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Hadrosauromorpha
Hadrosauromorpha
Hadrosauromorpha
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Hadrosauromorphs
Temporal range: Late Cretaceous, 96.7–66 Ma
Skeleton of Gobihadros
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Dinosauria
Clade: †Ornithischia
Clade: †Ornithopoda
Clade: †Hadrosauriformes
Superfamily: †Hadrosauroidea
Clade: †Hadrosauromorpha
Norman, 2015 vide Norman, 2014
Subgroups[1][2][3]

Hadrosauromorpha is a clade of iguanodontian ornithopods, defined in 2014 by David B. Norman to divide Hadrosauroidea into the basal taxa with compressed manual bones and a pollex, and the derived taxa that lack them. The clade is formally defined in the PhyloCode as "the largest clade containing Hadrosaurus foulkii, but not Probactrosaurus gobiensis".[4] This results in different taxon inclusion depending on the analysis.

Classification

[edit]

Hadrosauromorpha was first used in literature by David B. Norman in 2014 in a discussion of phylogenetics of Hypselospinus. In his 2014 paper Norman references another of his publications as the authority for Hadrosauromorpha, a chapter in the book Hadrosaurs.[1] However, the book was in fact published later, in 2015.[5] Following Article 19.4 of the PhyloCode, the authorship of the clade is thus Norman (2015), while the authorship of the definition is Norman (2014).[6]

Definition

[edit]

Hadrosauromorpha was first given a phylogenetic definition by Norman (2014 and 2015) as hadrosauroid taxa closer to Edmontosaurus regalis than Probactrosaurus gobiensis.[1][5] This definition was contested by Mickey Mortimer, who stated that to follow the PhyloCode the taxon Hadrosaurus must be included in the definition, as it is the type genus of Hadrosauromorpha.[7] By this definition, Norman (2015) considered Hadrosauromorpha to include Hadrosauridae, as well as the taxa Tethyshadros and Bactrosaurus.[5] Norman in 2014 had included more taxa in Hadrosauromorpha, those of Norman (2015) as well as Levnesovia, Gilmoreosaurus and Telmatosaurus, the last of which was considered inside Hadrosauridae by Norman in 2015.[1] Another phylogenetic analysis by Xing et al. in 2014 also found that Eolambia and Protohadros, both found outside Hadrosauromorpha by Norman, fell within his definition, as well as a large number of other taxa.[2]

Phylogeny

[edit]

Many different versions of phylogenies have been conducted on the group of hadrosauromorphs.[2] Norman (2014) created his own analysis, which includes 105 different morphological characters and 27 select ornithopod taxa. His phylogeny is shown below, using his specific clade definitions:[1]

Hadrosauriformes (Altirhinus + Edmontosaurus)

Norman's definitions have been heavily criticized by Mickey Mortimer as being unnecessary changes which cause more confusion to classification.[7] Other phylogenetic analyses, like the results of Madzia et al. in 2020, have placed Hadrosauromorpha in the middle of a long grade of stem-hadrosaurs, without any large groups of taxa unlike previous versions of the same analysis, apart from a large group of Eurasian taxa.[8]

Hadrosauroidea

Description

[edit]
Skeleton of Probactrosaurus, the taxon just outside Hadrosauromorpha

Probactrosaurus was selected as the outgroup to Hadrosauromorpha because of numerous differences that Norman (2014) thought to be significant. The tooth crowns in the dentary are asymmetrical and have multiple vertical ridges; there is a foramen in the surangular; and the quadrate bone has a more prominent depression for the articulation of the jugal. None of these features are found in the skulls of the more derived hadrosauromorphans. The premaxilla contacts the prefrontal, and the jugal contact with the ectopterygoid bone of the palate is reduced.[1]

