| Ankylopollexia | |
|---|---|
 | |
| Six ankylopollexian ornithopods (top left to bottom right):Shantungosaurus,Iguanodon,Tethyshadros,Uteodon,Olorotitan,Gongpoquansaurus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Clade: | †Dryomorpha |
| Clade: | †Ankylopollexia Sereno,1986 |
| Subgroups | |
| |
Ankylopollexia is anextinctclade ofornithischiandinosaurs that lived from theLate Jurassic to theLate Cretaceous. It is a derived clade ofiguanodontianornithopods and contains the subgroup Styracosterna.[1] The name stems from the Greek word, “ankylos”, mistakenly taken to mean stiff, fused (in fact the adjective means bent or curved; used of fingers, it can mean hooked), and the Latin word, “pollex”, meaning thumb. Originally described in 1986 by Sereno, a most likelysynapomorphic feature of a conical thumb spine defines the clade.[2]
First appearing around 156 million years ago, in theJurassic, Ankylopollexia became an extremely successful and widespread clade during theCretaceous, and were found around the world. The group died out at the end of theMaastrichtian.[1] They grew to be quite large, comparable to some carnivorous dinosaurs and they were universallyherbivorous.[3]
Ankylopollexians varied greatly in size over the course of their evolution.[citation needed]. Jurassic genusCamptosaurus was small, no more than 5 metres (16 ft) in length and half a tonne in weight.[4] The largest known ankylopollexian, dating to the lateCampanian age (around 70 million years ago), belonged to the hadrosaurid family, and is namedShantungosaurus. It was around 14.7 metres (48 ft) to 16.6 metres (54 ft) in length and weighed, for the largest individuals, up to 16 tonnes (18 short tons).[5][6]
Primitive ankylopollexians tended to be smaller as compared to the larger, more derivedhadrosaurs. There are, however, exceptions to this trend. A single track from a large ornithopod, likely a relative ofCamptosaurus, was reported from theLourinhã Formation, dating to theJurassic inPortugal. The corresponding animal had an estimated hip height of around 2.8 metres (9.2 ft), much larger than the contemporary relativeDraconyx.[7] The primitive styracosternanIguanacolossus was named for its distinct robustness and large size, likely around 9 metres (30 ft) in length.[citation needed] Regarding hadrosaurs, one of the more basal members ofHadrosauroidea, theChinese genusBolong, is estimated to have been around 200 kilograms (440 lb).[8] Another exception of this trend isTethyshadros, a more derived genus of Hadrosauroidea. Estimated to have weighed 350 kilograms (770 lb),Tethyshadros have been found only on certain islands in Italy. Its diminutive size is explained byinsular dwarfism.[9] In addition a 44 cm scapula belonging to an ankylopollexian has been found in the lourinha formation[10] the length of the scapula indicates an animal similar in size to camptosaurus.
About 157 million years ago, Ankylopollexia andDryosauridae are believed to have split into separate evolutionary branches.[11] Originally named and described in 1986 by Paul Sereno, Ankylopollexia would receive a phylogenetic definition in a later paper by Sereno in 2005.[2] In the 1986 paper, the groups Camptosauridae and Styracosterna were used to define the clade, but in the 2005 paper, aphylogenetic definition was given: the last common ancestor of the speciesCamptosaurus dispar andParasaurolophus walkeri and all its descendants. Ankylopollexia would receive a formalPhyloCode definition as "the smallest clade containingCamptosaurus dispar andIguanodon bernissartensis".[12] This clade contains the two subclades Camptosauridae and Styracosterna, which are both defined usingCamptosaurus dispar andIguanodon bernissartensis, creating a node-stem triplet with Ankylopollexia.[12]
The cladogram below follows the phylogenetic analysis of Bertozzoet al. (2017).[13]
| Ankylopollexia |
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Theneurobiology of ankylopollexians has been studied as far back as 1871, when a well preserved cranium (specimenNHMUK R2501[14]) discovered in September 1869 from theWealden Group on theIsle of Wight and tentatively referred to the genusIguanodon was described byJohn Hulke. He noted that due to the lesser correlation of the shape of the brain and wall of cranial cavity in reptiles, any deduction of the shape of the brain of the animal would be approximate.[15] The referral of this skull was reinforced in a later study, published in 1897. It was here inquired that the brain of the dinosaur may have been more closely associated to the cavity than that of modern reptiles, and so anendocast was created and studied.[16] This was not the first endocast of an ankylopolloxian brain, for in 1893, the skull of aClaosaurus annectens (today referred to the genusEdmontosaurus[17]) was used byOthniel Charles Marsh to create a cast of the brain cavity. Some basics remarks were made, including the small size of the organ, but interpreting minute features of the organ was noted to be difficult.[18] The 1897 paper noted the similarity of the two endocasts.[16]
