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Ankylopollexia
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Ankylopollexia
Temporal range: Late Jurassic–Late Cretaceous, 156.3–66 Ma
Six ankylopollexian ornithopods (top left to bottom right): Shantungosaurus, Iguanodon, Tethyshadros, Uteodon, Olorotitan, Gongpoquansaurus
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Clade: †Ornithischia
Clade: †Ornithopoda
Clade: †Dryomorpha
Clade: †Ankylopollexia
Sereno, 1986
Subgroups

Ankylopollexia is an extinct clade of ornithischian dinosaurs that lived from the Late Jurassic to the Late Cretaceous. It is a derived clade of iguanodontian ornithopods 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 likely synapomorphic feature of a conical thumb spine defines the clade.[2]

First appearing around 156 million years ago, in the Jurassic, Ankylopollexia became an extremely successful and widespread clade during the Cretaceous, and were found around the world. The group died out at the end of the Maastrichtian.[1] They grew to be quite large, comparable to some carnivorous dinosaurs and they were universally herbivorous.[3]

Size

[edit]
Size of three ankylopollexians (Edmontosaurus, Iguanodon, and Camptosaurus) compared to other ornithopods

Ankylopollexians varied greatly in size over the course of their evolution.[citation needed]. Jurassic genus Camptosaurus 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 late Campanian age (around 70 million years ago), belonged to the hadrosaurid family, and is named Shantungosaurus. 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]

Life restoration of Iguanacolossus

Primitive ankylopollexians tended to be smaller as compared to the larger, more derived hadrosaurs. There are, however, exceptions to this trend. A single track from a large ornithopod, likely a relative of Camptosaurus, was reported from the Lourinhã Formation, dating to the Jurassic in Portugal. The corresponding animal had an estimated hip height of around 2.8 metres (9.2 ft), much larger than the contemporary relative Draconyx.[7] The primitive styracosternan Iguanacolossus 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 of Hadrosauroidea, the Chinese genus Bolong, is estimated to have been around 200 kilograms (440 lb).[8] Another exception of this trend is Tethyshadros, 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 by insular dwarfism.[9] In addition a 44 cm scapula belonging to an ankylopollexian has been found in the Lourinhã Formation[10] the length of the scapula indicates an animal similar in size to Camptosaurus.

Classification

[edit]
Hand of Iguanodon, showing the distinctive thumb of the group

About 157 million years ago, Ankylopollexia and Dryosauridae 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, a phylogenetic definition was given: the last common ancestor of the species Camptosaurus dispar and Parasaurolophus walkeri and all its descendants. Ankylopollexia would receive a formal PhyloCode definition as "the smallest clade containing Camptosaurus dispar and Iguanodon bernissartensis".[12] This clade contains the two subclades Camptosauridae and Styracosterna, which are both defined using Camptosaurus dispar and Iguanodon bernissartensis, creating a node-stem triplet with Ankylopollexia.[12]

The cladogram below follows the phylogenetic analysis of Bertozzo et al. (2017).[13]

Ankylopollexia

Palaeobiology

[edit]

Brain

[edit]
Brain endocast of an Iguanodon, created in 1897 from specimen NHMUK R2501

The neurobiology of ankylopollexians has been studied as far back as 1871, when a well preserved cranium (specimen NHMUK R2501[14]) discovered in September 1869 from the Wealden Group on the Isle of Wight and tentatively referred to the genus Iguanodon was described by John 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 an endocast was created and studied.[16] This was not the first endocast of an ankylopolloxian brain, for in 1893, the skull of a Claosaurus annectens (today referred to the genus Edmontosaurus[17]) was used by Othniel 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 among ornithischian 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 as Edmontosaurus annectens, Edmontosaurus regalis, and Gryposaurus notabilis (at that time thought to be a synonym of its relative Kritosaurus) 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]

A 1905 diagram showing the small size of an Edmontosaurus annectens brain (bottom; alongside that of Triceratops horridus, top) commented on in early sources

