Small-bodied Dinosaurs Research Articles

An extraordinary fossil captures the struggle for existence during the Mesozoic

Dinosaurs and mammals have coexisted for the last ~ 230 million years. Both groups arose during the Late Triassic and diversified throughout the Mesozoic and into the Cenozoic (the latter in the form of birds). Although they undoubtedly interacted in many ways, direct fossil evidence for their interaction is rare. Here we report a new fossil find from the Lujiatun Member of the Lower Cretaceous Yixian Formation of China, showing a gobiconodontid mammal and psittacosaurid dinosaur locked in mortal combat. We entertain various hypothesized explanations for this association, but the balance of the evidence suggests that it represents a predation attempt on the part of the smaller mammal, suddenly interrupted by, and preserved within, a lahar-type volcanic debris flow. Mesozoic mammals are usually depicted as having lived in the shadows of their larger dinosaurian contemporaries, but this new fossil convincingly demonstrates that mammals could pose a threat even to near fully-grown dinosaurs. The Yixian Formation—and the Chinese fossil Jehol Biota more broadly—have played a particularly important role in revealing the diversity of small-bodied dinosaurs and other fauna. We anticipate that the volcanically derived obrution deposits specific to the Lujiatun Member will likewise continue to yield evidence for biotic interactions otherwise unknown from the rest of the fossil record.

New dinosaur swim tracks from the Middle Jurassic Zhanghe Formation in Yunnan, China

ABSTRACT Recently, dinosaur tracks were discovered in the vicinity of Zhemujiu village in Yimen County, Yunnan Province, representing the first dinosaur footprints found in the purplish-red sandy mudstone of Lower Jurassic Zhanghe Formation. This marks a significant discovery as evidence of dinosaurs from this formation has previously only been reported from body fossils. Nearly 100 tracks were observed at the site and two morphotypes were identified: Characichnos-like morphotype A, and Wintonopus-like morphotype B. Some dinosaurs scraped the substrate with the distal portions of their digits (morphotype A), while others punted by pushing either the digital plantar surface against the sediment or plunging their digits vertically into the sediment (morphotype B). These track morphotypes suggest small-bodied dinosaurs swam (fully buoyant) and punted (partially buoyant) aqueous palaeo-environments (respectively). The orientation of these Zhemujiu tracks indicates that some dinosaurs were moving along the same axis as the flow of the water while others were moving perpendicular to it, yet despite the high abundance and shared orientations of tracks, we find no evidence of gregarious behaviour. This Zhemujiu tracksite discovery expands our understanding of dinosaur swimming behaviour in the Mesozoic era, as well as the occurrence of these track morphologies in China.

Size-driven preservational and macroecological biases in the latest Maastrichtian terrestrial vertebrate assemblages of North America

AbstractThe end-Cretaceous (K/Pg) mass extinction event is the most recent and well-understood of the “big five” and triggered establishment of modern terrestrial ecosystem structure. Despite the depth of research into this event, our knowledge of upper Maastrichtian terrestrial deposits globally relies primarily on assemblage-level data limited to a few well-sampled formations in North America, the Hell Creek and Lance Formations. These assemblages disproportionally affect our interpretations of this important interval. Multiple investigations have quantified diversity patterns within these assemblages, but the potential effect of formation-level size-dependent taphonomic biases and their implications on extinction dynamics remains unexplored. Here, the relationship between taphonomy and body size of the Hell Creek Formation and Lance Formation dinosaurs and mammals are quantitatively analyzed. Small-bodied dinosaur taxa (<70 kg) are consistently less complete, unlikely to be articulated, and delayed in their description relative to their large-bodied counterparts. Family-level abundance (particularly skeletons) is strongly tied to body mass, and the relative abundance of juveniles of large-bodied taxa similarly is underrepresented. Mammals show similar but nonsignificant trends. The results are remarkably similar to those from the Campanian-aged Dinosaur Park Formation, suggesting a widespread strong taphonomic bias against the preservation of small taxa, which will result in their seemingly depauperate diversity within the assemblage. This taphonomically skewed view of diversity and abundance of small-bodied taxa amid our best late Maastrichtian samples has significant implications for understanding speciation and extinction dynamics (e.g., size-dependent extinction selectivity) across the K/Pg boundary.

