David J Varricchio

• PhD
• Professor at Montana State University

The Campanian Two Medicine Formation of northwestern Montana, USA, is richly fossiliferous, and discoveries made within the unit over the past century have greatly advanced our appreciation of dinosaur paleobiology and evolution. Previously undifferentiated from a lithostratigraphic perspective, the formation is now subdivided into four new members...

The vertebrate assemblages of the Albian to Cenomanian Wayan Formation of southeastern Idaho and southwestern Montana’s coeval Vaughn Member of the Blackleaf Formation are dominated by the small, burrowing orodromine dinosaur Oryctodromeus cubicularis. Here, we describe in detail the osteology of Oryctodromeus based on new specimens from Idaho and...

For most dinosaurs, clutches consisted of a single layer of spherical to sub-spherical, highly porous eggs that were probably fully buried. Both eggs and clutch form change drastically with pennaraptoran theropods, the clade that includes birds. Here, far less porous, more elongate eggs are arranged with additional complexity, and only partially bu...

The evolution of nest site use and nest architecture in the non-avian ancestors of birds remains poorly understood because nest structures do not preserve well as fossils. Nevertheless, the evidence suggests that the earliest dinosaurs probably buried eggs below ground and covered them with soil so that heat from the substrate fuelled embryo develo...

Hadrosauriformes is a group of ornithopod dinosaurs with a rich and extensive Cretaceous track record. However, their rear foot (pes) anatomy is rather conservative. Thus, distinguishing different putative track producers from morphological differences is often impeded unless their identification is corroborated by stratigraphic and geographic dist...

The Coniacian-Santonian stages in western North America are characterized by a sparse fossil record. We present here the first account of dinosaur tracks from nine sites in the Frontier Formation (Coniacian-Santonian) of southwestern Montana. Tracks are largely preserved in distal alluvial facies as sandstone casts, with a single example of shallow...

In amniotes, the predominant developmental strategy underlying body size evolution is thought to be adjustments to the rate of growth rather than its duration. However, most theoretical and experimental studies supporting this axiom focus on pairwise comparisons and/or lack an explicit phylogenetic framework. We present the first large-scale phylog...

The avian palaeognath phylogeny has been recently revised significantly due to the advancement of genome-wide comparative analyses and provides the opportunity to trace the evolution of the microstructure and crystallography of modern dinosaur eggshells. Here, eggshells of all major clades of Palaeognathae (including extinct taxa) and selected eggs...

In 2008, and subsequent collecting trips, the remains of a partial basal neornithischian were recovered from the Cenomanian Willow Tank Formation of southern Nevada. Bones identified include the proximal femora, a series of vertebrae missing neural arches, several pedal phalanges, fragments of ossified tendons, and some as yet unidentified elements...

The avian palaeognath phylogeny has been recently revised significantly due to the advancement of genome-wide comparative analyses and provides the opportunity to trace the evolution of the microstructure and crystallography of modern dinosaur eggshells. Here, eggshells of all major clades of Palaeognathae (including extinct taxa) and selected eggs...

Since their discovery in the 1920s, some asymmetric, elongated dinosaur eggs from the Upper Cretaceous of Mongolia have been interpreted as ceratopsian eggs. However, recent views support a maniraptoran affinity mainly based on macroscopic features. Technical advancements in palaeontology provide a novel approach to diagnose maniraptoran eggs, and...

Among amniotes, turtles are the only clade that lay aragonitic eggs. Because aragonite is a metastable mineral, unequivocal preservation of aragonite in fossil turtle eggs has only been reported from Pliocene deposits. Here, we report in situ preservation of aragonite in a turtle egg from the Judith River Formation (Campanian) of Montana, USA. We u...

The terrestrial feeding trace Edaphichnium lumbricatum is known from the Triassic to the Pleistocene and is characterized by tubular burrows with ellipsoidal fecal pellets, indicating substrate feeding by earthworms or other invertebrates. We describe 11 specimens attributable to Edaphichnium isp. from Egg Mountain, a terrestrial locality with a di...

Fossil gastric pellets (regurgitalites) have distinct taphonomic characteristics that facilitate inferences of behavioural ecology in deep time, despite their rarity in the fossil record. Using the taphonomic patterns of both extant and fossil small mammals from more recent geologic deposits as a guide, we assess the taphonomy of three unusual mult...

