The others are in preparation, so my attempts to synthesize data on the other orders is likely to be incomplete, although the arrangement of the and some characteristic descriptions were provided by Fortey (in Whittington et al 1997), and adjusted via recent articles (, papers dealing with higher classification of the Asaphida, Proetida, Harpetida, Agnostida, and Lichida).
The evolution of distinct pectoral and pelvic fins is prefigured in the fin-folds of anaspids and thelodonts. In , both pairs appear in the same animal. When we next encounter these structures in gnathostomes, both sets are proper fins with internal skeletons, however these differ:In fact, the pelvic fin is often in line with various other structures also derived from the postbranchial fin-fold, such as the series of spines in (right. Other examples below). Indeed, the pectoral fin is also part of an array of spines arranged perpendicular to the spines of the postbranchial fin-fold.
You can download a gargantuan series of pdfs of theentire book:Paleobiology: A Synthesis (1990)Edited by Derek Briggs and Peter Crowtherat:Don't even think about dialup.
Dr. Mullis joined the Cetus Corporation in Emeryville, California, as a DNA chemist in 1979. During his seven years there, he conducted research on oligonucleotide synthesis and invented the polymerase chain reaction.
Among living craniates, bone in any form only occurs among members of Vertebrata - craniates with vertebral elements protecting their spinal cords. What does the study of fossil organisms tell us about the distribution of bony tissue?
The braincase of adult gnathostomes ossifies in a variety of patterns, however the early development of the cartilages that give rise to it is relatively uniform. The first cartilages to appear give rise to the floor of the braincase or form the special sense capsules. From front to back we see: These define the basic landmarks of the braincase. For example, the hypophysis forms on the midline between parachordals and trabeculae. During growth, these cartilages grow together to form a trough-shaped braincase.