Abelisauroid cervical vertebrae from the Cretaceous Kem Kem beds of Southern Morocco and a review of Kem Kem abelisauroids

Isolated cervical vertebrae from the mid Cretaceous Kem Kem beds of south east Morocco are referred to the theropod dinosaur clade Abelisauroidea, and represent the first axial remains from this deposit referred to this group. An isolated axis is referred to Abelisauroidea on account of the invaginated spinopostzygapophyseal lamina; the extremely large, projecting and pointed epipophyses; and the anteroposteriorly long, transversely compressed neural spine with a gently convex and unexpanded dorsal margin. In addition, postzygapophyseal facets which completely overhang the centrum posteriorly and lack lateral orientation indicate abelisaurid affinities. An anterior cervical (C4?) is referred to Noasauridae based on an anteriorly-positioned, reduced neural spine and extremely well developed centroprezygapophyseal fossae. This specimen represents both the smallest dinosaur and the first definitive small-bodied dinosaur from the Kem Kem beds. The affinities of the new material are discussed in the context of other abelisauroid remains reported from the Kem Kem assemblage and elsewhere in Africa.

Evidence for Blood Vessels and Physiological Heat Exchange in the Heads of Dinosaurs

Extant diapsids share broad vascular patterns with known roles in thermoregulation, but different clades emphasize different sites of thermal exchange (oral, nasal, and orbital regions). We tested the hypothesis that dinosaurs emphasized different sites of thermal exchange. Evidence for dinosaur vascular anatomy was collected, and blood vessels were restored in Maya. Cross-sectional areas of canals offering evidence for blood flow (dorsal alveolar, cerebral carotid, premaxillary, and canals for palatine and nasal vessels) were measured and analyzed using principal components analysis and partial least squares. Small-bodied dinosaurs were compared with larger-bodied members of the same clade to assess the evolution of vascular networks associated with enhanced heat-exchange regions. Smaller-bodied dinosaurs with modest oral and nasal regions displayed balanced blood flow patterns. Large dinosaurs had less capacity to shed heat, resulting in selective pressure to evolve elaborate physiological heat-exchange regions. These regions displayed blood flow patterns with different sizes and combinations of blood vessels. For example, the large subnarial foramen in sauropods indicates that both the oral and nasal regions were highly vascularized, with little supply from the maxillary vessels. Yet, ankylosaurs show evidence for enhanced nasal and maxillary vessels, and unexpanded vascularization of the oral cavity. Theropods generally showed balanced blood flow patterns, but evidence indicates they may have used paranasal sinuses as a site of heat exchange. The observed phylogenetic patterns are consistent with the hypothesis that larger-bodied forms evolved anatomically expanded regions with enhanced blood flow to facilitate evaporative cooling.

A reappraisal of Notohypshilophodon comodorensis (Ornithischia: Ornithopoda) from the Late Cretaceous of Patagonia, Argentina.

The Bajo Barreal Formation (Cenomanian, Late Cretaceous; central Patagonia, Argentina) preserves an important and rich fossil record. Among the dinosaurs described, a small ornithischian, Notohypsilophon comodorensis, was named in the 1990s. This small-bodied dinosaur, the most complete representative of the group discovered in that formation, was described as a "hypsilophodontid" ornithopod based on close morphological affinities with other members of that group, which is currently regarded as paraphyletic. Within this context, we present a restudy of Notohypsilophodon. This dinosaur is considered a basal ornithopod, probably more basal than Gasparinisaura. Likewise, this analysis emends and provides additional unique anatomical characters that support its taxonomic validity and position. On the basis of weak evidence, Notohypsilophodon might share the presence of gastroliths with other basal ornithopods (Gasparinisaura and Haya), which could suggest a specific behavior in response to its food ingestion. Finally, this study increases our knowledge of the evolutionary dynamics of South American Cretaceous ornithopods, and therefore broadens our knowledge of the early Late Cretaceous continental vertebrate assemblages of central Patagonia and of the Southern Hemisphere in general.