James Hopson investigated the encephalization quotients (EQs) of various dinosaurs in 1977 study. Three ornithopods for which brain endocasts had previously been produced – Camptosaurus, Iguanodon, and Anatosaurus (now known as Edmontosaurus 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 of carnosaurian theropods and of modern crocodilians, but far lower than that of coelurosaurian theropods. The latter two genera, which lived later than Camptosaurus, had somewhat higher EQs than the Jurassic taxon, which, being at the lower end, was more comparable to the ceratopsian genus Protoceratops. Reasonings suggested for their comparably high intelligence were the need for acute senses in the lack of defensive weapons, and more complex intraspecific behaviours as indicated by their acoustic and visual display structures.[21]

In a first for any terrestrial fossil vertebrate, Brasier et al. (2017) reported mineralized soft tissues from the brain of an iguanodontian dinosaur, from the Valanginian age (around 133 million years ago) Upper Tunbridge Wells Formation at Bexhill, Sussex. Fragmentary ornithopod remains were associated with the fossil, and though assigning the specimen to any one taxon with certainty wasn't possible, Barilium or Hypselospinus were put forward as likely candidates. The specimen compared well to endocasts of similar taxa, such as one from a Mantellisaurus on display at the Oxford University Museum of Natural History. Detailed observations were made with the use of a scanning electron microscope. Only some parts of the brain were preserved; the cerebellar and cerebral expansions were best preserved, whereas the olfactory lobes and medulla oblongata were missing or nearly so. The neural 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]

Endocast of an Amurosaurus brain in right lateral (A), dorsal (B), and ventral (C) views

The advent of CT 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 various lambeosaurine 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 the cerebral hemispheres was, for the first time, remarked upon, being far larger than in other ornithischians and all large saurischian dinosaurs; maniraptorans Conchoraptor and Archaeopteryx 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] Lambeosaurine Amurosaurus 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% in Amurosaurus, significantly lower than in Hypacrosaurus, closer to that of theropods like Tyrannosaurus, 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]

Palaeobiogeography

[edit]
Life restoration of Camptosaurus

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 to Asia and Europe. An early example is the Chinese genus Bayannurosaurus, from the Berriasian.[24] The oldest genus, found in Wyoming, is Camptosaurus dispar, which dates to around the Callovian-Oxfordian, about 156-157 million years ago.[25]

References

[edit]
[edit]
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Ankylopollexia

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Ankylopollexia is an extinct clade of ornithischian dinosaurs within Iguanodontia, defined as the smallest clade containing dispar and bernissartensis. The name derives from the Greek ankylos (meaning "stiff" or "bent") and Latin pollex (meaning ""), referring to the characteristic stiff, cone-shaped thumb spike that evolved in this group for defense or display. Members of Ankylopollexia were large-bodied herbivores that lived from the to the epochs, spanning approximately 156 to 66 million years ago, and achieved a global distribution across , , , and . Phylogenetically, Ankylopollexia represents a derived branch of iguanodontians, positioned as the to within Dryomorpha, stemward to Hadrosauriformes, and more derived than basal groups like Tenontosauridae. It encompasses basal forms such as and Uteodon (often grouped in Camptosauridae) as well as the Styracosterna, which includes iguanodontids like and more derived hadrosauroids leading to duck-billed dinosaurs. The was first named informally by Sereno in 1986 and formally defined in 1998 as the node uniting and , though subsequent revisions by Norman (2014) and others refined its boundaries to emphasize its role in iguanodontian diversification. Ankylopollexians exhibited a range of adaptations for terrestrial herbivory, including robust limbs supporting bipedal to facultatively quadrupedal locomotion and specialized for efficient grinding of material. Body sizes varied from medium (around 4–6 meters in basal forms like ) to massive (up to 10–12 meters in advanced iguanodontids), with some species featuring sail-like neural spines for or display, as seen in . Their fossils, often found in and coastal deposits, indicate they inhabited diverse environments from forests to riverine systems, contributing significantly to terrestrial ecosystems before declining with the hadrosaurid radiation in the . Recent discoveries, including new taxa from in 2023 and 2025, continue to refine understanding of basal ankylopollexian diversity.

