The starting point of comparative evolutionary studies of the dermal skeleton of vertebrates is Hertwig’s series of papers (1874, 1876/1879/1882), which directly stimulated many dozens of papers, most of them in German and some of them long forgotten. The literature on comparative histology and histogenesis of the dermal skeleton and on regulatory and morphogenetic processes of the vertebrate integument is so voluminous that it can hardly be summarized. Ever since Hertwig, attempts have been made not only to contribute descriptive and experimental data, but also to arrive at a synthesis. Most of the synthetic papers, however, address only a small section of the theoretical problems, and it seems that some important questions have never been asked.

Evolution of Dermal Skeleton and Dentition in Vertebrates

Phosphatic dermal fragments of Anatolepis, interpreted as a heterostracan fish (class Agnatha), have been discovered in the Deadwood Formation of Late Cambrian age in northeastern Wyoming. This discovery extends back the age of the earliest known vertebrate fossils by approximately 40 million years. Other occurrences of Anatolepis in North America, Greenland, and Spitzbergen show that these fish had a widespread geographic distribution in Late Cambrian and Early Ordovician marine environments.

http://science.sciencemag.org/content/sci/200/4341/529.2.full.pdf

A fish from the upper cambrian of north america.

Size and distribution of dermal elements in Ordovician eriptychiids and astraspids are considered relative to phases of skeletal assimilation and regression. The phyletic significance of acellularity in aspidin is discussed, as also is the alleged relationship claimed to exist in initial developmental stages between that hard tissue and dentine proper. To judge from hard tissue histology the astraspids may not, like the eriptychiids, have belonged to the heterostracans but to another group, of early agnathans, still incompletely known.

Histologic studies of ostracoderms, placoderms and fossil elasmobranchs. 6. Hard tissues of Ordovician vertebrates

The palaeoecology of the Ordovician Harding Sandstone (Colorado, U.S.A.)

The early vertebrate Astraspis, habitat based on a lithologic association

Vertebrate fossil fragments from the northeast part of St. Joseph Island, Ontario, Canada have been referred to Astraspis desiderata Walcott on the basis of their stellate tubercles. They occur in shallow marine limestones of Middle Ordovician age. This is the first record of an Ordovician vertebrate in limestone. This discovery considerably extends the geographic range of Astraspis and lends some additional support to the theory of the marine origin of the vertebrates.

Ordovician Vertebrates from Ontario

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Ordovician Vertebrates from Ontario