Disentangling Arrhinoceratops brachyops' Skull Ontogeny & how it Relates to Other Ceratopsids

Skull ontogeny in Arrhinoceratops brachyops (Ornithischia: Ceratopsidae) and other horned dinosaurs

Authors:

Mallon et al

Abstract:

Disentangling ontogenetic from interspecific variation is key to understanding biodiversity in the fossil record, yet information on growth in the ceratopsid subfamily Chasmosaurinae is sparse. Here, we describe the partial skull of a juvenile chasmosaurine, attributed to Arrhinoceratops brachyops, within the context of more mature specimens of this species, to better understand the ontogenetic transformations therein. We show that as A. brachyops matured, the postorbital horncores became longer and shifted from a posterior to an anterior inclination, the delta-shaped frill epiossifications became lower and fused to the underlying frill, and the face became more elongate. In these respects, A. brachyops closely resembled Triceratops, suggesting that these ontogenetic changes may have been common to all long-horned chasmosaurines. However, an event-paired cladistic analysis of Chasmosaurinae using a standardized matrix of 24 developmental characters reveals that the relative timing of ontogenetic events in Arrhinoceratops was more like that of Chasmosaurus, particularly in the relatively late reduction in scalloping around the frill margins. Thus, the ontogenetic similarities between Arrhinoceratops and Triceratops appear to be plesiomorphic, partly related to the retention of the elongate postorbital horncores, which are primitive for Ceratopsidae. This study elucidates the otherwise contentious evolutionary relationships of Arrhinoceratops, and highlights the importance of ontogenetic data for resolving phylogenies when morphological data from adults alone are inadequate.

Evolutionary Trends in Triceratops

Evolutionary trends in Triceratops from the Hell Creek Formation, Montana

Authors:

Scannella et al

Abstract:

The placement of over 50 skulls of the well-known horned dinosaur Triceratops within a stratigraphic framework for the Upper Cretaceous Hell Creek Formation (HCF) of Montana reveals the evolutionary transformation of this genus. Specimens referable to the two recognized morphospecies of Triceratops, T. horridus and T. prorsus, are stratigraphically separated within the HCF with the T. prorsus morphology recovered in the upper third of the formation and T. horridus found lower in the formation. Hypotheses that these morphospecies represent sexual or ontogenetic variation within a single species are thus untenable. Stratigraphic placement of specimens appears to reveal ancestor–descendant relationships. Transitional morphologies are found in the middle unit of the formation, a finding that is consistent with the evolution of Triceratops being characterized by anagenesis, the transformation of a lineage over time. Variation among specimens from this critical stratigraphic zone may indicate a branching event in the Triceratops lineage. Purely cladogenetic interpretations of the HCF dataset imply greater diversity within the formation. These findings underscore the critical role of stratigraphic data in deciphering evolutionary patterns in the Dinosauria.

Bonebeds may not be Evidence of Gregarious Behavior in Triceratops

Paleobiological implications of a Triceratops bonebed from the Hell Creek Formation, Garfield County, northeastern Montana

Author:

Keenan

Abstract:

Ceratopsid dinosaurs are notable for their common occurrences in bonebeds; however, until recently, these have not been encountered for the chasmosaurine Triceratops. The aim of this investigation is to describe the taphonomy of Quittin' Time (Museum of the Rockies locality HC-430), a Triceratops bonebed in the Hell Creek Formation, Garfield County, Montana. Using site taphonomic descriptions with an evaluation of ontogeny, inferences regarding the paleobiology of this extinct taxon are possible. The locality is associated with abundant organic material, including woody debris, large seeds, and other fragments in isolated silty lenses, all incorporated within a siltstone matrix, indicating preservation within a floodplain environment. Based on the repetition and ontogenetic stages of cranial elements, the minimum number of individuals (MNI) is three. Evidence from the location and taphonomic condition of the bones preserved in close proximity within the same siltstone unit suggests that the individuals—one young adult, one subadult, and a juvenile—likely accumulated during distinct flooding events within a narrow region of the floodplain as a result of “bloat-and-float” transport. The relatively small scale of the bonebed, both in terms of total area and number of individuals, implies that future work on Triceratops sites requires careful scrutiny of cranial elements examined within an ontogenetic framework because they are potentially critical to establishing MNI. Preservation of multiple individuals within the same unit does not necessarily provide evidence of gregarious behavior in Triceratops but rather may be a reflection of site taphonomic history and accumulation processes.