Etymology and Definition

Naming

The clade Ankylopollexia was first named by paleontologist Paul C. Sereno in 1986 as a monophyletic subgroup within Iguanodontia, diagnosed primarily by shared derived morphology of the manus, including modifications to the thumb. The name derives from the Greek word ankylos (meaning stiff or fused) combined with the Latin pollex (thumb) and the suffix -ia (indicating a group), alluding to the characteristic stiff, cone-shaped thumb structure observed in derived members. This enlarged, conical ungual of the pollex represents the primary autapomorphy that unites the .

Phylogenetic Definition

Ankylopollexia is a of ornithischian dinosaurs defined as the smallest containing dispar Marsh, 1879, and bernissartensis Boulenger in Beneden, 1881, and all descendants of their . This phylogenetic definition was originally proposed by Sereno in 1998 using Parasaurolophus walkeri Parks, 1923, as the second specifier and formally revised by Madzia et al. (2021) to better reflect the clade's content. Within the broader ornithischian phylogeny, Ankylopollexia represents a derived lineage of iguanodontian ornithopods, positioned as the sister group to , with their divergence estimated around 157 million years ago during the . Together, Ankylopollexia and form the node-based clade Dryomorpha, which branches from more basal iguanodontians such as rhabdodontoids. The clade encompasses a range of subclades, including basal forms assigned to Camptosauridae—such as Camptosaurus and related taxa like Uteodon—and more derived groups within Styracosterna, which includes hadrosauroids and their descendants leading to the duck-billed hadrosaurids. This internal structure highlights Ankylopollexia's evolutionary progression from facultatively bipedal herbivores in the Jurassic to more specialized, often quadrupedal forms in the Cretaceous.

Description

Anatomy

Ankylopollexians exhibit distinctive cranial morphology adapted for herbivory, featuring an elongated preorbital region that extends the forward, enhancing reach during feeding. The dentary houses a series of closely packed, leaf-shaped teeth with asymmetrical crowns, typically bearing a prominent median primary ridge flanked by smaller secondary ridges or denticles, facilitating the grinding and shearing of tough material. These dental features represent an advancement over more basal ornithopod , allowing for efficient processing of fibrous . The postcranial of Ankylopollexia is characterized by robust limb s, including a strong pectoral with a bearing a deltoid nearly parallel to its long axis and a with a prominent deltopectoral crest, supporting capabilities. The manus displays asymmetrical proportions, with the enlarged pollex modified into a conical ungual spine—likely serving defensive or manipulative functions during —and reduced outer digits (IV and V) that are vestigial or absent. Additionally, the pelvic shows fusion of sacral ribs to the ilium, bolstering stability for facultative quadrupedality. Diagnostic autapomorphies of the include caudal vertebrae with chevron facets where the distal articulation surface is significantly larger than the proximal one, contributing to flexibility and support. The further features an ulnar flange wrapping around the and partial fusion of carpals, enhancing forearm rigidity. These traits underscore the clade's evolutionary shift toward more robust, versatile locomotion. Basal ankylopollexians, such as , possess simpler dental arrangements with a single functional row of replacement teeth, differing markedly from the elaborate, multi-layered dental batteries in derived hadrosauroids that enable rapid tooth replacement and continuous wear.