In the appendicular regions, hadrosauromorphans the scapulae are not J-shaped, instead having an overhanging projection. The lower forelimb bones are more slender in both hadrosauromorphans and Probactosaurus, unlike their more robust ancestors. Probactosaurus, however, possesses the basal condition of having a small, conical pollex, like in earlier ornithopods such as Iguanodon or Hypselospinus. This absence of a pollex is also linked to a reduction of the carpal bones, and a less mobile manus. The ilium bones of Probactrosaurus are more angular than in hadrosauromorphs, which lack a brevis shelf. It was also identified that the femoral shaft is straight in hadrosauromorphans and the pedal bones are truncated[1]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hadrosauromorpha

View on Grokipedia
from Grokipedia
Hadrosauromorpha is a of ornithischian dinosaurs within the ornithopod subgroup Iguanodontia, defined as the largest complete crown containing Hadrosaurus foulkii Leidy, 1858, but not Probactrosaurus gobiensis Rozhdestvensky, 1966. This phylogenetic definition, established by David B. Norman in 2014, distinguishes advanced ornithopods more closely related to the duck-billed hadrosaurs from earlier basal forms. Members of Hadrosauromorpha represent a key evolutionary lineage of herbivorous dinosaurs that thrived during the period, bridging simpler iguanodontian ancestors to the highly specialized hadrosaurids. The encompasses a range of basal hadrosauroids and the derived family , including taxa such as Bactrosaurus johnsoni, Datonglong tianzhenensis, Eolambia caroljonesa, Gilmoreosaurus mongoliensis, Jeyawati rugoculus, Jintasaurus meniscus, Levnesovia transoxiana, Nanyangosaurus zhugeii, Orthomerus dolloi, Plesiohadros djadokhtaensis, Protohadros byrdi, Shuangmiaosaurus gilmorei, Tanius sinensis, Tethyshadros insularis, and Zhanghenglong yangchengensis. These dinosaurs are primarily known from discoveries in Laurasian landmasses, with records spanning the (e.g., Protohadros from ) to the , when hadrosaurids became dominant herbivores in North American, Asian, and European ecosystems. Hadrosauromorpha played a pivotal role in ornithischian , exhibiting progressive adaptations in cranial and dental morphology that facilitated efficient herbivory. Basal members retained relatively simple and structures akin to earlier iguanodontians, while trends toward the complex, multi-row dental batteries of hadrosaurids emerged within the , enabling the processing of tougher . This evolutionary innovation contributed to the ecological success of hadrosauromorphs, which often reached lengths of 5–12 meters and weights exceeding several tons in their more derived forms.

Taxonomy

Definition and Diagnosis

Hadrosauromorpha is a of iguanodontian ornithopods phylogenetically defined as the largest complete crown containing Parasaurolophus walkeri Parks, 1923, but not Probactrosaurus gobiensis (Rozhdestvensky, 1966). This crown definition, initially proposed by David B. Norman in 2014, was updated in 2015 to a stem-based definition as Edmontosaurus regalis (Lambe, 1917) and all taxa sharing a more recent common ancestor with E. regalis than with Probactrosaurus gobiensis. The 2015 stem-based definition adheres to the standards of the and emphasizes the evolutionary lineage leading to advanced hadrosauroids while excluding more basal iguanodontians like Probactrosaurus. The was initially outlined by Norman in 2014 as part of broader ornithopod revisions. Key diagnostic traits of Hadrosauromorpha distinguish it from basal iguanodontians through modifications associated with enhanced herbivory and bipedal efficiency. These include slender forelimbs with a reduced pollex (thumb) ungual, reflecting a shift away from quadrupedal support; straight femoral shafts lacking the sigmoid curvature typical of outgroups like Probactrosaurus; and precursors to the complex dental battery, such as the development of multiple functional tooth rows and prismatic tooth morphology in more derived members. Compared to Probactrosaurus, hadrosauromorph femora exhibit reduced lateral bowing and a straighter overall profile, with limb proportions showing a relatively longer tibia to femur ratio (approximately 0.95–1.05 in core taxa versus 0.85–0.90 in Probactrosaurus), supporting greater cursorial capabilities. The core membership of Hadrosauromorpha encompasses (the crowngroup of duck-billed dinosaurs), along with basal genera such as Bactrosaurus (Gilmore, 1933) from and Tethyshadros (Dalla Vecchia, 2009) from . Phylogenetic analyses have variably included additional taxa like Eolambia (Kirkland, 1998) and Protohadros (Head, 1998) within the , particularly in studies recovering them as closer to hadrosaurids than to Probactrosaurus.

