John R. Horner

Verified

John verified their affiliation via an institutional email.

Learn more

Verified

John verified their affiliation via an institutional email.

Learn more

• D.Sc. h. c.
• Professor at Chapman University

Background: A dinosaur census recorded during the Hell Creek Project (1999-2009) incorporates multiple lines of evidence from geography, taphohistory, stratigraphy, phylogeny and ontogeny to investigate the relative abundance of large dinosaurs preserved in the Upper Cretaceous Hell Creek Formation of northeastern Montana, USA. Overall, the dinosa...

The development of modern birds provides a window into the biology of their dinosaur ancestors. We investigated avian postnatal development and found that sterile inflammation drives formation of the pygostyle, a compound structure resulting from bone fusion in the tail. Inflammation is generally induced by compromised tissue integrity, but here is...

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...

A partial skull of a pachycephalosaurid from the Upper Cretaceous Hell Creek Formation, Montana, is interpreted as a new taxon, Platytholus clemensi gen. et sp. nov. MOR 2915 does not fit into an ontogenetic continuum of known pachycephalosaurids from the Hell Creek Formation, Montana, and contemporaneous sediments from the Western Interior. Compar...

The thickened frontoparietal in pachycephalosaurid dinosaurs expands dramatically during ontogeny from a flat-headed to a domed state. This expansion results in the formation of zonal tissue characterized largely by differences in vascularity and bony tissue structure that changes through ontogeny. Void space identified in CT scans of the frontopar...

The radiation of archosauromorph reptiles in the Triassic Period produced an unprecedented collection of diverse and disparate forms with a mix of varied ecologies and body sizes. Some of these forms were completely unique to the Triassic, whereas others were converged on by later members of Archosauromorpha. One of the most striking examples of th...

Abstract: Tyrannosaurids were the apex predators of Late Cretaceous Laurasia and their status as dominant carnivores has garnered considerable interest since their discovery, both in the popular and scientific realms. As a result, they are well studied and much is known of their anatomy, diversity, growth, and evolution. In contrast, little is know...

Montana is renowned for its rich paleontological treasures, particularly those of vertebrate animals such as fishes, dinosaurs, and mammals. For example, the most speciose fish fauna in the world comes from Fergus County. The first dinosaur remains noted from the western hemisphere came from an area near the mouth of the Judith River in what would...

Here, we describe an ethylenediaminetetraacetic acid (EDTA)-based bone demineralization procedure that uses cation-exchange resin and dialysis tubing. This method does not require solution changes or special equipment, is faster than EDTA alone, is cost-effective, and is environmentally friendly. Like other EDTA-based methods, this procedure yields...

The enigmatic Alvarezsauria (Dinosauria: Theropoda) are characterized by extremely short forelimbs with a single functional digit bearing a large, robust ungual. Alvarezsauria are first recorded from the Jurassic of China, but are otherwise mostly known from the Cretaceous of South America and Asia, including a number of relatively complete skeleto...

The avian transition from long to short, distally fused tails during the Mesozoic ushered in the Pygostylian group, which includes modern birds. The avian tail embodies a bipartite anatomy, with the proximal separate caudal vertebrae region, and the distal pygostyle, formed by vertebral fusion. This study investigates developmental features of the...

A histological ground-section from a duck-billed dinosaur nestling (Hypacrosaurus stebingeri) revealed microstructures morphologically consistent with nuclei and chromosomes in cells within calcified cartilage. We hypothesized that this exceptional cellular preservation extended to the molecular level and had molecular features in common with extan...

Despite its iconic status as the king of dinosaurs, Tyrannosaurus rex biology is incompletely understood. Here, we examine femur and tibia bone microstructure from two half-grown T. rex specimens, permitting the assessments of age, growth rate, and maturity necessary for investigating the early life history of this giant theropod. Osteo-histology r...

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)

Modern avian cranial kinesis played a significant role in the adaptive radiation of birds as they evolved flexible skulls from their presumably less kinetic, non‐avian theropod ancestors. Avian cranial kinesis is partly mediated by novel synovial joints, which differ from those of other extant sauropsids in that they form secondary articular cartil...

The avian tail played a critical role in the evolutionary transition from long- to short-tailed birds, yet its ontogeny in extant birds has largely been ignored. This deficit has hampered efforts to effectively identify intermediate species during the Mesozoic transition to short tails. Here we show that fusion of distal vertebrae into the pygostyl...