Size

Ankylopollexia encompasses a broad spectrum of body sizes, ranging from modest basal iguanodontians to some of the largest ornithischian dinosaurs. Basal taxa, such as Bolong yixianensis from the of , represent smaller forms, estimated at approximately 4 m in length and 200–300 kg in body mass based on the . Similarly, dispar from the of attained lengths of about 5 m and masses around 0.5 tonnes, as inferred from skeletal scaling and biomechanical analyses of partial skeletons. Uteodon aphanoecetes, another basal ankylopollexian from the same formation, is estimated to have reached 6–7 m in length, comparable to closely related camptosaurids. Likewise, Oblitosaurus bunnueli from the of reached an estimated 6–7 m in length, representing one of the largest basal ankylopollexians from . Among more derived ankylopollexians, size variation persists, with Iguanacolossus fortis from the Cedar Mountain Formation of representing a larger basal hadrosauroid at roughly 9 m in length, based on partial skeletal reconstructions. In contrast, Tethyshadros insularis from the of exemplifies , with adults measuring about 4 m in length and weighing approximately 350 kg, as determined from nearly complete skeletons suggesting adaptation to an island environment in the . The clade's upper size limit is exemplified by advanced hadrosauroids like Shantungosaurus giganteus from the of China, which achieved lengths of 14.7–16.6 m and masses up to 16 tonnes, estimated through volumetric modeling of multiple specimens from bonebed assemblages. histology in larger ankylopollexians, such as the hadrosaurid Maiasaura peeblesorum, reveals rapid juvenile growth rates, with highly vascularized woven tissue indicating accelerated somatic expansion during early to attain adult sizes quickly. This pattern of fast growth is consistent across derived hadrosauroids, supporting high metabolic rates and enabling the evolution of gigantism in the clade.

Taxonomy

History

The discovery of ankylopollexian dinosaurs traces back to early 19th-century explorations of iguanodontian fossils in , with the genus first recognized in 1825 by Gideon Algernon Mantell based on teeth collected from the Wealden Group in , . These initial finds, which included fragmentary remains suggesting large herbivorous reptiles, marked the beginning of understanding basal ornithopods but were initially classified under broader reptilian categories without recognizing the specific . In , key specimens emerged in the late 1870s from the , where described Camptosaurus dispar in 1879 from bones collected in , providing early evidence of basal ankylopollexian morphology such as robust limbs and dental adaptations. The clade Ankylopollexia was formally established in 1986 by Paul C. Sereno, who introduced it as part of a cladistic framework for ornithischian phylogeny, emphasizing shared derived traits like a stiffened thumb in iguanodontians. This naming built on prior iguanodontian studies but shifted focus toward monophyletic groups rather than traditional Linnaean hierarchies, incorporating Camptosaurus as a reference taxon. Early classifications grouped these taxa loosely under Iguanodontidae, but Sereno's work highlighted their position as a stem to more derived forms like hadrosauroids. By the 1990s, the adoption of cladistic methods revolutionized iguanodontian , moving away from paraphyletic Linnaean arrangements toward explicit phylogenetic hypotheses that integrated new discoveries from and . For instance, finds such as basal forms from the Lower of and reassessed European specimens from the of prompted revisions that expanded Ankylopollexia's scope, recognizing its global distribution and diversity beyond North American Morrison deposits. These shifts, driven by analyses from researchers like David B. Norman, emphasized character-based clades over rank-based systems. In 2021, revisions to ornithischian phylogenetic nomenclature by Daniel Madzia and colleagues formally defined Ankylopollexia as the smallest clade containing Camptosaurus dispar and Iguanodon bernissartensis, confirming its status and resolving ambiguities from earlier informal uses. Subsequent discoveries, such as Oblitosaurus bunnueli from the Upper of described in 2023, have further expanded understanding of basal ankylopollexian diversity in .