Dale Russell described the osteology, morphology, and ecology of the small theropod “Stenonychosaurus inequalis” in two papers, speculating on its life habits, brain power, vision, movement, feeding, and hand capabilities. Russell even pondered a tool-using dinosauroid, the hypothetical troodontid descendant if the lineage had survived the Cretaceo...

A new Cretaceous ootaxon (eggshell type) from the Kaiparowits Formation of Grand Staircase-Escalante National Monument is among a growing number of very small eggs described from the Mesozoic. Analyses of two partial eggs (~ 17.7 mm in diameter) and 29 eggshell fragments reveal that this new ootaxon exhibits nodose ornamentation with distinctive br...

The Coniacian non-marine portion of the Frontier Formation in southwestern Montana has been primarily studied stratigraphically, whereas its trace fossil record and relative sedimentological significance have remained overlooked so far. We present the first description of a trace fossil assemblage reported from Frontier Formation deposits exposed i...

When sociality evolved and in which groups remain open questions in mammalian evolution, largely due to the fragmentary Mesozoic mammal fossil record. Nevertheless, exceptionally preserved fossils collected in well-constrained geologic and spatial frameworks can provide glimpses into these more fleeting aspects of early mammalian behaviour. Here we...

Troodon formosus , a theropod from the Late Cretaceous, is one of the few species of dinosaurs with multiple nest sites uncovered. It has been consistently demonstrated that eggs within these nests would have been partially buried in life—an exceedingly rare state in modern vertebrates. There has been debate over Troodon 's capacity to engage in th...

Calcified eggshells protect developing embryos against environmental stress and contribute to reproductive success¹. As modern crocodilians and birds lay hard-shelled eggs, this eggshell type has been inferred for non-avian dinosaurs. Known dinosaur eggshells are characterized by an innermost membrane, an overlying protein matrix containing calcite...

With abundant well-preserved clutches and several adult-clutch associations, oviraptorids provide some of the most detailed information on reproduction for dinosaurs. Here, we describe an oviraptorosaur closely associated with two eggs from the Upper Cretaceous Nanxiong Formation of Jiangxi Province, China, and discuss its implications for various...

A locality in the Upper Cretaceous Two Medicine Formation of Montana preserves abundant and variable horseshoe crab tracks and trails of the ichnotaxon Kouphichnium isp. These specimens span six morphologies differing in track form and trail configuration. These differences likely reflect variations in track-maker locomotion and behavior, substrate...

A new trace fossil, Feoichnus martini new ichnospecies, from the Two Medicine Formation is here described. This ichnotaxon is reported from the upper Campanian deposits of the Egg Mountain locality (Montana) and consists of a hemispherical to hemiellipsoidal structure with a truncated upper edge, and a regular, rounded lower edge marked by a lined...

The geological and paleoenvironmental setting and the vertebrate taxonomy of the fossiliferous, Cenomanian-age deltaic sediments in eastern Morocco, generally referred to as the “Kem Kem beds”, are reviewed. These strata are recognized here as the Kem Kem Group, which is composed of the lower Gara Sbaa and upper Douira formations. Both formations h...

Reports on nesting debris generated by great blue heron (Ardea herodias), arboreal nesting birds with ‘semi-altricial’ young, are limited. In this study, surface and subsurface sampling were conducted in 2012 and 2013 in a heronry near the Missouri River in Montana, including bi-weekly collections in 2013 documenting accumulations over the breeding...

Fossils from the Jehol Group (Early Cretaceous, Liaoning Province, China) are integral to our understanding of Paraves, the clade of dinosaurs grouping dromaeosaurids, troodontids, and avialians, including living birds. However, many taxa are represented by specimens of unclear ontogenetic age. Without a more thorough understanding of ontogeny, evo...

An incomplete specimen assigned to the genus Neurankylus sp. was collected from Coniacian deposits of the Frontier Formation of Montana (USA). The material consists of anterior plastron, a costal plate and neural plate fragment, and several undetermined skeletal elements. The identification is based on the relatively large size of the specimens, sc...