Where and When for Triceratops Discoveries Between 2006 to 2010 in Hell Creek, Montana

A stratigraphic survey of Triceratops localities in the Hell Creek Formation, northeastern Montana (2006–2010)

Authors:

Scannella et al

Abstract:

Here we provide a survey of Triceratops localities and accompanying stratigraphic data from the Hell Creek Formation of northeastern Montana. The majority of the sites discussed here were relocated or discovered during the last 5 yr of the Hell Creek Project (1999–2010), a multi-institutional effort to record a large volume of faunal, floral, and geologic data on the Hell Creek Formation in order to test evolutionary, paleoecological, and geological hypotheses. Triceratops is the most abundant dinosaur in the Hell Creek Formation and one of the most common nonavian dinosaurs of the Upper Cretaceous. It is known from hundreds of specimens, which have been collected since it was first described in 1889. Although these specimens provide a wealth of morphological data on Triceratops, many lack detailed stratigraphic information and context. Detailed stratigraphic and contextual data for more than 70 specimens of Triceratops collected during the Hell Creek Project make this data set among the most comprehensive for any nonavian dinosaur.

Torosaurus is NOT Triceratops

Is Torosaurus Triceratops? Geometric Morphometric Evidence of Late Maastrichtian Ceratopsid Dinosaurs

Authors:

Maiorino et al

Abstract:

Background

Recent assessments of morphological changes in the frill during ontogeny hypothesized that the late Maastrichtian horned dinosaur Torosaurus represents the “old adult” of Triceratops, although acceptance of this finding has been disputed on several lines of evidence.

Methodology/Principal Findings

Examining the cranial morphology of 28 skulls in lateral view and 36 squamosals of Nedoceratops hatcheri, Triceratops spp. and Torosaurus spp. by means of landmark-based geometric morphometrics, we compared ontogenetic trajectories among these taxa. Principal Component Analysis and cluster analysis confirmed different cranial morphologies. Torosaurus shape space is well separated from Triceratops, whereas Triceratops horridus and Triceratops prorsus partially overlap within Triceratops shape space. Linear regressions between shape and size suggest different ontogenetic trajectories among these taxa. Results support the “traditional” taxonomic status of Torosaurus. We hypothesize that ontogeny drives cranial morphology with different patterns between Torosaurus and Triceratops.

Conclusions/Significance

Torosaurus is a distinct and valid taxon. Whether looking at entire skulls, skulls without the frill, frills alone, or squamosals, Torosaurus has different morphologies and distinct allometric trajectories compared to Triceratops. This new approach confirms the taxonomic status of Torosaurus as well as the comparatively low diversity of ceratopsids at the end of the Maastrichtian in North America.

Fossil Site Yields Four Very Complete Triceratops

Somewhere south of Newcastle, amid the wide-open prairie and rolling hills, rests a mass grave. A femur here. A tooth there. A tip of a tail barely poking through the ground somewhere else.

The cause of death is unknown. It could have been a lightning strike, disease or an attack by a band of marauding T. rexes.

The victims: At least four U-Haul-sized, plant-eating triceratopses.

Paleontologists worked for two months this summer and found 250 bones. Only 950 more to go.

On a hot day in mid-August, one paleontologist held up a pterygoid for inspection. A pterygoid is a portion of a triceratops palette in its skull. It’s roughly the size of a loaf of bread, and had never previously been found complete and alone.

link.

Dimorphism in Ceratopsians

Triceratops Trio (Family?) Found in Wyoming: Two Adults, One Juvenile

Paleontologists digging in northeastern Wyoming have uncovered fossilized remains of three triceratops—possibly the most complete specimens of the dinosaurs yet found.

The triceratops bones were initially discovered by a rancher who owns the land in Newcastle, Wyo., just west of Mt. Rushmore. What is grassland today would have looked more like a subtropical flatland when the dinosaurs called it home during the Cretaceous period. So far the dig has uncovered bones from three triceratops—two adults and a juvenile about half their size.

[...]

Paleontologists think the three dinosaurs may have been a family, which could offer new insights into triceratops behavior and development. But there isn’t a happy ending to their story. Some of the largest dinosaur’s bones appear to have been bitten through, and researchers from the Black Hills Institute of Geological Research are pointing fingers at Tyrannosaurus rex...

HA! Toroceratops takes a hit.

World's Largest Triceratops Skull Found?

Its rumored to be 8 ft in length.  Check for fenestrae!  That sucker might be bigger than a Torosaurus skull.

Hope it was collected on the up and up.  Probably, but given the issues other fossil dealers have had recently...