Abstract
AbstractThe fossil record of chondrichthyans (sharks, rays and chimaeroids) principally consists of isolated teeth, spines and dermal denticles, their cartilaginous skeleton being rarely preserved. Several Late Jurassic chondrichthyan assemblages have been studied in Europe based on large bulk samples, mainly in England, France, Germany and Spain. The first study of this kind in Switzerland is based on controlled excavations in Kimmeridgian deposits related to the construction of the A16 motorway in the Swiss Jura (Porrentruy, NW Switzerland). This study is based on more than 2000 isolated chondrichthyan remains (teeth, dental plates, spines and dermal denticles) and adds to our knowledge of the chondrichthyan distribution at a regional scale in Europe. We describe and identify this new fauna, define a new species of hybodont with crushing‐type dentition (Asteracanthus udulfensis sp. nov.) and report for the first time the carcharhiniform Corysodon cirinensis in Switzerland. By the Late Jurassic, modern neoselachian sharks had overtaken hybodonts in European marine realms, the latter being gradually confined to brackish or freshwater environments. However, while the associated fauna of the Porrentruy platform indicates marine conditions, neoselachian sharks are surprisingly rare. The chondrichthyan assemblage is largely dominated by hybodonts, guitarfishes (rays) and chimaeroids that are all known to be euryhaline. This unexpected chondrichthyan faunal composition questions the presence of fresh to brackish water in the vicinity of the platform, and the occurrence of salinity fluctuations within a general context marine. This could explain the scarcity of neoselachian sharks and the extended success of hybodonts in the Porrentruy area as late as the Late Jurassic.
Highlights
The fossil record of chondrichthyans principally consists of isolated teeth, spines and dermal denticles, their cartilaginous skeleton being rarely preserved
Late Jurassic chondrichthyan assemblages of Europe are generally characterized by the reduced presence of hybodont sharks in marine deposits (e.g. Duffin & Thies 1997; Kriwet & Klug 2004) gradually replaced by the socalled modern sharks and rays (Neoselachii) (Cuny & Benton 1999) that underwent a strong diversification in the Middle Jurassic, an epoch when most of their groups emerged (Underwood 2006; Kriwet et al 2009)
Euryhaline character of the hybodont Asteracanthus, recently demonstrated through stable isotope analyses, so far only in the study area (Leuzinger et al 2015). This is the first study of Kimmeridgian chondrichthyans using a large and diverse dataset from a Swiss locality
Summary
The fin spines of the second kind are massive and are five in number They all show star-shaped tubercles (Cappetta 1987) organized in longitudinal rows on the lateral and anterior surfaces (Fig. 4K–L). The best-preserved fin spine (MJSN TCH007-89, Fig. 4K–M) shows that the anterior and lateral tubercles cover more than half of its length. Our material differs from cephalic spines identified as Planohybodus (Rees & Underwood 2008) by having a basal plate with a more curved anterior edge, and a crown less curved posteriorly. Several small hybodont teeth lacking nodes or protuberances on their labial and/or lingual sides share
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