Historical Development

The concept of Hadrosauromorpha emerged from early paleontological efforts to classify ornithopod dinosaurs, initially grouping hadrosaurs closely with iguanodonts. In 1881, included hadrosaurs within a broad , alongside iguanodonts and camptosaurids, reflecting a pre-cladistic approach that emphasized shared anatomical features like bipedal locomotion and herbivorous dentition without strict phylogenetic boundaries. This lumping persisted into the early , but subsequent revisions began to separate hadrosaurs; had established the family in 1869 based on cranial and dental specializations, while Richard Swann Lull's 1904 monograph on North American hadrosaurs further distinguished them from basal iguanodonts through detailed osteological comparisons. ’s 1912 description of osborni highlighted unique crested morphologies, reinforcing the split by emphasizing derived traits absent in earlier iguanodonts. The advent of cladistic methods in the late transformed these groupings, shifting from Linnaean hierarchies to node- and stem-based definitions that revealed in traditional "hadrosauroids." Jason Head's 1998 analysis of Protohadros byrdi positioned it as a basal hadrosaurid, implicitly establishing Hadrosauroidea as a stem-based encompassing forms more closely related to than to , thus critiquing earlier paraphyletic assemblages. This was formalized in Horner, Weishampel, and Forster's 2004 chapter, which defined Hadrosauroidea phylogenetically to include pre-hadrosaurid ornithopods with advanced dental batteries, integrating fossil evidence from Asia and to highlight its stem-group nature. David B. Norman's 2014 phylogenetic revision refined this framework by introducing Hadrosauromorpha as a maximum-clade (crown) definition, comprising hadrosauroid taxa closer to than to Probactrosaurus gobiensis. Norman's 2015 update changed it to a stem-based definition with as the internal specifier, addressing inconsistencies in prior stem definitions and incorporating new data on basal iguanodontians to emphasize clade stability amid increasing taxon sampling. Recent nomenclatural updates, such as Madzia et al.'s 2020 reassessment of Orthomerus dolloi, have underscored Hadrosauromorpha's role in a diverse stem-hadrosaur grade, particularly incorporating Eurasian taxa like those from the of the to resolve long-branch attractions in prior analyses.

Phylogeny

Position in Ornithopoda

represents a major clade of ornithischian dinosaurs characterized by herbivorous adaptations and bipedal to quadrupedal locomotion, with origins tracing back to the Late Jurassic. Basal iguanodontians such as , known from formations like the in , exemplify early ornithopods with slender builds and dental batteries suited for grinding vegetation. By the , ornithopods had diversified into more advanced forms, including larger-bodied iguanodontians that exhibited enhanced cranial and dental specializations for complex herbivory. Hadrosauromorpha constitutes a derived within Iguanodontia, the dominant subgroup of during the . Defined phylogenetically as the largest containing Hadrosaurus foulkii but not Probactrosaurus gobiensis, it encompasses basal hadrosauroids and the crown group . This likely stemmed from ancestors resembling Tenontosaurus, a basal ornithopod from the of that shares primitive iguanodontian features such as a robust postcranial and early dental complexity, positioning Hadrosauromorpha between more basal iguanodonts like Iguanodon and the specialized duck-billed hadrosaurids. Phylogenetic analyses consistently recover Hadrosauromorpha as a monophyletic group within Hadrosauroidea, supported by synapomorphies including compressed manual digits and advancements in jaw mechanics for efficient mastication. Key evidence for this positioning derives from cladistic analyses incorporating cranial, dental, and postcranial characters, which demonstrate Hadrosauromorpha's divergence from basal iguanodontians through shared herbivorous adaptations such as increased row length and incipient dental batteries. Time-calibrated phylogenetic trees indicate that Hadrosauromorpha originated in the , around 100 Ma, with subsequent radiation leading to by the . These trees highlight a gradual evolutionary progression from generalized ornithopod ancestors toward the highly derived feeding mechanisms of hadrosaurids. In comparison to non-hadrosauromorph iguanodontians such as , Hadrosauromorpha exhibits distinct pelvic modifications, including a more elongate and slender pubis with reduced proximal robusticity, reflecting adaptations for quadrupedal support and locomotion efficiency absent in the broader, less specialized pelvic girdle of .

