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524 FOSSIL VERTEBRATES/Cenozoic Amphibians accumulations Notable lagerstaă tte assemblages are the Eocene brown coals from Messel and Geiseltal in Germany, and the Oligocene–Miocene freshwater limestones from Oeningen in Switzerland and from Bechlejovice in the Czech Republic These produce large numbers of complete, articulated frogs and tadpoles and a smaller number of salamanders Modified body tissues may be preserved, but the skeletons are generally crushed flat In contrast, microvertebrate assemblages may contain few to abundant amphibian bones that are three-dimensionally preserved but are not associated, resulting in problems of association unless there is significant similarity to living taxa The Cenozoic record of caecilians is negligible and that of albanerpetontids is too patchy to evaluate in the absence of living descendants The record of frogs and salamanders predominantly derives from North America and Eurasia, with frogs also well represented in South America For salamanders, this ‘Laurasian’ range may represent a genuine pattern, because most living genera of salamander are found in the same area, but for frogs it is undoubtedly an artefact of the distribution of microvertebrate collecting There is ample morphological and molecular evidence that the Neobatrachia or higher frogs originated in Gondwana and that they diversified during the breakup of that supercontinent It can be anticipated that a substantial record will eventually be found in Africa, India, and Australia as well as in South America Aquatic amphibians living in lakes and ponds are preserved most readily and so are disproportionately well represented in the fossil record The frog families Pipidae and Palaeobatrachidae (Figure 1) and the salamander family Cryptobranchidae fall into this category At the other extreme, stream-dwelling amphibians and those living in damp terrestrial environments are hardly represented at all The salamander family Plethodontidae, for example, comprised mainly of such forms, is the largest family of living salamanders but has one of the poorest fossil records Figure The palaeobatrachid frog Palaeobatrachus grandipes, from the Oligocene, Bechlejovice, Czech Republic Specimen at the National Museum, Prague ß Andrew Milner several successful Gondwanan families, the ranid frogs (Figure 2), bufonid toads, and hylid tree frogs, extended their ranges into Laurasia, diluting the original pattern First appearances of fossils of these families in the northern continents show this to have happened in the Paleocene and Eocene, for ranids and bufonids Much of the Cenozoic fossil record of frogs comprises earlier representatives of the extant frog faunas of each continent, e.g., leptodactylids in South America, pipids and ranids in Africa, myobatrachids in Australia, and discoglossids and pelobatids in Europe Occasionally, however, frog fossils reveal that past faunas had distinctive components Latonia gigantea, for example, a discoglossid from the Oligocene– Pliocene of Europe, was twice as large as any modern discoglossid and had a heavily sculptured skull Frogs Salamanders Frogs occur on all non-polar continents and on many islands, particularly those such as New Zealand, Madagascar, and the Seychelles, which are relicts of larger continents Early in Mesozoic frog evolution, there appears to have been a major dichotomy between Laurasian and Gondwanan frogs, with the discoglossids, pelobatids, pelodytids, and palaeobatrachids evolving in Laurasia, and the pipids and neobatrachians ($20 families) evolving in Gondwana By the Cenozoic and up to the present day, this division is still recognizable in the distribution of frog families However, during the Cenozoic, The major diversity of living salamanders occurs in North America, central America, and Eurasia A few genera are found in South America and North Africa, and these are believed to represent Neogene range extensions The Cenozoic record is consistent with this, with much of the fossil material belonging to the same families and genera that occur in each continent today Most of the European fossil record is of salamandrids and proteids; the Asian record comprises salamandrids, hynobiids, and cryptobranchids; and the North American record is of cryptobranchids, sirenids, amphiumids, proteids, ambystomatids, and
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