Small Theropod Teeth from the Late Cretaceous of the San Juan Basin, Northwestern New Mexico and Their Implications for Understanding Latest Cretaceous Dinosaur Evolution

Studying the evolution and biogeographic distribution of dinosaurs during the latest Cretaceous is critical for better understanding the end-Cretaceous extinction event that killed off all non-avian dinosaurs. Western North America contains among the best records of Late Cretaceous terrestrial vertebrates in the world, but is biased against small-bodied dinosaurs. Isolated teeth are the primary evidence for understanding the diversity and evolution of small-bodied theropod dinosaurs during the Late Cretaceous, but few such specimens have been well documented from outside of the northern Rockies, making it difficult to assess Late Cretaceous dinosaur diversity and biogeographic patterns. We describe small theropod teeth from the San Juan Basin of northwestern New Mexico. These specimens were collected from strata spanning Santonian – Maastrichtian. We grouped isolated theropod teeth into several morphotypes, which we assigned to higher-level theropod clades based on possession of phylogenetic synapomorphies. We then used principal components analysis and discriminant function analyses to gauge whether the San Juan Basin teeth overlap with, or are quantitatively distinct from, similar tooth morphotypes from other geographic areas. The San Juan Basin contains a diverse record of small theropods. Late Campanian assemblages differ from approximately co-eval assemblages of the northern Rockies in being less diverse with only rare representatives of troodontids and a Dromaeosaurus-like taxon. We also provide evidence that erect and recurved morphs of a Richardoestesia-like taxon represent a single heterodont species. A late Maastrichtian assemblage is dominated by a distinct troodontid. The differences between northern and southern faunas based on isolated theropod teeth provide evidence for provinciality in the late Campanian and the late Maastrichtian of North America. However, there is no indication that major components of small-bodied theropod diversity were lost during the Maastrichtian in New Mexico. The same pattern seen in northern faunas, which may provide evidence for an abrupt dinosaur extinction.

The oldest North American pachycephalosaurid and the hidden diversity of small-bodied ornithischian dinosaurs

Taphonomic biases dictate how organisms are represented in the fossil record, but their effect on studies of vertebrate diversity dynamics is poorly studied. In contrast to the high diversity and abundance of small-bodied animals in extant ecosystems, small-bodied dinosaurs are less common than their large-bodied counterparts, but it is unclear whether this reflects unique properties of dinosaurian ecosystems or relates to taphonomic biases. A new, fully domed pachycephalosaurid dinosaur, Acrotholus audeti, from the Santonian of Alberta predates incompletely domed taxa, and provides important new information on pachycephalosaur evolution and the completeness of the ornithischian fossil record. Here we provide the first empirical evidence that the diversity of small-bodied ornithischian dinosaurs is strongly underestimated based on ghost lineages and the high proportion of robust and diagnostic frontoparietal domes compared with other pachycephalosaur fossils. This suggests preservational biases have a confounding role in attempts to decipher vertebrate palaeoecology and diversity dynamics through the Mesozoic.

New data on the diversity and abundance of small-bodied ornithopods (Dinosauria, Ornithischia) from the Belly River Group (Campanian) of Alberta

ABSTRACT Relative to large-bodied dinosaurs, the diversity of small-bodied dinosaurs from the Campanian of North America is poorly understood due to a lack of well-preserved skeletons. We document the first articulated remains, as well as the first cranial bones, of non-iguanodontian ornithopods from the Belly River Group of Alberta. The geologically oldest specimen consists of the posterior half of an articulated skeleton from the middle unit of the Oldman Formation and shares many anatomical features with the contemporaneous Orodromeus makelai and the older Oryctodromeus cubicularis. A second, younger specimen from the upper Oldman Formation is distinct from other ornithopods in having a reduced distal portion of the fibula that is fused to the anterior surface of the tibia; it is designated as the type of a new taxon, Albertadromeus syntarsus, gen. et sp. nov. Numerous isolated elements from small ornithopods from the Dinosaur Park Formation are also identified, but cannot be assigned to the generic level with confidence. Although small-bodied ornithopod material is rare, their known postcranial material outnumbers those of taphonomically equivalent and contemporaneous pachycephalosaurs, which are known to be abundant and diverse due to their robust and frequently recovered cranial domes. These findings suggest considerable undiscovered diversity of small-bodied ornithopods, and highlight biases against the preservation of small taxa in this system. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Bone Histology of the Stegosaur Kentrosaurus aethiopicus (Ornithischia: Thyreophora) from the Upper Jurassic of Tanzania