Major Clades and Relationships

Hadrosauromorpha encompasses a diverse array of basal ornithopods that form a paraphyletic grade leading to the monophyletic Hadrosauridae. Phylogenetic analyses, such as those employing extensive character matrices, position several genera as stem hadrosauromorphs, including Bactrosaurus johnsoni from the of and Tethyshadros insularis from the of . These taxa exhibit transitional features between more primitive iguanodontians and derived hadrosaurids, such as incipient dental batteries and modifications to the jaw apparatus. The crown group is robustly supported as a defined by advanced cranial adaptations for complex mastication, splitting into two primary subclades: and . includes non-crested forms like and , characterized by solid cranial structures, while comprises crested taxa such as and , with hollow nasal crests linked to vocalization or thermoregulation. Eolambia caroljonesa, from the Early of , serves as a transitional form near the base of , bridging stem hadrosauromorphs and the crown through features like an elongate dentary and partial dental battery development, as shown in updated matrices incorporating postcranial data. Several taxa remain debated within Hadrosauromorpha due to fragmentary remains and conflicting character scorings, particularly Protohadros byrdi from the Cenomanian of Texas. Its advanced dental features, including a complex battery with multiple tooth rows and high-crowned teeth suggestive of hadrosaurid-level grinding efficiency, have led to proposals for its inclusion as an early hadrosaurid; however, recent analyses favor a stem position based on primitive postcranial traits. Prieto-Márquez et al. (2019) resolved early divergences by highlighting separate Asian and North American lineages, with Bactrosaurus anchoring an Asian stem clade and North American forms like Eolambia and Protohadros contributing to the radiation of Hadrosauridae, supported by Bayesian tip-dating that estimates branch lengths reflecting Cenomanian origins. Consensus topologies from phylogenetic studies consistently depict Hadrosauromorpha as a ladder-like grade, with low to moderate support values (Bremer indices of 1–3) for basal nodes but higher posterior probabilities (0.8–1.0) for the crown in Bayesian analyses. For instance, matrices yield a strict consensus where Bactrosaurus and Tethyshadros branch sequentially before Eolambia, leading to the Saurolophinae– split, while Prieto-Márquez et al. (2019) incorporate evolutionary rate shifts to refine intercontinental relationships, emphasizing rapid skull diversification post-Santonian.