We describe the diversity and abundance of insect (specifically hymenopterans and coleopterans) pupation structures in the Upper Cretaceous (Campanian) Two Medicine Formation at the Egg Mountain locality, western Montana, U.S.A., an important dinosaur nesting site. The study interval comprises a massive calcareous siltstone and indurated silty lime...

During the mid-Cretaceous, multituberculate mammal communities were restructured, as 'plagiaulacidan' multituberculates, common from the Middle Jurassic through Early Cretaceous, began to be replaced by the Cimolodonta, a clade that would come to be the most abundant and diverse group of mammals during the Late Cretaceous and early Paleocene. Altho...

The first intact North American Macroelongatoolithus specimen was excavated from sediments in the Wayan Formation of southeastern Idaho and initially reported by Krumenacker et al. (2017, Historical Biology 29:170–186). Here, we present a description of the specimen, including a zonal analysis of microstructural variation and comparison with an egg...

Analysis of ontogenetic changes in long bone microstructure aid in vertebrate life history reconstructions. Specifically, osteohistological examination of common fauna can be used to infer growth strategies of biologically uncommon, threatened, or extinct vertebrates. Although nine-banded armadillo biology has been studied extensively, work on grow...

Transverse sections of sampled femora under linearly polarized light. (A) UTCM 802, (B) UTCM 801, (C) OMNH 39188, (D) UTCM 1557, (E) OMNH 40173, and (F) OMNH 40175. All sections are stained with toluidine blue. FL—femur length. Note–some sections were flipped along the horizontal axis to allow for easier comparisons. (TIF)

Transverse sections of sampled tibiae under linearly polarized light. (A) UTCM 802, (B) UTCM 801, (C) OMNH 39188, (D) UTCM 1557, (E) OMNH 40173, (F) OMNH 40175. All sections stained with toluidine blue. TL = tibia length. Note–some sections were flipped along the horizontal axis to allow for easier comparisons. (TIF)

Transverse sections of sampled femora under circularly polarized light. (A) UTCM 802, (B) UTCM 801, (C) OMNH 39188, (D) UTCM 1557, (E) OMNH 40173, and (F) OMNH 40175. All sections are stained with toluidine blue. FL—femur length. Note–some sections were flipped along the horizontal axis to allow for easier comparisons. (TIF)

Transverse sections of sampled tibiae under circularly polarized light. (A) UTCM 802, (B) UTCM 801, (C) OMNH 39188, (D) UTCM 1557, (E) OMNH 40173, (F) OMNH 40175. All sections stained with toluidine blue. TL = tibia length. Note–some sections were flipped along the horizontal axis to allow for easier comparisons. (TIF)

Oryctodromeus cubicularis is a small-bodied ornithischian dinosaur from the mid-Cretaceous Blackleaf and Wayan Formations of Montana and Idaho, respectively, and is the only documented dinosaur with evidence of a burrowing ecology. The type locality, from the Blackleaf Formation of southwestern Montana, consisted of an infilled burrow structure con...

Nesting localities of extant birds and reptiles may provide taphonomic models for interpreting nesting sites of ancient archosaurs. Here we describe assemblages of nesting gulls (Larus delawarensis and L. californicus), American white pelicans (Pelecanus erythrorhynchos), and double-crested cormorants (Phalacrocorax auritus). Seventy nests yielded...

Here we expand on prior findings by describing a new locality and fossil eggs within the Upper Two Medicine Formation of North Western Montana. The lithology of the site consists of a grey bentonitic mudstone with aragonite veins. Terrestrial gastropods are common throughout. Igneous samples extracted from an ash layer directly above the site are b...

Non-avian dinosaurs such as oviraptorosaurs and troodontids share several important reproductive characters with modern birds, including eggshell microstructure and iterative egg production. Nevertheless, debate exists concerning their incubation strategies. Here we estimate incubation period for the troodontid, Troodon formosus, by examining a nea...

Fluvial transport is recognized as a common manner by which bone assemblages forming in or near moving water can be taphonomically modified. Here we study an assemblage of 38 cow carcasses, killed during a mass-mortality event and deposited in a fluvial system (the Yellowstone River of eastern Montana), over the course of three years. Seven of the...