Using bone histology, a slow growth rate, uncommon for most dinosaurs, has been interpreted for the highly derived stegosaur Stegosaurus (Ornithischia: Thyreophora) and the basal thyreophoran Scutellosaurus. In this study, we examine whether this slow growth rate also occurs in the more basal stegosaur Kentrosaurus from the Tendaguru beds of Tanzania. The bone histology of six femora of Kentrosaurus representing an ontogenetic series from subadult to adult was studied, as well as one scapula. The primary bone is mainly highly vascularized fibro-lamellar bone with some reticular organization of the vascular canals. In addition to LAGs and annuli, distinctive shifts in the pattern of vascularization occur, which have been interpreted as potential growth marks. The variation in the development of growth marks may reflect annual climatic fluctuations. The overall bone depositional rate, and hence growth rate in Kentrosaurus appears to be higher than in Stegosaurus and Scutellosaurus. Considering that Stegosaurus is the larger-sized of the two stegosaurs, this would be contrary to an earlier supposition that small-bodied dinosaurs have slower growth rates than larger ones. Our finding of rapid rates of bone deposition in Kentrosaurus suggests that slow growth rates previously reported in Scutellosaurus and Stegosaurus are not a phylogenetic characteristic of the Thyreophora. Thus, slow growth rates are not plesiomorphic for the Thyreophora. We propose that the slow growth rates documented in the highly derived Stegosaurus could have been secondarily derived or alternatively that Kentrosaurus is the exception having increased growth rates.

Reduced plumage and flight ability of a new Jurassic paravian theropod from China

Feathered theropods were diverse in the Early Cretaceous Jehol Group of western Liaoning Province, China. Recently, anatomically distinct feathered taxa have been discovered in the older Middle-Late Jurassic Tiaojishan Formation in the same region. Phylogenetic hypotheses including these specimens have challenged the pivotal position of Archaeopteryx in bird phylogeny. Here we report a basal troodontid from the Tiaojishan Formation that resembles Anchiornis, also from Jianchang County (regarded as sister-taxa). The feathers of Eosinopteryx are less extensive on the limbs and tail than Anchiornis and other deinonychosaurians. With reduced plumage and short uncurved pedal claws, Eosinopteryx would have been able to run unimpeded (with large foot remiges cursorial locomotion was likely problematic for Anchiornis). Eosinopteryx increases the known diversity of small-bodied dinosaurs in the Jurassic, shows that taxa with similar body plans could occupy different niches in the same ecosystem and suggests a more complex picture for the origin of flight.

Anatomy and cranial functional morphology of the small-bodied dinosaur Fruitadens haagarorum from the Upper Jurassic of the USA.

BackgroundHeterodontosaurids are an important but enigmatic and poorly understood early radiation of ornithischian dinosaurs. The late-surviving heterodontosaurid Fruitadens haagarorum from the Late Jurassic (early Tithonian) Morrison Formation of the western USA is represented by remains of several small (<1 metre total body length, <1 kg body mass) individuals that include well-preserved but incomplete cranial and postcranial material. Fruitadens is hypothesized to represent one of the smallest known ornithischian dinosaurs.Methodology/Principal FindingsWe describe the cranial and postcranial anatomy of Fruitadens in detail, providing comparisons to all other known heterodontosaurid taxa. High resolution micro-CT data provides new insights into tooth replacement and the internal anatomy of the tooth-bearing bones. Moreover, we provide a preliminary functional analysis of the skull of late-surviving heterodontosaurids, discuss the implications of Fruitadens for current understanding of heterodontosaurid monophyly, and briefly review the evolution and biogeography of heterodontosaurids.Conclusions/SignificanceThe validity of Fruitadens is supported by multiple unique characters of the dentition and hindlimb as well as a distinct character combination. Fruitadens shares highly distinctive appendicular characters with other heterodontosaurids, strengthening monophyly of the clade on the basis of the postcranium. Mandibular morphology and muscle moment arms suggest that the jaws of late-surviving heterodontosaurids, including Fruitadens, were adapted for rapid biting at large gape angles, contrasting with the jaws of the stratigraphically older Heterodontosaurus, which were better suited for strong jaw adduction at small gapes. The lack of wear facets and plesiomorphic dentition suggest that Fruitadens used orthal jaw movements and employed simple puncture-crushing to process food. In combination with its small body size, these results suggest that Fruitadens was an ecological generalist, consuming select plant material and possibly insects or other invertebrates.