Anatomy

Cranial Characteristics

Hadrosauromorphs exhibit a distinctive morphology adapted for advanced herbivory, characterized by an elongated preorbital region that lengthens progressively during , accommodating larger orbits and facilitating enhanced sensory capabilities. This elongation contributes to a more streamlined facial profile compared to basal iguanodontians, with the premaxillae often forming a broadened, edentulous beak-like structure for cropping . The quadrate features a ball-and-socket articulation with the articular, permitting mediolateral rotation of the mandibular corpus and enabling a wide gape essential for processing tough plant material. The dental system in hadrosauromorphs represents a precursor to the complex batteries of derived hadrosaurids, featuring multiple rows of replacement teeth that differ markedly from the simpler, single-row of basal iguanodonts. Non-hadrosaurid hadrosauromorphs typically possess up to three per tooth family, arranged in a shallow battery that allows for gradual wear and replacement, supporting efficient grinding of fibrous . In more advanced forms, this evolves toward deeper batteries with increased tooth rows—up to five in some taxa—enhancing mastication through heterochronic shifts that elongate the dental arcade during growth. Crest variations are prominent in derived hadrosauromorphs, particularly within , where lambeosaurines display hollow nasal formed by the premaxillae and nasals, enclosing elongated and chambered nasal passages. These structures, often S-shaped or tubular, increase the surface area for respiratory and sensory functions. In contrast, saurolophines bear solid composed solely of nasals, lacking internal cavities and serving primarily structural or display roles without enclosing the nasal apparatus. Biomechanically, the hollow lambeosaurine likely amplified olfactory sensitivity by expanding epithelial surfaces, while solid forms provided minimal additional support. Sensory features in hadrosauromorph skulls include large orbits indicative of well-developed vision, with the expansive orbital regions supporting acute suited to complex social environments. Braincase expansions, particularly in lambeosaurines, feature elongated olfactory tracts integrated with nasal crest chambers, enhancing chemosensory detection for foraging and social interactions. These adaptations reflect evolutionary pressures for heightened sensory integration in herbivorous lifestyles.

Postcranial Skeleton

The postcranial skeleton of hadrosauromorphs exhibits adaptations supporting a versatile bipedal-quadrupedal lifestyle, with a robust axial column and specialized limb girdles. The axial skeleton comprises a series of elongated cervical vertebrae, typically numbering 10–15 (up to 18 in some derived forms), that provide enhanced neck flexibility through strongly opisthocoelous centra and reduced zygapophyses, as documented in basal taxa like Tanius sinensis and Gobihadros mongoliensis. The dorsal vertebral series is robust and often features tall neural spines, particularly in basal forms such as Tanius sinensis, which help accommodate the expanded ribcage and support the voluminous gut necessary for fermenting plant material. The pelvic girdle is characterized by a retroverted pubis that forms a prominent prepubic , a synapomorphy distinguishing hadrosauromorphs from more basal ornithopods like , where the pubis remains propubic. This retroversion partially closes the and is evident in specimens of Lophorhothon atopus, where partial pubes preserve the caudally directed shaft and expanded distal blade. The ilium typically bears a long preacetabular and a pronounced supraacetabular crest for muscle attachment, enhancing stability during locomotion. Limb proportions reflect facultative quadrupedality, with slender forelimbs adapted for auxiliary support. The and are gracile relative to the hindlimbs, and the manus retains three functional digits (II-IV) with reduced phalangeal counts, as seen in hadrosauroids like Bactrosaurus johnsoni, where digit I is vestigial or absent. In contrast, the hindlimbs are robustly constructed for primary weight-bearing, featuring a straight femoral shaft—a derived trait in hadrosauromorphs—with the comprising approximately 50-60% of total length in basal forms like Tanius sinensis. The overall length often represents 50-60% of body length, supporting efficient bipedal progression when needed. Size variation across Hadrosauromorpha underscores evolutionary trends toward gigantism in advanced lineages. Basal hadrosauromorphs, such as Tanius sinensis and Probactrosaurus mazongshanensis, typically measure 5–8 m in length, with volumetric body mass estimates around 1–3.5 tons based on skeletal scaling and . Advanced hadrosaurs, including giganteus, reach up to 13 m in length, with mass estimates exceeding 7 tons derived from volumetric reconstructions that account for torso expansion and limb robusticity. These estimates highlight the clade's diversification in body size during the .

