New in Paleobiology

["Jaime A. Headden" <qilongia@yahoo.com>]

  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.

Abstract:
  "The evolution of scutes in thyreophoran dinosaurs, based on
*Scutellosaurus*,
   *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
tissues
   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
   incremental growth.

  "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
the
   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
   facultative thermoregulation."

--

  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.

Abstract:
  "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:
The
    significance of reduced functional tooth replacement in marsupial and
    placental mammals. _Paleobiology_ 3(2):324-346.

Abstract:
  "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
suppression
   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
diverse
   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."

  Cheers,

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

"Human beings, who are almost unique in having the ability to learn from the 
experience of others, are also remarkable for their apparent disinclination to 
do so." --- Douglas Adams


                
__________________________________ 
Discover Yahoo! 
Use Yahoo! to plan a weekend, have fun online and more. Check it out! 
http://discover.yahoo.com/