Cracks in Fossil Enamels Resulting from Premortem vs. Postmortem Events

Vertebrate enamel preserves a record of fracture-producing strain. Fracturing during the life of the individual is potentially a source of selection for stronger enamel in the course of evolution. To determine if it is possible to recognize such fractures in fossil enamel, cracks in a variety of fossil materials, including enamel-covered holostean scales, crocodilian teeth, theropod and hadrosaurid dinosaur teeth, and mammalian teeth were examined. Cracks that occurred during the life of the individual could be recognized by abrasive wear on edges exposed at the surface of the enamel in areas worn by oral or locomotor abrasion. Certain distinctive crack patterns were identified as results of specific stress states occurring during life. Transverse cracks on the anterior parts of Lepisosteus scales were probably caused by external loading. Hertzian cracks and shallow, arcuate, lateral cracks on the occlusal edges of tooth enamel appear to be caused by stress concentrating impacts. Horizontal cracks arranged asymmetrically on the sides of conical teeth were reproduced in models subjected to bending stresses. Oblique cracks near the tips of conical fossil teeth were produced in models by oblique loads near the tip. Vertical cracks around cylindrical or conical tooth surfaces may be caused by several different sources of stress, including lateral wind loads and vertical snow loads. Of the postmortem causes of fracturing of fossil enamel, drying cracks seem to be the most important. Experimental drying produced from 25% to 50% of the cracks in dry teeth

Microscopic Effects of Predator Digestion on the Surfaces of Bones and Teeth

Concentrations of small fossil mammals are frequently encountered in Cenozoic deposits, but the causes for such accumulations have seldom been determined. In many cases the tooth, jaw, and limb fragments appear to be well-preserved under light microscopy, and it is difficult to differentiate damage due to predator digestion from breakage and abrasion due to physical agents. In order to find more specific evidence of predator digestion, we used a scanning electron microscope (SEM) to examine the surface microstructure of bones and teeth consumed by Bubo virginianus (great horned owl) and Canis latrans (coyote), which prey upon similar species. Effects of digestion were found on all the digested bones and teeth examined. The effects on bone include distinctive sets of pits and fissures, dissolution, and physical polishing. The pits and fissures are apparently caused by solution that commences in canals beneath the surf ace of the bone. The most conspicuous effects on teeth are island-like pillars of dentin surrounded by deep solution fissures. The effects of digestion by coyote and owl are fundamentally the same but differ in degree of development. Bone digested by the owl shows a greater degree of polishing and rounding of edges but has less extensive fissuring. Wide variation in the degree of surface damage occurs in bones digested by the coyote, even within a single fecal pellet

Enamel Structure in Astrapotheres and Its Functional Implications

Astrapotheres, large extinct ungulates of South America, share with rhinoceroses vertical prism decussation in the cheek tooth enamel. The similarity extends beyond merely the direction of the planes of decussation. The vertical decussation in astrapotheres is confined to the inner part of the enamel and has uniformly well-defined zones in which the prism direction differs by nearly 90° and the zones are separated by narrow transitional borders of intermediate prism direction. The outer enamel consists of predominantly occlusally and outwardly directed prisms. Within the outer enamel is a region of horizontally decussating prisms; here the angle of decussation is usually smaller than that of the inner vertically decussating prisms. Except for the horizontal decussation in the outer enamel, these conditions match structures that have been described for rhinocerotoids. These features, together with the similarity in premortem crack direction and gross shape of the cheek teeth, imply that astrapotheres and rhinocerotoids shared essentially the same system of cheek tooth mechanics. However, the microstructure of the canine enamel in the astrapotheres is distinct. The lower canine enamel of the Oligocene Parastrapotherium exhibits a form of vertical decussation modified by a wavelike bending of prism zones, whereas the decussation in the rhinocerotoid canine is horizontal. The lower canine in Parastrapotherium was subjected to different loading conditions, judging from multiple sets of premortem crack directions. The modified vertical decussation would in theory resist cracking under different directions of tensile stresses. This is confirmed by the sinuous paths of cracks that run in directions differing by up to 90°. That diverse stresses were generated in the enamel during life is confirmed by the pattern of premortem cracks in Parastrapotherium. The enamel in the upper canine of a late Miocene astrapothere lacks decussation but may have resisted cracking under varied loading conditions by virtue of a 3-dimensional wavelike bending of the prisms