Paleobiology

Locomotion and Behavior

Hadrosauromorphs exhibited a range of locomotor strategies, with basal forms primarily employing bipedal locomotion while more derived taxa, particularly within , demonstrated facultative quadrupedality. Early hadrosauromorphs, such as iguanodontians, relied on bipedal s for efficient movement, supported by elongated hindlimbs relative to forelimbs. In contrast, advanced hadrosaurids could shift to quadrupedal stances during slow progression or foraging, using robust forelimbs to bear weight, as evidenced by trackways preserving both pedal-only (bipedal) and manus-pedal (quadrupedal) impressions. A notable example comes from the Jindong Formation in Korea, where a trackway of a quadrupedal ornithopod—likely a hadrosauromorph—indicates this transitional during low-speed activities. This bipedal-to-quadrupedal shift reflects in ornithischians, enhancing stability for larger body sizes without sacrificing agility. Locomotor capabilities varied with body size and phylogeny, with limb proportions suggesting moderate speeds for hadrosauromorphs compared to more agile theropods. Analyses of hindlimb-to-foot ratios and stride lengths from trackways indicate that hadrosaurids could achieve bursts of speed up to 20-30 km/h, particularly in subadult individuals with proportionally longer lower limbs. These estimates derive from biomechanical models incorporating skeletal scaling, which highlight hadrosaurs' relatively elongated femora and tibiae as adaptations for rapid escape from predators, though their bulky builds limited sustained high velocities. Trackway data further confirm gaits transitioning from walking quadrupedality to bipedal trotting at higher speeds. Social behavior in hadrosauromorphs is inferred from monodominant bonebeds, which suggest herding as a primary for predator avoidance and resource exploitation. In peeblesorum, multiple bonebeds preserve mixed-age assemblages, indicating gregarious living in large groups where juveniles and adults coexisted, likely forming migratory herds during seasonal movements. Cranial crests in lambeosaurine hadrosaurids, such as , likely served as visual and auditory display structures, facilitating intra-group communication, mate attraction, and individual recognition through resonant vocalizations or visual signaling. Evidence for nesting and parental care is particularly robust in hadrosaurids, with colonies revealing structured family units. Fossil nests contain eggs, hatchlings, and juveniles alongside adults, implying extended biparental care where young remained dependent for months post-hatching, growing rapidly under protection. This , evidenced by numerous nests (at least 15) in close proximity, suggests colonial nesting that minimized predation risks and allowed for communal defense, with juveniles often found in age-segregated groups within the colony.

Diet and Feeding Mechanisms

Hadrosauromorphs were herbivores adapted to process tough, fibrous through specialized cranial and dental features that facilitated efficient mastication. Their diet primarily consisted of low-lying , including ferns, cycads, and in earlier floras, with evidence from coprolites and stomach contents indicating opportunistic consumption of emerging angiosperms in later ecosystems. This low-browse strategy is inferred from their battery-like and jaw morphology, which suited ground-level on abrasive plant matter rather than high . The mechanics of hadrosauromorphs enabled transverse chewing, a motion powered by pleurokinetic skulls where the maxillae rotated laterally during occlusion, guided by robust pterygoid flanges that stabilized the adductor musculature. In advanced hadrosaurs, this mechanism worked in concert with a complex dental battery, comprising stacked columns of up to 60 alveolar positions per quadrant, each holding multiple replacing teeth that formed a continuous grinding surface; patterns on these teeth, often showing heavy attrition from foods, confirm their role in pulverizing plant material. Adult hadrosaurids possessed over 1,000 teeth in total across both , with continual eruption and replacement ensuring sustained functionality despite rapid . Gastrolith evidence further supports a digestive system involving mechanical breakdown and potential microbial . In basal ornithopods, clusters of small, polished stones (averaging 8 mm in diameter, totaling up to 180 per individual) were found in the abdominal regions of articulated skeletons, positioned to function as a gastric mill for triturating ingested in a bird-like . These , primarily igneous and quartzose, imply that hadrosauromorphs ingested stones to aid of fibrous , enhancing extraction through grinding prior to hindgut . The evolutionary progression of feeding in hadrosauromorphs reflects increasing specialization from basal forms to derived hadrosaurs. Early ornithopods relied on simple cropping with single-row dentition and limited jaw mobility for shearing soft vegetation, as seen in taxa like with low-crowned, denticle-bearing teeth. Over time, iguanodontian hadrosauromorphs developed multi-row teeth and enhanced transverse motion, culminating in the hadrosaurid dental battery and full grinding capability, which allowed processing of tougher, more abrasive flora and contributed to their ecological dominance. This shift is evidenced by increasing dental complexity and disparity rates in the fossil record, correlating with body size growth and dietary breadth.