Phylogeny

Phylogenetic analyses of Ankylopollexia, based on comprehensive morphological datasets, consistently recover the as a monophyletic group within Iguanodontia, sister to within the broader Dryomorpha. Recent studies utilizing a 323-character matrix across 73 operational taxonomic units (OTUs) demonstrate that Ankylopollexia originated in the , encompassing approximately 20 valid genera distributed across various formations from the to the stages. These analyses, employing both parsimony (yielding 114 most parsimonious trees of 1394 steps) and , highlight key synapomorphies such as a reduced or absent premaxillary and tightly packed cheek teeth with tapered crowns. Within Ankylopollexia, Uteodon and Camptosaurus represent successive basal outgroups to the more derived clade Styracosterna, forming a grade of early-diverging taxa characterized by relatively primitive postcranial features, including less robust forelimbs compared to later members. Camptosauridae emerges as a major basal branch, including Camptosaurus from the Morrison Formation and Iguanacolossus from North America, distinguished by features like a prominent deltopectoral crest on the humerus and moderately sized body plans adapted for bipedal locomotion. Styracosterna, in contrast, comprises more advanced lineages, including Iguanodontidae and Hadrosauriformes; the latter encompasses non-hadrosaurid forms such as Ouranosaurus from the Aptian of Africa and the duck-billed hadrosaurs, exemplified by Parasaurolophus from the Campanian of North America, marked by innovations like increased maxillary tooth count (18-28) and the absence of an antorbital fenestra. Debates persist regarding the exact placement of certain fragmentary taxa as basal ankylopollexians. For instance, Orthomerus dolloi from the of , known from isolated appendicular elements, has been variably positioned as a basal iguanodontian or within Ankylopollexia, though recent reassessments suggest it may be a due to insufficient diagnostic material. Similarly, Barilium dawsoni from the of is often recovered as a basal styracosternan in robust-forelimb clades alongside taxa like Hypselospinus and , but its precise affinities remain contentious owing to incomplete specimens and varying character scorings in phylogenetic matrices. These uncertainties underscore the need for additional discoveries to refine branching patterns at the base of the .

Distribution and Ecology

Temporal Range

Ankylopollexia fossils span from the to the , encompassing the and stages approximately 156.3 million years ago to the stage ending at 66 million years ago. This extended duration reflects the clade's evolutionary success as a diverse group of ornithopods, with basal members appearing early and advanced forms persisting until the end of the . The earliest known records of Ankylopollexia include basal European taxa such as Draconyx loureiroi and Oblitosaurus bunnueli from the Lourinhã and Villar del Arzobispo Formations in and , dating to approximately 150 million years ago, alongside dispar from . These initial appearances mark the origin of the within iguanodontian ornithopods, transitioning from more primitive dryosaurids. Diversity within Ankylopollexia peaked during the Barremian–Aptian interval of the , with a proliferation of styracosternan forms following lower counts in the and earliest . This surge gave way to a relative decline in basal ankylopollexian lineages through the , as the became increasingly dominated by derived hadrosaurs. The final representatives, including and , occurred in the , with the extinguishing at the 66 million years ago.

Geographic Range

Fossils of Ankylopollexia are primarily known from Laurasian landmasses, with the clade exhibiting a widespread distribution across , , and from the to the , alongside limited records from northern . This distribution reflects an origin within , where vicariance associated with the breakup of the supercontinent contributed to the divergence of North American and Eurasian lineages during the . Gondwanan occurrences are rare, represented mainly by African taxa such as from the Aptian-aged in . In , basal ankylopollexians like are documented from the in states such as and , while more derived forms, including hadrosauroids such as Gryposaurus and , occur in the of , . European records include basal ankylopollexians such as Draconyx loureiroi and Oblitosaurus bunnueli from the Lourinhã and Villar del Arzobispo Formations in and , as well as evidence of large-sized indeterminate forms from the Upper Jurassic of (as of 2025), and well-known taxa such as Iguanodon from the Wealden Group in , highlighting faunal similarities with contemporaneous North American assemblages. Asia hosts the highest diversity of ankylopollexians, particularly during the , with basal forms like Bayannurosaurus perfectus from the Berriasian Zuunbayan locality in , , and more advanced hadrosauroids such as Bolong yixianensis and Jinzhousaurus yangi from the in . Late Cretaceous representatives include the giant hadrosaurid Shantungosaurus giganteus from the Wangshi Group in . These Asian sites underscore the clade's and adaptation across diverse fluvial and lacustrine environments.