Stegosauria is a clade of ornithischian dinosaurs characterized by a bizarre array of dermal armor that extends from the neck to the end of the tail. Two genera of stegosaur are currently recognised from North America: the well-known Stegosaurus stenops and the much rarer Hesperosaurus mjosi. A new specimen of Hesperosaurus mjosi was discovered in...

I'll begin with the obvious question, how many people saw the movie “Jurassic Park”? How many people believed it? In the movie, “Jurassic Park”, there were basically two stars. Tyrannosaurus rex was one of them and the Velociraptors (or Deinonychus , or whatever they happened to be) were the others. What you might also know is that I was an advisor...

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...

Determining maturity in sauropod dinosaurs histologically is problematic as rapid growth leads to remodeling of Lines of Arrested Growth (LAGs). Although a complimentary system has been devised utilizing several factors including relative amounts of remodeling (Histologic Ontogenetic Stage [HOS]), most assessments of sauropod maturity are based on...

Rugose projections on the anterior and posterior aspects of vertebral neural spines appear throughout Amniota and result from the mineralization of the supraspinous and interspinous ligaments via metaplasia, the process of permanent tissue-type transformation. In mammals , this metaplasia is generally pathological or stress induced, but is a normal...

Sutures and synchondroses, the fibrous and cartilaginous articulations found in the skulls of vertebrates, have been studied for many biological applications at the morphological scale. However, little is known about these articulations at the microscopic scale in non-mammalian vertebrates, including extant archosaurs (birds and crocodilians). The...

The sutures of the skulls of vertebrates are generally open early in life and slowly close as maturity is attained. The assumption that all vertebrates follow this pattern of progressive sutural closure has been used to assess maturity in the fossil remains of non-avian dinosaurs. Here, we test this assumption in two members of the Extant Phylogene...

Strict Consensus Phylogenetic Tree for A. mississippiensis. The numbers (1 through 4) following each museum specimen number indicate ontogenetic categories: 1 for juveniles, 2 for sub-adults, 3 for sexually mature adults and 4 for skeletally mature adults. (TIF)

Relationship between ontogeny and the averaged degree of sutural closure in the four different anatomical groups of sutures in D. novaehollandiae and A. mississippiensis. In the emus, the braincase synchondroses are the first to be completely obliterated (at skeletal maturity), while cranial sutures reach their highest degree of closure later, arou...

Relationship between ontogeny and the averaged degree of interdigitation in D. novaehollandiae and A. mississippiensis. In A. mississippiensis, the degree of interdigitation increases drastically as ontogeny proceeds. In emus, values are much lower (meaning that sutures are more straight than in the alligators overall) and they increase until sub-a...

Supporting Information. Character list for the sutures and synchondroses of D. novaehollandiae and A. mississippiensis (Text A). Taxon-character matrix for D. novaehollandiae (Dataset A). Taxon-character matrix for A. mississippiensis (Dataset B). Sutural closure scores and averages for D. novaehollandiae (Table A). Sutural closure scores and avera...

Brachylophosaurini is a clade of hadrosaurine dinosaurs currently known from the Campanian (Late Cretaceous) of North America. Its members include: Acristavus gagslarsoni, which lacks a nasal crest; Brachylophosaurus canadensis, which possesses a flat paddle-shaped nasal crest projecting posteriorly over the dorsal skull roof; and Maiasaura peebles...

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...

Fossil bone microanalyses reveal the ontogenetic histories of extinct tetrapods, but incomplete fossil records often result in small sample sets lacking statistical strength. In contrast, a histological sample of 50 tibiae of the hadrosaurid dinosaur Maiasaura peeblesorum allows predictions of annual growth and ecological interpretations based on m...

Predator confrontation or predator evasion frequently produces bone fractures in potential prey in the wild. Although there are reports of healed bone injuries and pathologies in non-avian dinosaurs, no previously published instances of biomechanically adaptive bone modeling exist. Two tibiae from an ontogenetic sample of fifty specimens of the her...

We propose the hypothesis that in the long bones of large, rapidly growing animals, secondary osteons may form to a greater degree in smaller bones than in larger ones for reasons that may have more to do with the interplay between element-specific growth rates and whole-body metabolic rates than with mechanical or environmental factors. We predict...