Paleontologists have characterized the Two Medicine Formation as possessing three stratigraphically distinct ecological communities representing the Lower, Middle and Upper portions of the unit. These three communities were originally recognized based on skeletal data. Whereas, fossil eggs and eggshells are known from the Lower and Middle Two Medic...

Two closely associated egg types occur at the same locality in the Upper Cretaceous (Maastrichtian) St. Mary River Formation in north central Montana. These specimens represent the first fossil eggs described from this formation. At least fifteen small ovoid eggs or egg portions are scattered through a 25 cm interval of rock. Five significantly lar...

Analysis of ontogenetic changes in long bone microstructure aid in vertebrate life history reconstructions. Specifically, osteohistological examination of common fauna can be used to infer growth strategies of biologically uncommon, threatened, or extinct vertebrates. Although armadillo biology has been studied extensively, work on growth history i...

Nesting behaviors of extant vertebrates can serve as taphonomic models for interpreting extinct archosaurian reproduction. Past studies have examined birds with open nests and nest-bound young and tortoises with buried nests and precocial young. Here we taphonomically describe nesting sites of two crocodylians, American crocodile (Crocodylus acutus...

A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. In concert with other anagenetic lineages of dinosaurs from the same time and place, this suggests that anagenesis could have been a widespread mechanism ge...

Iguanomorpha (stem + crown Iguania) is a diverse squamate clade with members that predominate many modern American lizard ecosystems. However, the temporal and palaeobiogeographic origins of its constituent crown clades (e.g. Pleurodonta (basilisks, iguanas, and their relatives)) are poorly constrained, mainly due to a meagre Mesozoic-age fossil re...

Iguanomorpha (stem + crown Iguania) is a diverse squamate clade with members that predominate many modern American lizard ecosystems. However, the temporal and palaeobiogeographic origins of its constituent crown clades (e.g. Pleurodonta (basilisks, iguanas, and their relatives)) are poorly constrained, mainly due to a meagre Mesozoic-age fossil re...

Taphonomy of extant birds in combination with similar studies of modern reptiles provides a range of models for the interpretation of reproduction in ancient archosaurs. Here we describe the reproductive assemblages generated by nesting American white pelican (Pelecanus erythrorhynchos), double-crested cormorant (Phalacrocorax auritus), and two gul...

The reproductive biology of living birds differs dramatically from that of other extant vertebrates. Although some attributes of modern avian reproduction had their origin within theropod dinosaurs like oviraptors and troodontids, even the most derived non-avian theropods lack key features of modern birds. We review the current knowledge of reprodu...

Diapsids show an extremely wide range of reproductive strategies. Offspring may receive no parental care, care from only one sex, care from both parents, or care under more complex regimes. Young may vary from independent, super-precocial hatchlings to altricial neonates needing much care before leaving the nest. Parents can invest heavily in a few...

The complete phylogeny used in these analyses including both extant and extinct taxa. For analyses where data for extinct taxa was not available, these were pruned from the tree. (PDF)

Ancestral character estimation of parental care strategy in diapsids using only extant data. The proportions of colors in each the bars at each node of the phylogeny represents the likelihood of each care strategy at that node. Blue = no care, red = maternal care, orange = biparental care, yellow = paternal care. (PDF)

Ancestral character estimation of hatchling precociality state in diapsids. The proportions of colors in each the bars at each node of the phylogeny represents the likelihood of each care strategy at that node. Blue = no care, red = maternal care, orange = biparental care, yellow = paternal care. Light blue = superprecocial, green = precocial, yell...

Clades demonstrating marked heterogeneity in hatchling precociality. A–Caprimulgimorpha + Otidimorphae, B–Aequiornithia + Phaethontimorphae. Green = precocial, yellow = semiprecocial, orange = subaltricial, red = altricial. The proportion of the colored bar at each node represents the likelihood of each precociality state at that node. (PDF)

Complete parental care strategy model comparison data for derived theropods, sauropodomorphs and hadrosaurs. (PDF)

Ancestral character estimation of parental care strategy in diapsids using complete dataset (extinct and extant taxa). The proportions of colors in each the bars at each node of the phylogeny represents the likelihood of each care strategy at that node. Blue = no care, red = maternal care, orange = biparental care, yellow = paternal care. (PDF)

Complete dataset used for GEE and ACE analyses. (PDF)