Distribution

Temporal Range

The temporal range of Hadrosauromorpha spans from the stage of the Early Late Cretaceous to the end of the . The earliest records date to approximately 100 million years ago during the stage, represented by the basal hadrosauroid Protohadros byrdi from the Woodbine Formation in . This taxon provides the oldest definitive evidence of the clade in , marking the initial diversification of hadrosauromorphs following the evolution of more basal iguanodontians. Hadrosauromorpha achieved peak diversity during the , particularly in the and stages (approximately 83 to 66 million years ago), when numerous genera and species proliferated across . This radiation coincided with the widespread proliferation of angiosperms, which expanded into diverse habitats and likely influenced the dietary adaptations and ecological success of hadrosauromorphs. The clade became extinct during the end-Cretaceous mass extinction event at 66 million years ago, triggered by the Chicxulub asteroid impact and associated environmental catastrophes. The final records occur in uppermost deposits, including the in , where hadrosauromorphs such as represent some of the last non-avian dinosaurs. Biostratigraphic correlations refine this timeline through key formations worldwide. In western , the of , , dated to the middle (76.5–74.4 million years ago), yields diverse hadrosauromorph assemblages including and , aiding precise dating via and radiometric methods. In Asia, the of , assigned to the late (approximately 70 million years ago), contains advanced hadrosauromorphs like and Barsboldia, with its age constrained by isotopic dating of associated theropod remains and sedimentary correlations. These formations highlight the global extent and temporal resolution of the hadrosauromorph fossil record.

Geographic Occurrence

Hadrosauromorph fossils are predominantly known from Laurasian continents, with the majority of discoveries concentrated in and during the . North American occurrences are particularly abundant in formations of the western interior, such as the Judith River Group in , where multiple hadrosauromorph taxa including Gryposaurus and have been documented from fluvial and floodplain deposits. Similarly, the in has yielded extensive remains of peeblesorum, including nesting sites that highlight localized population densities in coastal plain environments. In , the region of and preserves some of the earliest and most diverse hadrosauromorph assemblages, reflecting an origin and early radiation on this continent. Key sites include the Bayan Shireh Formation, which has produced basal hadrosauroids like Gobihadros mongoliensis, known from nearly complete skeletons in riverine sediments dating to the Cenomanian-Turonian. Nearby, the contains Bactrosaurus johnsoni, a primitive hadrosauromorph with robust postcranial elements suggestive of adaptation to arid floodplains. European records are sparser but indicate insular within the fragmented Tethyan . The most notable example is Tethyshadros insularis from the Liburnian Formation (Calcare di Aurisina) in northeastern , represented by an articulated juvenile skeleton that points to in isolated island settings. This underscores limited but recurrent dispersals from Asian mainland populations across marine barriers. Dispersal patterns show strong Laurasian dominance, with hadrosauromorphs originating in before radiating to via Beringian land connections during the . Rare Gondwanan outliers include a partial hadrosaurid dentary from the late Lopez de Bertodano Formation on Vega Island, , representing a late southward migration across widening seaways between and the . Biogeographic models, such as those employing dispersal-vicariance , suggest that post-Pangean continental fragmentation isolated populations, promoting in and facilitating opportunistic incursions into southern landmasses via ephemeral land bridges.

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