Evolutionary biologists define “metaplasia” as the permanent transformation of a cell identity, and there are many examples of such transformations in living vertebrates (e.g., chondrocytes transforming directly into osteoblasts). These metaplasias have been observed during the mineralization of “ossified” tendons of living birds. In the present st...

In histology textbooks, the vertebrate skeleton is represented as almost entirely made of bone and cartilage. This is a false dichotomy and in fact, a continuum of intermediate tissues between bone and cartilage exists. Chondroid bone ([CB] or chondroid tissue), one of the most well-known intermediate tissues, has been reported in mammals, birds an...

Les écrits de Darwin nécessitent d’être vus dans leur ensemble et non à partir de phrases retirées de leur contexte. Quelquefois, il arrive que Darwin écrive longuement et apparemment favorablement sur des idées qu’il abandonne ensuite en les replaçant dans une perspective plus intégrative qui reflète sa vaste propre vue d’ensemble. Darwin a compri...

A bs t r ac t Hypacrosaurus stebingeri is known from one of the most complete ontogenetic series of dinosaurs, yet the adult skeletal material has never been fully described. Here, the first detailed description of all well-known H. stebingeri crania is presented, accompanied by the first multivariate statistical analysis for lambeosaurine crania t...

A particularly critical event in avian evolution was the transition from long- to short-tailed birds. Primitive bird tails underwent significant alteration, most notably reduction of the number of caudal vertebrae and fusion of the distal caudal vertebrae into an ossified pygostyle. These changes, among others, occurred over a very short evolutiona...

Significance The deciphering of evolutionary trends in nonavian dinosaurs can be impeded by a combination of small sample sizes, low stratigraphic resolution, and lack of ontogenetic (developmental) details for many taxa. Analysis of a large sample ( n > 50) of the famous horned dinosaur Triceratops from the Hell Creek Formation of Montana incorpor...

Bone microanalyses of extant vertebrates provide a necessary framework from which to form hypotheses regarding the growth and skeletochronology of extinct taxa. Here, we describe the bone microstructure and quantify the histovariability of appendicular elements and osteoderms from three juvenile American alligators (Alligator mississippiensis) to a...

A soft tissue structure has been discovered on the head of the duck-billed dinosaur Edmontosaurus. Its similarity to a cock's comb and other sexually dimorphic structures of birds suggests that potential sexual signals existed in these extinct animals.

Here, we describe a juvenile Triceratops sp. skull, UCMP 136306, from the Hell Creek Formation, McCone County, Montana. The relative completeness and superb preservation of this skull contribute to an improved understanding of the cranial ontogeny, morphology, and individual variation in Triceratops. Total skull length is 120 cm long (est.). UCMP 1...

► Sexual selection requires sexual dimorphism; this was integral to Darwin's original definition. ► Claims of sexual selection in extinct animals often rely on vague or untested assertions. ► There is at present no documented case of sexual dimorphism or sexual selection among dinosaurs and their relatives. ► A test of sexual dimorphism requires an...

The skull and jaws of extant birds possess secondary cartilage, a tissue that arises after bone formation during embryonic development at articulations, ligamentous and muscular insertions. Using histological analysis, we discovered secondary cartilage in a non-avian dinosaur embryo, Hypacrosaurus stebingeri (Ornithischia, Lambeosaurinae). This fin...

Certaines hypothèses ont été proposées pour expliquer des « structures bizarres » chez les dinosaures ou chez d’autres animaux disparus (par exemple fonction mécanique, différentes sortes d’affichage intra- et interspécifique). Des preuves et tests récents pour une reconnaissance de l’espèce en tant que facteur déterminant de ces structures ont été...

The Cretaceous-Paleogene (K-Pg) extinction is one of the major events in the history of life on Earth, with the loss of up to 75% of species. Extinction rates were not uniform, however; although all clades suffer lineage extinctions, some are wiped out entirely. Such differential survivorship is suggestive of ‘species selection’ acting on attribute...

Bone and calcified cartilage can be fossilized and preserved for hundreds of millions of years. While primary cartilage is fairly well studied in extant and fossilized organisms, nothing is known about secondary cartilage in fossils. In extant birds, secondary cartilage arises after bone formation during embryonic life at articulations, sutures and...

Informations about Mammalian and Teleostean secondary cartilages. (DOC)

Toothmarked bone, created by the feeding behavior of carnivorous dinosaurs, provides direct evidence of ecological interaction between predator and prey. However, examples are typically limited to small sample sizes, restricting behavioral inferences for specific taxa. In this study, we present one of the largest samples (n=18) linking a single car...

