyosauria in sense motani gave concept, appear 245 million years ago. these later diversified variety of forms, including still sea serpent-like cymbospondylus, problematic form reached ten metres in length, , smaller, more typical forms mixosaurus. mixosauria fish-like pointed skull, shorter trunk, more vertical tail fin, dorsal fin, , short flippers containing many phalanges. sister group of mixosauria more advanced merriamosauria. late triassic, merriamosaurs consisted of both large, classic shastasauria , more advanced, dolphin-like euichthyosauria. experts disagree on whether these represent evolutionary continuum, less specialised shastosaurs paraphyletic grade evolving more advanced forms, or whether 2 separate clades evolved common ancestor earlier on. euichthyosauria possessed more narrow front flippers, reduced number of fingers. basal euichthyosaurs californosaurus , toretocnemus. more derived branch parvipelvia, reduced pelvis, basal forms of hudsonelpidia , macgowania.

shonisaurus popularis

during carnian , norian, shastosauria reached huge sizes. shonisaurus popularis, known number of specimens carnian of nevada, 15 m (49 ft) long. norian shonisauridae known both sides of pacific. himalayasaurus tibetensis , tibetosaurus (probably synonym) have been found in tibet. these large (ten fifteen metres long) ichthyosaurs have been placed genus shonisaurus. gigantic shonisaurus sikanniensis (sometimes considered shastasaurus sikanniensis) remains found in pardonet formation of british columbia elizabeth nicholls, has been estimated as 21 m (69 ft) in length—if correct, largest marine reptile known date.

in late triassic, ichthyosaurs attained peak of diversity. occupied many ecological niches. apex predators, others hunters of small prey. several species perhaps specialised in suction feeding or ram feeders; durophagous forms known. towards end of late triassic, decline of variability seems have occurred. giant species seemed have disappeared @ end of norian. rhaetian (latest triassic) ichthyosaurs known england, , these similar of jurassic. possible explanation increased competition sharks, teleostei, , first plesiosauria. dinosaurs, ichthyosaurs , contemporaries, plesiosaurs, survived end-triassic extinction event, , diversified again fill vacant ecological niches of jurassic.

jurassic

stenopterygius resembled modern dolphin

during jurassic, ichthyosaurs still showed large variety of species, ranging 1 ten metres in length. epoch, original british discoveries made, among ichthyosaurs, names ones familiar general public. genera include eurhinosaurus, ichthyosaurus, leptonectes, stenopterygius, , large predator temnodontosaurus, along basal parvipelvian suevoleviathan, little changed norian ancestors. general morphological variability had been reduced, however. giant forms, suction feeders , durophagous species absent. of these animals streamlined, dolphin-like forms, although more basal animals perhaps more elongated advanced , compact stenopterygius , ichthyosaurus. latter belonged parvipelvian subgroup, thunnosauria. these adapted efficient thunniform locomotion, propelling end of tail only, equipped vertical tail fin. front flippers had more 5 fingers. parvipelvian branch eurhinosauria leptonectes , eurhinosaurus, specialised forms having elongated , pointy snout.

few ichthyosaur fossils known middle jurassic. might result of the, in general, poor fossil record of epoch. strata of late jurassic seem indicate further decrease in diversity had taken place. ichthyosaurs belonged thunnosaurian clade ophthalmosauria. represented 4 m-long (13 ft) ophthalmosaurus , related genera, similar in general build ichthyosaurus. eyes of ophthalmosaurus huge, , these animals hunted in dim , deep water. however, new finds cretaceous indicate ichthyosaur diversity in late jurassic must have been underestimated.

cretaceous

many ichthyosaur lineages continued cretaceous



restoration of platypterygius kiprijanovi – albian-cenomanian of kursk region (russia)

traditionally, ichthyosaurs seen decreasing in diversity further cretaceous, though had worldwide distribution. fossils period referred single genus: platypterygius. last ichthyosaur genus thought have become extinct in late cretaceous, during cenomanian 95 million years ago. ichthyosaurs have disappeared earlier other large mesozoic reptile groups survived until end of cretaceous. 2 major explanations given this. firstly, have been matter of chance. second explanation had extinction caused competition. less hydrodynamically efficient animals, mosasauridae , long-necked plesiosaurs, flourished. ichthyosaurian overspecialisation contributing factor extinction, possibly being unable keep fast-swimming , highly evasive new teleost fish, had become dominant @ time, against sit-and-wait ambush strategies of mosasaurids proved superior. model emphasised evolutionary stagnation, innovation shown platypterygius being ten fingers.

recent studies, however, show ichthyosaurs far more diverse in cretaceous thought. fragments referred platypterygius in fact represented diverse species. in 2012, concluded @ least 8 lineages spanned jurassic-cretaceous boundary, among them acamptonectes, sveltonectes, caypullisaurus, , maiaspondylus. in 2013, cretaceous basal thunnosaurian revealed: malawania. indeed, there radiation during cretaceous due increase of coastlines when continents further broke up.

the demise of ichthyosaurs has been described two-step process. first extinction event in beginning of cenomanian eliminated 2 of 3 ichthyosaur feeding guilds present, soft-prey specialists , generalists , leaving apex predator group, not particularly specialized. second major ichthyosaur extinction took place during cenomanian-turonian boundary event, anoxic event , eliminating apex predators, after single lineage survived, platypterygius hercynicus, disappeared shortly afterwards, 93 million years ago. ichthyosaur extinction pair of abrupt events rather long decline, related environmental upheavals , climatic changes in cenomanian , turonian. if so, not caused mosasaurid competition; large mosasaurs did not appear until 3 million years after ichthyosaur extinction, fill resulting ecological void. plesiosaurian polycotylidae perhaps filled of niches occupied ichthyosaurs, although had coexisted 19 million years. extinction result of ecological change , volatility caused changes in migration, food availability, , birthing grounds. part of cretaceous 1 in many other marine extinctions occurred, including of types of microplankton, ammonites, belemnites, , reef-building bivalves.