Chondrichthyans from the spanish triassic (westermost tethys) with remarks on the evolution of chondrichthyan enameloid

Abstract

Iberian Peninsula has been the focus of Geological and Paleontological studies for more than a century and the stratigraphic sequence and the invertebrate assemblages are well established for a number of different basin. However, works focused on chondrichthyans faunas were non-existent until recently. As a consequence, it has been long time assumed that chondrichthyans were very scarce (or inexistent) in the coastal environments of the current Iberian Peninsula during Triassic times. This view changed at the beginning of the last decade, with the studies of paleontologist from the department of Geology of the University of Valencia (Dra. Márquez-Aliaga, Dr. Héctor Botella, Dr. Pablo Plasencia and Dr. Martínez-Pérez). Their first studies already demonstrated that the shark faunas from Middle Triassic of the Iberian Chains ( East of Spain) are comparable (in terms of diversity and abundance) with other worldwide localities of the same age, and even a number of endemic taxon were erected. Preliminary studies at Bugarra section yield abundants disarticulated remains of chondrichthyans exoesqueleton (mainly teeth and scales) including teeth of Pseudodalatias henarejensis, a new endemic species of pseudodalatiid. But it wasn’t until 2013, when the first comprehensive taxonomic study of the chondrichthyan assemblage of several Middle Triassic sections from the Iberian Range was published. The assemblage consisted of isolated remains of seven species of five non-neoselachian shark genera (Palaeobates, Hybodus, Pseudodalatias, Omanoselache, and Lissodus), including a new species of hybodontiform shark, Hybodus bugarensis. This assemblage represented a varied group from a paleogeographic point of view made up of common genus from Middle Triassic shark faunas of northern Europe together with species only known previously from North America and China. This disparity in the ‘paleobiogeographical’ distribution of the species was suggested to be linked with the Ladinian marine transgression, which allowed for the colonization of the shallow epicontinental seas of the Iberian Peninsula by faunas from the Germanic domain to the north and the Sephardic domain to the south of the Tethys sea. However further studies were required to test this hypothesis and to fully understand the faunal colonization patterns of the “new-developed” epicontintental seas of the Iberian seas, which can be an excellent proxy to compare differences in colonization process between differences groups in a long-time ecological context. After this, the project of the present thesis chondrichthyan faunas found in the Betic and the Catalanonian Coastal Ranges are taxonomical similar to that described from the Ladinian of the Iberain Range , the relationship with other Triassic localities of the Tethys sea and the possible pathways for the colonization of the new shallow epicontinental seas of the western Tethys followed by coastal sharks and to compare it with other faunas, adjusting of previous palaeogeographic reconstructions of the westernmost Tethyan realm for the Middle Triassic. Due to the cartilaginous nature of the skeleton of chondricthyans, their most common fossil remains are their teeth and scales. So, apart from the systematic classification of the chondrichthyan remains based on morphological characteristics, histological studies of the tissues that composed them has been realizing since the 19th century. Chondrichthyes teeth are covered by a thin layer of hypermineralized tissue known as enameloid with an intricate hierarchical organization. The smallest repeating structural units are individualized elongated fluoroapatite crystallites [Ca5(PO4)3F], each formed by a number of hexagonal fluoroapatite unit cells. In a higher level of structural complexity, crystallites are arranged in parallel, forming tightly packed bundles, each with an envelope of organic matrix. Bundles with different orientations make assemblies that can be found in different layers, forming the whole enameloid layer of the teeth. The acquisition of these hierarchical structural levels has been progressive along the phylogeny of Chondrichthyans and traditionally has been related to the emergence of new feeding strategies in the group (i.e grasping and shallowing, grinding, crushing, cutting, etc). Through the last century, the study of the the micro and ultrastructure of the enameloid layer has been of great importance in order to differentiate teeth of neoselachian (batomorphs and selachimorphs), and in understanding higher-level taxonomic relationships and the evolutionary history of the group. It was assumed that the teeth of neoselachian, more precisely selachimorphs, are organized in a triple layer enameloid, whereas for batoid different works reported the presence of a single layer of TBE (Tangled Bundled Enameloid) or of SCE (Single Crystallite Enameloid). This resulted in the emergence of a debate centred around whether this ‘simplified’ enameloid is a derived or a retained ancestral character for the group. However, enameloid microstructure has never been studied in a broad phylogenetically and/or anatomical context. Accordingly, the study of the enameloid layer has demonstrated the presence of an homogeneus SCE layer in euselachian scales, the teeth of Lonchidion derenzii and most of extant batoid; whereas all Archaeobatoidae and Rhyna ancylostoma present a bundled enameloid