The holotype and only specimen of the chasmosaurine ceratopsid dinosaur 'Nedoceratops hatcheri' has been the source of considerable taxonomic debate since its initial description. At times it has been referred to its own genus while at others it has been considered synonymous with the contemporaneous chasmosaurine Triceratops. Most recently, the de...

An external fundamental system (EFS) is a form of bone microstructure present in the outermost cortex of long bones in animals that have attained skeletal maturity. It indicates an effective cessation of any significant periosteal growth (i.e., growth in circumference or girth). Although an EFS has been noted in several reptile taxa, the idea that...

A new hadrosaurid dinosaur, Acristavus gagslarsoni, is here named on the basis of several autapomorphic characteristics of the frontal, postorbital, and dentary. Acristavus is a member of the newly erected clade Brachylophosaurini, which along with its other members, Brachylophosaurus and Maiasaura, constitutes the earliest hadrosaurine hadrosaurid...

The parietal frill of Triceratops, one of the largest cranial ornamental features known, undergoes extraordinary morphological changes late in ontogeny — progressing from a large, thickened, solid frill to a substantially larger, thin, fenestrated frill. To understand how this structure changed so dramatically we undertook a histological examinatio...

Specimens of Stegoceras validum used in the allometric analyses and their measurements. (DOC)

Description of measurements. (DOC)

HRCT methods. (DOC)

Historically, studies of pachycephalosaurs have recognized plesiomorphically flat-headed taxa and apomorphically domed taxa. More recently, it has been suggested that the expression of the frontoparietal dome is ontogenetic and derived from a flat-headed juvenile morphology. However, strong evidence to support this hypothesis has been lacking. Here...

The carnivorous Tyrannosauridae are among the most iconic dinosaurs: typified by large body size, tiny forelimbs, and massive robust skulls with laterally thickened teeth. The recently described small-bodied tyrannosaurid Raptorex kreigsteini is exceptional as its discovery proposes that many of the distinctive anatomical traits of derived tyrannos...

‘Bizarre structures’ in dinosaurs have four main traditional explanations: mechanical function, sexual selection, social selection and species recognition. Any of these can be plausible for individual species, but they fail to be persuasive when other lines of evidence cannot adequately test them. The first three also fail as general propositions w...

A well-preserved skull from a previously unknown growth stage for the lambeosaurine hadrosaurid Hypacrosaurus stebingeri from the Campanian of western North America is described. This skull is equivalent in size to the smallest known growth stages for Corythosaurus, Hypacrosaurus altispinus and Lambeosaurus, and allows for a direct comparison of th...

Lower Hell Creek Formation (L3) dinosaur skeletons in order of abundance. (DOC)

Upper Hell Creek Formation (U3) skeletons in order of abundance. (DOC)

Upper Hell Creek Formation (U3) Triceratops skulls recorded but not collected. (DOC)

Census from the Doldrum's lag deposit (MOR loc. HC-530) at the base of the Apex sandstone in the upper Hell Creek Formation (U3.AS) in order of abundance. (DOC)

Upper Hell Creek Formation (U3) Triceratops skulls collected. These specimens are either whole skulls or isolated elements and do not qualify to be counted under the “three bone rule.” (DOC)

Dinosaur census from 3B1 lag deposit (MOR loc. HC-312) at the base of the Jen-rex sand in order of abundance. (DOC)

Prieto-Márquez, A., Minjin B. & Horner, J.R. iFirst article. A diminutive deinonychosaur (Dinosauria: Theropoda) from the Early Cretaceous of Öösh (Övörkhangai, Mongolia). Alcheringa, 1–19. ISSN 0311-5518.We describe a new specimen of small maniraptoran theropod from the Early Cretaceous Öösh locality, Övörkhangai province, central Mongolia. This e...

A “diversity first” approach to dinosaur taxonomy interprets morphological variation as diagnostic of unique taxa, leading to an explosion of species. Description of differing morphologies is useful and important for our understanding of evolution; however, disparity is not always indicative of diversity. An alternative to “diversity first” is “Uni...

EARLY OSSIFICATION AND CALCIFIED TISSUES IN THE SKULL OF HYPACROSAURUS STEBINGERI (ORNITHISCHIA, LAMBEOSAURINAE): A PRELIMINARY STUDY.