Tooth-like structures on the rostrum of the Cretaceous batoid Schizorhiza

Author(s): Charlie J. Underwood, Moya Meredith Smith, Zerina Johanson, Alex Riley, Gareth J. Fraser, Jürgen Kriwet
Abstract: Chondrichthyans possess both true teeth within their mouth and tooth-like denticles on their skin, both outside and inside the mouth. Understanding the relationships between teeth and denticles is fundamental to the interpretation of the origins of teeth within the gnathostomes. Superficially tooth-like structures on the body surface of sharks and rays typically comprise body-covering denticles and various structures that appear to be derived from them (such as dermal thorns in many rays and tail spines in stingrays). In addition, elongate 'teeth' are present on lateral edges of the rostrum of several clades; pristiophorids (sawsharks), pristids (sawfishes) and sclerorhynchids (extinct sawfish-like rays). Despite their highly derived form, these appear to represent convergent evolution of modified dermal denticles, either growing continuously (pristids) or added to/replaced through life (pristiophorids, sclerorhynchids). Rostral 'teeth' of the Cretaceous batoid Schizorhiza differ from those of other chondrichthyans, and challenge the idea that all rostral 'teeth' are derived from dermal denticles and do not represent 'teeth outside the mouth', as they show a series of characters otherwise only present within oral teeth. Several partial to near complete articulated rostra were obtained from the Maastrichtian of Morocco. These were studied by surface observation, thin sectioning and micro CT scanning. Individual 'teeth' have a unique morphology with a short crown and deeply divided root. 'Teeth' do not reach the tip of the rostrum, and become progressively smaller as the rostral tip is approached. These 'teeth' form an alternating pattern along the edges of the rostrum, a pattern otherwise seen in oral teeth of chondrichthyans. In addition, replacement 'teeth' being present within the root cavity of the preceding 'tooth', with replacement occurring from below, a replacement pattern otherwise unknown in chondrichthyans but present in teeth of osteichthyans. Below the well and partially developed rostral 'teeth', developing crowns are seen to form parallel with the edge of the rostral cartilage before undergoing two rotations sets of before moving into replacement position. Rotation into position is present in modified denticles of some chondrchthyans (such as rostral 'teeth' of pristiophorids) and the oral teeth of some osteichthyans. We will discuss the significance of these characters and propose a likely model for genetic control of the development of these unique structures. Grant Information NERC
Organisation(s): Department of Palaeontology
External organisation(s): University of London, Natural History Museum London, University of Florida, Gainesville, King's College London
No. of pages: 8
DOI: https://doi.org/10.13140/RG.2.1.4960.0729
Publication date: 2015
Peer reviewed: Yes
Austrian Fields of Science 2012: 105118 Palaeontology
Portal url: https://ucrisportal.univie.ac.at/en/publications/toothlike-structures-on-the-rostrum-of-the-cretaceous-batoid-schizorhiza(73d93672-77d3-4f15-856d-6c4849fac531).html

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