References used to compile reproductive ecology data. (PDF)

References used to produce composite phylogeny. (PDF)

Ancestral character estimation of polygamy in diapsids. The proportions of colors in each the bars at each node of the phylogeny represents the likelihood of each care strategy at that node. Green = both sexes polygamous, blue = socially monogamous. (PDF)

Despite over a hundred years of intense paleontological exploration, the terrestrial rocks of the Upper Cretaceous (Maastrichtian) Hell Creek Formation of eastern Montana are remarkable for the absence of fossil eggs. Here, we describe the first fossil egg and additional eggshell fragments from the formation. The two-layered structure of the smooth...

Nesting ground assemblages generated by extant birds can potentially serve as taphonomic models for the interpretation of dinosaur reproduction. Here, we describe the nesting material associated with American white pelican (Pelecanus erythrorhynchos), double-crested cormorant (Phalacrocorax auritus), and two gulls (Larus). Nest surveys focused on p...

Taphonomic and morphological evidence suggests that Oryctodromeus cubicularis, a basal ornithopod dinosaur from the mid-Cretaceous of Montana, constructed burrows by digging with its forelimbs. Reconstruction of the forelimb musculature of Oryctodromeus was carried out using the extant phylogenetic bracket method, with crocodilians and ratites as t...

The record of terrestrial vertebrates in the upper Albian to Cenomanian Wayan Formation of Idaho is sparse, with most fossils recovered belonging to the small orodromine neornithischian Oryctodromeus cubicularis and the maniraptoran ootaxon Macroelongatoolithus carlylei. Here we report on a diversity of theropod forms now recognised from various is...

The nesting behavior in extant animals can potentially serve as either analogs or as taphonomic models for the interpretation of reproduction habits in extinct organisms. Past studies have examined birds with open nests and nest-bound, altricial young and turtles with buried clutches and precocial young. However, no taphonomic research covers croco...

Increasing importance has been placed on bone microstructure studies of extant organisms to better interpret the fossil record. For instance, studies examining extant crocodylians, aves, and mammals help describe and interpret extinct tetrapod growth. Nine-banded armadillos (Dasypus novemcinctus) are common taxa throughout the southern United State...

The interpretation of fossil eggshells can be problematic because eggshells may be transported by hydraulic flow in floodplains, making it difficult to interpret the reproductive behavior and ecology of parent animals. A series of flume studies was conducted to establish analytical techniques for assessing eggshell hydraulic transport in the fossil...

The basal ornithopod Oryctodromeus cubicularis was described as burrowing due to its discovery in a burrow structure, and the presence of several morphological features considered consistent with burrowing. Using traditional and geometric morphometric analyses, the morphology of the humerus and scapula of ornithopods, basal ornithischians, and marg...

Embryonic remains within a small (4.75 by 2.23 cm) egg from the Late Cretaceous, Mongolia are here re-described. High-resolution X-ray computed tomography (HRCT) was used to digitally prepare and describe the enclosed embryonic bones. The egg, IGM (Mongolian Institute for Geology, Ulaanbaatar) 100/2010, with a three-part shell microstructure, was o...

ABSTRACT—Whereas ‘biological site fidelity’ refers to the regular reuse of a favored locale (e.g., breeding ground or nest) by an individual animal, ‘paleontological site fidelity’ typically refers to repeated use of a nesting locality by a herd or species over geologic time scales. Two new Cretaceous specimens from the Two Medicine Formation of Mo...

A nesting trace preserved in alluvial floodplain deposits in the Upper Cretaceous Two Medicine Formation at the Willow Creek anticline in north-central Montana contains four crushed theropod eggs referable to the oospecies Continuoolithus canadensis. These eggs immediately overlie the lower surface of a 35-cm-long × 7-cm-thick, dark-green mudstone...

Dinosaur reproductive biology is often inferred from the biology of extant taxa; however, taphonomic studies of modern nest sites have focused exclusively on avian, rather than reptilian species. We documented eight Agassiz's desert tortoise (Gopherus agassizii) nests and ten loggerhead sea turtle (Caretta caretta) nests. Gopherus agassizii excavat...

