Incremental lines of von Ebner in dinosaurs and the assessment of tooth replacement rates using growth line counts

Dinosaur dentine exhibits growth lines that are tens of micrometers in width. These laminations are homologous to incremental lines of von Ebner found in extant mammal and crocodilian teeth (i.e., those of amniotes). The lines likely reflect daily dentine formation, and they were used to infer tooth development and replacement rates. In general, dinosaur tooth formation rates negatively correlated with tooth size. Theropod tooth replacement rates negatively correlated with tooth size, which was due to limitations in the dentine formation rates of their odontoblasts. Derived ceratopsian and hadrosaurian dinosaurs retained relatively rapid tooth replacement rates through ontogeny. The evolution of dental batteries in hadrosaurs and ceratopsians can be explained by dentine formation constraints and rapid tooth wear. In combination with counts of shed dinosaur teeth, tooth replacement rate data can be used to assess population demographics of Mesozoic ecosystems. Finally, it is of historic importance to note that Richard Owen appears to have been the first to observe incremental lines of von Ebner in dinosaurs more than 150 years ago.

Impacts of Aluminum on the Cytoskeleton of the Maize Root Apex. Short-Term Effects on the Distal Part of the Transition Zone1

Using monoclonal tubulin and actin antibodies, Al-mediated alterations to microtubules (MTs) and actin microfilaments (MFs) were shown to be most prominent in cells of the distal part of the transition zone (DTZ) of an Al-sensitive maize (Zea mays L.) cultivar. An early response to Al (1 h, 90 μm) was the depletion of MTs in cells of the DTZ, specifically in the outermost cortical cell file. However, no prominent changes to the MT cytoskeleton were found in elongating cells treated with Al for 1 h in spite of severe inhibition of root elongation. Al-induced early alterations to actin MFs were less dramatic and consisted of increased actin fluorescence of partially disintegrated MF arrays in cells of the DTZ. These tissue- and development-specific alterations to the cytoskeleton were preceded by and/or coincided with Al-induced depolarization of the plasma membrane and with callose formation, particularly in the outer cortex cells of the DTZ. Longer Al supplies (>6 h) led to progressive enhancements of lesions to the MT cytoskeleton in the epidermis and two to three outer cortex cell files. Our data show that the cytoskeleton in the cells of the DTZ is especially sensitive to Al, consistent with the recently proposed specific Al sensitivity of this unique, apical maize root zone.

Tooth histology and ultrastructure of a Paleozoic shark, Edestus heinrichii / Katherine Taylor -- and Thomas Adamec --.

v.33:no.24(1977)

A mastodont tooth from Szechwan, China / Dirk A. Hooijer -- and Edwin H. Colbert --.

v.10:no.12(1951)

Ichthyolith (fish tooth and denticle) counts from DSDP Site 91-596 and ODP Site 145-886

While the history of taxonomic diversification in open ocean lineages of ray-finned fish and elasmobranchs is increasingly known, the evolution of their roles within the open ocean ecosystem remains poorly understood. To assess the relative importance of these groups through time, we measured the accumulation rate of microfossil fish teeth and elasmobranch dermal denticles (ichthyoliths) in deep sea sediment cores from the North and South Pacific gyres over the past 85 million years. We find three distinct and stable open ocean ecosystem structures, each defined by the relative and absolute abundance of elasmobranch and ray-finned fish remains. The Cretaceous Ocean (pre-66 Ma), was characterized by abundant elasmobranch denticles, but low abundances of fish teeth. The Paleogene Ocean (66-20 Ma), initiated by the Cretaceous/Paleogene Mass Extinction, had nearly 4 times the abundance of fish teeth compared to elasmobranch denticles. This Paleogene Ocean structure remained stable during the Eocene greenhouse (50 Ma) and the Eocene-Oligocene glaciation (34 Ma), despite large changes in overall accumulation of both groups during those intervals, suggesting that climate change is not a primary driver of ecosystem structure. Dermal denticles virtually disappeared from open ocean ichthyolith assemblages about 20 Ma, while fish tooth accumulation increased dramatically in variability, marking the beginning of the Modern Ocean. Together, these results suggest that open ocean fish community structure is stable on long timescales, independent of total production and climate change. The timing of the abrupt transitions between these states suggests that the transitions may be due to interactions with other, non-preserved pelagic consumer groups.