Correction: Bone Cells in Birds Show Exceptional Surface Area, a Characteristic Tracing Back to Saurischian Dinosaurs of the Late Triassic

<p>Correction: Bone Cells in Birds Show Exceptional Surface Area, a Characteristic Tracing Back to Saurischian Dinosaurs of the Late Triassic</p

Bone Cells in Birds Show Exceptional Surface Area, a Characteristic Tracing Back to Saurischian Dinosaurs of the Late Triassic

<div><p>Background</p><p>Dinosaurs are unique among terrestrial tetrapods in their body sizes, which range from less than 3 gm in hummingbirds to 70,000 kg or more in sauropods. Studies of the microstructure of bone tissue have indicated that large dinosaurs, once believed to be slow growing, attained maturity at rates comparable to or greater than those of large mammals. A number of structural criteria in bone tissue have been used to assess differences in rates of osteogenesis in extinct taxa, including counts of lines of arrested growth and the density of vascular canals.</p><p>Methodology/Principal Findings</p><p>Here, we examine the density of the cytoplasmic surface of bone-producing cells, a feature which may set an upper limit to the rate of osteogenesis. Osteocyte lacunae and canaliculi, the cavities in bone containing osteocytes and their extensions, were measured in thin-sections of primary (woven and parallel fibered) bone in a diversity of tetrapods. The results indicate that bone cell surfaces are more densely organized in the Saurischia (extant birds, extinct Mesozoic Theropoda and Sauropodomorpha) than in other tetrapods, a result of denser branching of the cell extensions. The highest postnatal growth rates among extant tetrapods occur in modern birds, the only surviving saurischians, and the finding of exceptional cytoplasmic surface area of the cells that produce bone in this group suggests a relationship with bone growth rate. In support of this relationship is finding the lowest cell surface density among the saurischians examined in Dinornis, a member of a group of ratites that evolved in New Zealand in isolation from mammalian predators and show other evidence of lowered maturation rates.</p></div

Total number of canalicular branching points per sample.

<p>See also legend for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119083#pone.0119083.g003" target="_blank">Fig. 3</a>.</p

Phyletic differences in cytoplasmic density and lamellar quality.

<p>Exceptionally dense branching of the osteocyte cytoplasmic processes characterizes all of the sampled saurischians except the moa <i>Dinornis</i>. Note that reduction of lamellar quality had not yet appeared in the earliest theropod and sauropodomorph members. Phyletic relationships from [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119083#pone.0119083.ref033" target="_blank">33</a>]. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119083#pone.0119083.ref034" target="_blank">34</a>]</p

Differences in quality (boundary definition and continuity) of bone lamellae.

<p>Top row—late Cretaceous to Recent saurischians (poorly formed lamellae). Left to right: Ornithurae (<i>Phalacrocorax</i>, Recent); Ornithomimidae (small theropod, late Cretaceous); <i>Tyrannosaurus</i> (large theropod, late Cretaceous); Sauropoda (titanosaurid, late Cretaceous). Middle row—Saurischians with well-formed distinct lamellae. <i>Dinornis</i> (Moa, sub-Recent); <i>Coelophysis</i> (small theropod, late Triassic); <i>Herrerasaurus</i> (basal theropod, late Triassic); <i>Adeopapposaurus</i> (basal sauropodomorph, early Jurassic). Bottom row—Non-saurischians. Ornithischia (hadrosaurid, late Cretaceous); Lacertilia (<i>Tupinambis</i>, lizard, Recent); Chelonia (testudinid, late Cretaceous); Mammalia (<i>Puma</i>, Recent).</p

Total lengths of canaliculi per sample.

<p>Each sample is 770,884 μm² in area; null probabilities were calculated with Fisher's Exact test; red frequency distributions identify Saurischia in this and subsequent figures. Original measurements for Figs. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119083#pone.0119083.g003" target="_blank">3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119083#pone.0119083.g004" target="_blank">4</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119083#pone.0119083.g005" target="_blank">5</a> are listed in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119083#pone.0119083.s001" target="_blank">S1 Table</a>.</p