Handbook of Paleoichthyology, Volume 3B: Chondrichthyes II

From the introduction:

"Mesozoic and Cenozoic selachians (the group comprising sharks and dogfishes) are known mainly by isolated teeth which are found abundantly in many deposits. Skeletons are much rarer and have been preserved in only a few exceptional deposits. In the Lias, one may mention Lyme Regis in England and Bad Boll – Holzmaden in Germany; in the Upper Jurassic, Cérin in France, Solnhofen and the deposits in the area of Eichstätt and in Nusplingen in Germany; in the Cretaceous, the Cenomanian and Santonian deposits of Lebanon and the Campanian of Bavaria; in the Tertiary, only the famous Eocene deposits of Monte-Bolca in Italy and the Green River limestones in Wyoming have furnished complete skeletons of selachians.

Since the publication of Agassiz’s classic work (1833–1844), a great many works have gradually made known the selachian faunas which have succeeded one another in the course of geologic time. In the first phase, description of the genera and species was based on small samples of teeth which, in view of the heterodont character of the elasmobranch dentition, resulted in the creation of many taxa which were later shown to be invalid; in a second phase, researchers who based their studies on large series of teeth and Recent comparative material endeavoured to circumscribe the species and their variations.

However, these palaeontologists adhered to a foregone conclusion, namely that their fossil forms belong to Recent genera; during this phase, which lasted until 1957, almost all the Cretaceous and Tertiary species were referred to Recent genera except for a few distinctive extinct forms such as Ptychodus, Squalicorax, Rhombodus and the Sclerorhynchidae. Glückman started a new phase in the study of fossil selachians, which is particularly important for the study of Cretaceous and Tertiary forms; indeed this author demonstrated many cases of dental morphological convergence which induced him to propose many new genera for species previously attributed to Recent genera. I will return later to the classification established by Glückman (1964) which can be criticized on many points; however, it made it possible to reorientate the study of Cretaceous and Tertiary selachians and to understand better the phylogeny of some groups. Casier (1967) was the first to cautiously follow Glückman’s lead.

One must also emphasize that during the past fifteen or so years, the development of new techniques of collection and preparation of sediments has made it possible to collect rich faunal assemblages of small and very small teeth which contained many new genera. The inventory of fossil faunas is far from complete; the ichthyofaunas of many regions still remain practically unknown and the exploration of these areas will result in the discovery of new faunas, probably rich in new genera. The aim of the present work is to give an idea of what is presently known about the genera of Mesozoic and Cenozoic selachians."