In 1991, Sabath described “larger avian eggs” from the Upper Cretaceous Barun Goyot and Djadokhta Formations of Mongolia. These were later included in the ootaxon Gobioolithus major. Here we recognize the larger avian eggs of Sa- bath as a distinct ootaxon, Styloolithus sabathi, oogen. et oosp. nov. These eggs differ from those of Gobioolithus in b...

Recent discoveries of spectacular dinosaur fossils overwhelmingly support the hypothesis that birds are descended from maniraptoran theropod dinosaurs, and furthermore, demonstrate that distinctive bird characteristics such as feathers, flight, endothermic physiology, unique strategies for reproduction and growth, and a novel pulmonary system origi...

We describe an extensive ichnofossil assemblage from the likely Cenomanian-age 'lower' and 'upper' units of the 'Kem Kem beds' in southeastern Morocco. In the lower unit, trace fossils include narrow vertical burrows in cross-bedded sandstones and borings in dinosaur bone, with the latter identified as the insect ichnotaxon Cubiculum ornatus. In th...

Mesozoic non-avian theropod dinosaurs displayed a diverse range of egg types and clutch forms, suggesting a variety of nesting behaviours, some of which may be shared with birds. More accurate inferences of these behaviours require taphonomic studies of modern nesting sites. Here, we document common tern (Sterna hirundo) nesting sites on Poplar Isl...

Numerous discoveries in the Tiantai basin of Zhejiang Province, China, enrich our understanding of the parataxonomy, paleobiology and taphonomic histories of fossil eggs from a diverse array of Cretaceous oofamilies. We describe the most abundant of these egg types catalogued in the Zhejiang Museum of Natural History, Spheroolithus cf. zhangtoucaoe...

Partial eggs from the Eocene Willwood Formation of Wyoming contain unidentifiable embryonic remains; the eggs are referable to a theropod on the basis of their structural layering of calcite and to an avian theropod because of their Eocene age. We assign the specimens to the oofamily Medioolithidae as Microolithus wilsoni, oogen. et oosp. nov., on...

Oological material recovered from Egg Mountain and other localities in the Upper Cretaceous (Campanian) Two Medicine Formation in Montana (USA) provides a wealth of information about the reproductive behaviour of the small maniraptoran theropod Troodon formosus. Studies of embryos, eggs, clutches, a nesting trace, and eggs associated with adult ske...

Lack of stratigraphic context for dinosaur eggs inhibits understanding of dinosaur reproductive biology and the taphonomic processes of egg preservation. Past taphonomic work suggests two features, compression ridges (sharp edge of broken eggshell around egg circumference) and deformation asymmetry (proportion of crushed to rounded sides of the egg...

Using tangential thin sections, we examined variation in porosity and water vapor conductance across two eggs of Troodon formosus, a small (similar to 50 kg) theropod dinosaur from the North American Upper Cretaceous, testing two hypotheses of egg incubation: (1) full burial within sediments or vegetation and (2) partial burial with exposed upper e...

Varricchio (2002) described some forelimb bones from the Late Cretaceous (Campanian) Two Medicine Formation, Glacier County, Montana (USA), as the holotype of Piksi barbarulna, a supposed ornithothoracine bird. However reevaluation of Piksi Varricchio, 2002 instead recognizes this genus as belonging to Pterosauria Kaup, 1834 and not Ayes Linnaeus,...

A second nearly complete, articulated specimen of the basal troodontid Mei long (DNHM D2154) is reported from the Early Cretaceous (Hauterivian-Valanginian) lower Yixian Formation, Liaoning Province, China. New diagnostic features of Mei long are identified, including: a uniquely shaped maxilla, low with small, low maxillary fenestra; sacrum with a...

A nexus file containing the matrix used to produce the trees seen in Figures 8 and S1. This file is formatted for use with the program TNT. (NEX)

The non-paravian portion of the phylogenetic analysis of Coelurosauria described in the text. Strict (Nelson’s) Consensus of 36 most parsimonious trees with a length of 1321 steps resulting from the analysis of the matrix from Xu et al. [24] with Talos sampsoni added. Numbers below branches represent Bremer decay indices greater than one. Protocol...

Selected measurements of Mei long specimens in millimeters. R and L indicate right and left elements, when applicable. *denotes estimated lengths. (DOC)