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Available from: Denver W Fowler, Jun 14, 2015
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    ABSTRACT: This is the first cranial ontogenetic assessment of Triceratops, the well-known Late Cretaceous dinosaur distinguished by three horns and a massive parietal-squamosal frill. Our analysis is based on a growth series of 10 skulls, ranging from a 38 cm long baby skull to about 2 m long adult skulls. Four growth stages correspond to a suite of ontogenetic characters expressed in the postorbital horns, frill, nasal, epinasal horn and epoccipitals. Postorbital horns are straight stubs in early ontogeny, curve posteriorly in juveniles, straighten in subadults and recurve anteriorly in adults. The posterior margin of the baby frill is deeply scalloped. In early juveniles, the frill margin becomes ornamented by 17-19 delta-shaped epoccipitals. Epoccipitals are dorsoventrally compressed in subadults, strongly compressed and elongated in adults and ultimately merge onto the posterior frill margin in older adults. Ontogenetic trends within and between growth stages include: posterior frill margin transitions from scalloped to wavy and smooth; progressive exclusion of the supraoccipital from the foramen magnum; internal hollowing at the base of the postorbital horns; closure of the midline nasal suture; fusion of the epinasal onto the nasals; and epinasal expansion into a morphologically variable nasal horn. We hypothesize that the changes in horn orientation and epoccipital shape function to allow visual identity of juveniles, and signal their attainment of sexual maturity.
    Proceedings of the Royal Society B: Biological Sciences 12/2006; 273(1602):2757-61. DOI:10.1098/rspb.2006.3643 · 5.29 Impact Factor
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    ABSTRACT: FLUCTUATIONS (transgressions and regressions) of intercon-tinental sea-ways during the Mesozoic Era affected the habitat area of coastal plains by alternately restricting the space during transgressive phases and expanding the space during regressive phases. In western North America a coastal plain stretched along the eastern front of the young Rocky Mountains during most of the Late Cretaceous. Here we report results of a six-year field study of the sediments and dinosaur remains from this coastal plain in Montana that provide evidence that anagenesis characterized dinosaur evolution at this time, and that the evolutionary pulse coincided with a marine transgression.
    Nature 07/1992; 358(6381):59-61. DOI:10.1038/358059a0 · 42.35 Impact Factor
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    ABSTRACT: In horned dinosaurs, taxonomy is complicated by the fact that the cranial ornament that distinguishes species changes with age. Based on this observation, it has been proposed that the genera Triceratops and Torosaurus are in fact synonymous, with specimens identified as Torosaurus representing the adult form of Triceratops. The hypothesis of synonymy makes three testable predictions: 1) the species in question should have similar geographic and stratigraphic distributions, 2) specimens assigned to Torosaurus should be more mature than those assigned to Triceratops, and 3) intermediates should exist that combine features of Triceratops and Torosaurus. The first condition appears to be met, but it remains unclear whether the other predictions are borne out by the fossil evidence. We assessed the relative maturity of Torosaurus and Triceratops specimens by coding skulls for characters that vary with maturity, and then using a clustering analysis to arrange them into a growth series. We found that a well-defined sequence of changes exists in horned dinosaurs: development of cranial ornament occurs in juveniles, followed by fusion of the skull roof in subadults, and finally, the epoccipitals, epijugals, and rostral fuse to the skull in adults. Using this scheme, we identified mature and immature individuals of both Torosaurus and Triceratops. Furthermore, we describe the ventral depressions on the frill of Triceratops, and show that they differ in shape and position from the parietal fenestrae of Torosaurus. Thus, we conclude that these structures are not intermediates between the solid frill of Triceratops and the fenestrated frill of Torosaurus. Torosaurus is a distinct genus of horned dinosaur, not the adult of Triceratops. Our method provides a framework for assessing the hypothesis of synonymy through ontogeny in the fossil record.
    PLoS ONE 02/2012; 7(2):e32623. DOI:10.1371/journal.pone.0032623 · 3.23 Impact Factor
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