Hadrosaurs have been noted as having the most complex brains among ankylopollexians, and indeed amongornithischian dinosaurs as a whole. The brains of a large variety of taxa have been studied.John Ostrom, would, in 1961, provide what was then the most extensive and detailed review and work on hadrosaur neuro-anatomy. This area of hadrosaur study was in its infancy at this point, and only the species known today asEdmontosaurus annectens,Edmontosaurus regalis, andGryposaurus notabilis (at that time thought to be a synonym of its relativeKritosaurus) had specimens suitable at the time to be examined (Lambeosaurus was listed as having a briefly described braincase, but this was a mistake originating in Lull and Wright (1942)).[19][20] Ostrom supported the view that the brains of hadrosaurs and other dinosaurs would've likely only filled a portion of the cranial cavity, therefore hindering the ability to learn from endocasts, but noted they were still useful. He noted, similar to Marsh, noted the small predicted size of the organ, but also that it was significantly developed. A number of similarities to the brains of modern reptiles were noted.[20]
James Hopson investigated theencephalization quotients (EQs) of various dinosaurs in 1977 study. Three ornithopods for which brain endocasts had previously been produced –Camptosaurus,Iguanodon, andAnatosaurus (now known asEdmontosaurus annectens[17]) – were investigated. It was found that they had relatively high EQs compared to many other dinosaurs (ranging from 0.8 to 1.5), comparable to that ofcarnosauriantheropods and of moderncrocodilians, but far lower than that ofcoelurosaurian theropods. The latter two genera, which lived later thanCamptosaurus, had somewhat higher EQs than theJurassic taxon, which, being at the lower end, was more comparable to theceratopsian genusProtoceratops. Reasonings suggested for their comparably high intelligence were the need for acute senses in the lack of defensive weapons, and more complexintraspecific behaviours as indicated by their acoustic and visual display structures.[21]
In a first for any terrestrial fossilvertebrate, Brasieret al. (2017) reported mineralized soft tissues from the brain of an iguanodontian dinosaur, from theValanginian age (around 133 million years ago)Upper Tunbridge Wells Formation atBexhill,Sussex. Fragmentaryornithopod remains were associated with the fossil, and though assigning the specimen to any one taxon with certainty wasn't possible,Barilium orHypselospinus were put forward as likely candidates. The specimen compared well to endocasts of similar taxa, such as one from aMantellisaurus on display at theOxford University Museum of Natural History. Detailed observations were made with the use of ascanning electron microscope. Only some parts of the brain were preserved; thecerebellar andcerebral expansions were best preserved, whereas theolfactory lobes andmedulla oblongata were missing or nearly so. Theneural tissues seemed to be very tightly packed, indicating an EC closer to five (with hadrosaurs having even higher ECs), nearly matching that of the most intelligent non-avian theropods. Though it was noted this was in-line with their complex behaviour, as had been noted by Hopson, it was cautioned the dense packing may have been an artifact of preservation, and the original lower estimates were considered more accurate. Some of the complex behaviours ascribed can be seen to some extent in modern crocodilians, who fall near the original numbers.[14]
The advent ofCT scanning for use in palaeontology has allowed for more widespread application of this without the need for specimen destruction. Modern research using these methods has focused largely on hadrosaurs. In a 2009 study by palaeontologist David C. Evans and colleagues, the brains of variouslambeosaurine hadrosaur genera were scanned and compared to each other, related taxa, and previous predictions. Contra the early works, Evans' studies indicate that only some regions of the hadrosaur brain were loosely correlated to the brain wall. As with previous studies, EQ values were investigated; even the lowest end of the determined EQ range was still higher than that of modern reptiles and most non-maniraptoran dinosaurs, though fell well short of maniraptorans themselves. The size of thecerebral hemispheres was, for the first time, remarked upon, being far larger than in other ornithischians and all largesaurischian dinosaurs; maniraptoransConchoraptor andArchaeopteryx had very similar proportions. This lends further support to the idea of complex behaviours and relatively high intelligence, for non-avian dinosaurs, in hadrosaurids.[19] LambeosaurineAmurosaurus was the subject of a 2013 paper once again looking into a cranial endocast. A once again high EQ range was found, higher than that of living reptiles,sauropods and other ornithischians, but different EQ estimates for theropods were cited, placing the hadrosaur numbers significantly below the majority of theropods. Additionally, the relative cerebral volume was only 30% inAmurosaurus, significantly lower than inHypacrosaurus, closer to that of theropods likeTyrannosaurus, though still distinctly larger than previously estimated numbers for more primitive iguanodonts. This demonstrated a previously unrecognized level of variation in neuro-anatomy within Hadrosauridae.[22]
Ankylopollexians would in the Cretaceous become one of the most successful groups on the planet, being both widespread and numerous in nature.[23] Around this time, ankylopollexians spread toAsia andEurope. An early example is theChinese genusBayannurosaurus, from theBerriasian.[24] The oldest genus, found inWyoming, isCamptosaurus dispar, which dates to around theCallovian-Oxfordian, about 156-157 million years ago.[25]
Data related toIguanodontia at Wikispecies
