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New in Paleobiology
Main, R. P., A. de Ricqlés, J. R. Horner, and K. Padian. 2005. The evolution
and function of thyreophoran dinosaur scutes. _Paleobiology_ 31(2):291-314.
"The evolution of scutes in thyreophoran dinosaurs, based on
*Scelidosaurus*, *Stegosaurus*, and several ankylosaurs, began with small
rounded or ovoid structures that typically had slight, anteroposteriorly
oriented keels. These scutes were elaborated in two general and overlapping
ways: they could flare laterally and asymmetrically beneath the keels that
mark the anteroposterior axis, and they could be hypertrophied in their
distal growth to produce plates, spikes, and other kinds of ornamentation.
*Stegosaurus* plates and spikes are thus primarily hypertrophied keels of
primitive thyreophoran scutes, sometimes with elaboration of dermal bone
around their pustulate bases. Histologically, most thyreophoran scute
comprise secondary trabecular medullary bone that is sandwiched between
layers of compact primary bone. Some scutes partly or mostly comprise
anatomically metaplastic bone, that is, ossified fibrous tissue that shows
"The 'plumbing' of *Stegosaurus* plates was not apparently built to support a
'radiator' system of internal blood vessels that communicated with the
outside of the plates and coursed along their external surfaces to return
heated or cooled blood to the body core. Possibly a purely external system
supported this function but there is no independent evidence for it. On the
other hand, many of the vascular features in stegosaurian plates and spikes
reflect _bautechnisches_ artifacts of growth and production of bone. Surface
vascular features likely supported bone growth and remodeling, as well as
blood supply to a keratinous covering. When the gross and microstructural
features of the plates and spikes are viewed in phylogenetic context, no
clear pattern of thermoregulatory function emerges, though an accessory role
cannot be eliminated in certain individual species. It seems more likely, as
in other groups of dinosaurs, that the variation of dermal armor form in
stegosaurs was primarily linked to species individuation and recognition,
perhaps secondarily to inter- and intraspecific display, and rarely to
Srivastava, R. A. Sahni, S. A. Jafar, and S. Mishrad. 2005.
Microstructure-dictated resistance properties of some Indian dinosaur
eggshells: finite element modeling. _Paleobiology_ 31(2):315-323.
"Finite element modeling (FEM) has been used to evaluate
microstructure-controlled stability of selected eggshells of Indian
dinosaurs. Our study suggests that under static load the eggshell
microstructure of *Megaloolithus cylindricus* displays a low magnitude of
tensile stress over most of the spherolith. The magnitude of this tensile
stress is lower than that displayed in *M. jabalpurensis*, *M. baghensis*,
and *Subtiliolithus kachchhensis*. In *M. cylindricus*, a shell thickness
matching the length of the spheroliths prevents the failure of eggshells,
whereas in *M. jabalpurensis* and *M. baghensis*, which have thinner shells,
the development of additional subspheroliths compensates for the relatively
higher magnitude of tensile stresses. Extremely thin eggshell in *S.
kachchhensis* shows a still higher magnitude of tensile stresses, thereby
making it prone to cracking, but the propagation of cracks is apparently
checked and stability reinforced by wider spacing of pore canals."
van Nievelt, A. F. H. and K. K. Smith. 2005. To replace or not to replace:
significance of reduced functional tooth replacement in marsupial and
placental mammals. _Paleobiology_ 3(2):324-346.
"Marsupial mammals are characterized by a pattern of dental replacement
thought to be unique. The apparent primitive therian pattern is two
functional generations of teeth at the incisor, canine, and premolar loci,
and a series of molar teeth, which by definition are never replaced. In
marsupials, the incisor, canine, and first and second premolar positions
possess only a single functional generation. Recently this pattern of dental
development has been hypothesized to be a synapomorphy of metatherians, and
has been used to diagnose taxa in the fossil record. Further, the
of the first generation of teeth has been linked to the marsupial mode of
reproduction, through the mechanical suppression of odontogenesis during the
period of fixation of marsupials, and has been used to reconstruct the mode
of reproduction of fossil organisms. Here we show that dental development
occurs throughout the period of fixation; therefore, the hypothesis that
odontogenesis is mechanically suppressed during this period is refuted.
Further, we present comparative data on dental replacement in eutherians and
demonstrate that suppression of tooth replacement is fairly common in
groups of placental mammals. We conclude that reproductive mode is neither a
necessary nor a sufficient explanation for the loss of tooth replacement in
marsupials. We explore possible alternative explanations for the loss of
replacement in therians, but we argue that no single hypothesis is adequate
to explain the full range of observed patterns."
Jaime A. Headden
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