Paleobiology

Locomotion and Feeding

Basal members of Ankylopollexia, such as , exhibited facultative , allowing them to alternate between bipedal and quadrupedal postures depending on activity, a trait inherited from more primitive ornithischians. This locomotor flexibility supported efficient foraging and escape behaviors in environments. In contrast, more derived forms within the , particularly hadrosaurs, shifted toward predominant quadrupedality, evidenced by osteological adaptations including robust forelimbs with columnar metacarpals capable of bearing significant body weight. Trackway evidence from hadrosaur localities reveals walking speeds typically ranging from 1 to 3 m/s, with stride lengths and pes-manus impressions indicating a stable, four-limbed gait suited to traversing varied terrains. Feeding in Ankylopollexia was adapted for herbivory through specialized dental mechanisms, featuring a battery of tightly packed, diamond-shaped teeth that formed a shearing surface for processing tough plant matter. These teeth, with asymmetrical enamel and precise occlusion between upper and lower jaws, enabled efficient grinding and slicing, as inferred from microwear patterns showing transverse jaw motion during mastication. The prominent thumb spines on the forelimbs of iguanodontians like Iguanodon likely served dual purposes, potentially aiding in stripping foliage from branches or providing defense against predators by delivering stabbing wounds. Dietary reconstructions for Ankylopollexia draw from analyses and biomechanical studies of function, indicating a browser's diet dominated by ferns, cycads, and gymnosperms in and taxa, with later hadrosaurs incorporating emerging angiosperms such as leaves and fruits. associated with ornithopod localities contain undigested plant fragments, including fern spores and cycad pollen, supporting selective feeding on low- to mid-height in forested or habitats. mechanics further suggest adaptations for cropping and pulverizing fibrous stems, allowing exploitation of a broad range of unavailable to less specialized herbivores. Social behavior in Ankylopollexia is inferred from monospecific bone beds, such as those of at Bernissart, , where clusters of over 30 individuals of varying ages suggest or gregarious living to enhance predator avoidance and resource access. These accumulations, preserved in a single depositional event, indicate group mortality—possibly from flash floods—rather than isolated deaths, pointing to coordinated movement in social units. Such aligns with trackway associations showing multiple individuals traveling together, reinforcing interpretations of dynamics in derived ankylopollexians.

Brain Structure

The of Ankylopollexia is primarily inferred from natural and artificial endocasts, which provide impressions of the brain cavity, as well as rare instances of preserved . These structures reveal a progressive expansion of the and relative to basal ornithopods, with the cerebral hemispheres showing lateral expansion and increased volume in derived forms such as iguanodontians and hadrosaurs. This evolutionary trend contributed to higher encephalization quotients (EQs) in ankylopollexians, ranging approximately from 1 to 3.7 in hadrosaurs, exceeding those of most other groups and indicating relatively greater -to-body mass ratios. Early studies of ankylopollexian brain structure date to the late , with from skulls collected from the Bernissart in revealing prominent olfactory bulbs and tracts, suggestive of a well-developed . Similarly, of (formerly Anatosaurus) from deposits in exhibit small olfactory bulbs comprising about 5% of the volume, underscoring a conserved trait across the for olfactory processing. A landmark discovery in 2016 involved mineralized brain tissues adhering to an from a iguanodontian specimen near Bexhill, , preserving fine details of cerebral folds and vascular impressions that confirm the filled a substantial portion of the endocranial cavity. Derived ankylopollexians, particularly hadrosaurs, display neuroanatomical adaptations for advanced sensory integration, including expanded optic lobes indicative of enhanced and elongated lagenae suggesting improved hearing sensitivity. The , a cerebellar structure prominent in endocasts of active species like , supported gaze stabilization and balance during bipedal or quadrupedal locomotion. Compared to basal ornithopods such as those in , ankylopollexian brains exhibit greater complexity, with larger cerebral hemispheres and more pronounced olfactory and cerebellar regions, features that likely facilitated sophisticated social interactions in herd-living hadrosaurs.

References

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