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Morphometric Discriminant Analysis of Isolated Chondrichthyan Scales for Palaeoecological Inferences: the Middle Triassic Of

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Morphometric Discriminant Analysis of Isolated Chondrichthyan Scales for Palaeoecological Inferences: the Middle Triassic Of Journal of Iberian Geology 40 (1) 2014: 87-97 http://dx.doi.org/10.5209/rev_JIGE.2014.v40.n1.44089 www.ucm.es /info/estratig/journal.htm ISSN (print): 1698-6180. ISSN (online): 1886-7995 Morphometric Discriminant Analysis of isolated chondrichthyan scales for palaeoecological inferences: the Middle Triassic of the Iberian Chain (Spain) as a case of study H. Ferrón1, C. Pla1, C. Martínez-Pérez1,2, M.J. Escudero-Mozo3,4, H. Botella1* 1Departamento de Geología, Universidad de Valencia, Avda. Dr. Moliner, 50, 46100 Burjassot, Spain. 2School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, United Kingdom. 3 Departamento de Estratigrafía, Universidad Complutense de Madrid, C/José Antonio Novais 12, 28040 Madrid, Spain 4 Instituto de Geociencias IGEO (CSIC,UCM), C/José Antonio Novais 12, 28040 Madrid, Spain e-mail addresses: [email protected] (H.F.), [email protected] (C.P.), [email protected] (C.M.-P.); [email protected] (M.J.E.-M.) [email protected] (H.B., *corresponding author) Received: 12 December 2012 / Accepted: 4 December 2013 / Available online: 25 February 2014 Abstract Palaeontological studies on exosqueletal disarticulated remains of chondrichthyans have focused on teeth and only less interest has been paid to scales due their limited taxonomic and systematic significance. However, classical works linking the morphology and the function of the squamation in extant sharks suggest that, despite their limited taxonomic value, the study of isolated scales can be a useful tool for palaeoenvironmental and palaeoecological inferences. Following this idea, we have analyzed the fossil record of shark scales from two Middle Triassic sections of the Iberian Chain (Spain), identifying different functional types by means of a morphometric discriminant analysis. From a total of 1136 isolated chondrichthyan scales, 25% were identified as abrasion resistant scales, 62% as drag reduction scales and 13% as scales of generalized functions. The elevated proportion of abrasion resistant scales suggests that this chondrichthyan palaeocommunity was highly dominated by benthic sharks that lived over a hard sea floor. However, one of the stratigraphical levels studied (He-20), presents statistically significant differences from the others, showing a lower percentage of abrasion resistant scales and a larger percentage of drag reduction scales. This level can be linked with storm episodes that could introduce remains of bentho-pelagic or pelagic forms in the inner platform.. Finally, partial correlation analysis between relative abundances of functional scale types and tooth-based taxa from the same sections provide positive correlation between teeth of Hybodus and Pseudodalatias and drag reduction scales, and teeth of Prolatodon and abrasion strength scales. Keywords: Triassic, Chondrichthyes, scales, teeth, Iberian Chain Resumen Los estudios paleontológicos de restos desarticulados de condrictios se han centrado en los dientes, no prestando prácticamente interés al estudio de sus escamas debido a su limitada importancia taxonómica y sistemática. Sin embargo, algunos trabajos clásicos que han relacionado su morfología y función en base al estudio de la escamación de tiburones actuales, sugieren que, a pesar de su limitado valor taxonómico, el estudio de las escamas aisladas puede ser una herramienta útil para obtener inferencias paleoambientales y paleoecológicas. Siguiendo estas ideas, se ha analizado el registro fósil de escamas de tiburón de dos secciones del Triásico Medio de la Cordillera Ibérica (España), identificando diferentes tipos funcionales por medio de un análisis morfométrico discriminante. De un total de 1.136 escamas aisladas de condrictios, el 25% de ellas fueron identificadas como escamas resistentes a la abrasión, el 62% como de reducción de la fricción con el agua y el 13% de las escamas como de función generalizada. La elevada proporción de las escamas resistentes a la abrasión sugiere que esta paleocomunidad de condrictios estaba claramente dominada por tiburones bentónicos que habitaban sobre un sustrato rocoso. Sin embargo, uno de los niveles estratigráficos estudiados (He-20), presenta diferencias estadísticamente significativas con los demás, mos- trando un porcentaje más bajo de escamas resistentes a la abrasión y un porcentaje mayor de las escamas de reducción de la fricción con el agua. Este nivel se relaciona con episodios de tormenta que pudieron haber introducido los restos de formas bento-pelágicas o pelágicas en la plataforma interna. Por último, el análisis de correlación parcial entre la abundancia relativa de los tipos funcionales de escamas y taxones en basado dientes de las mismas secciones proporcionan una correlación positiva entre los dientes de Hybodus y Pseudodalatias y escamas de reducción de la fricción con el agua, y dientes de Prolatodon y escamas resistencia a la abrasión. Palabras clave: Triásico, condrictios, escamas, dientes, Cordillera Ibérica JIGE 40 1.indb 87 24/03/2014 21:25:58 88 Ferrón et al. / Journal of Iberian Geology 40 (1) 2014: 87-97 1. Introduction enabling the skin to carry photophores and permitting light to pass between them (Reif, 1985b). This functional type is rep- Due to the cartilaginous nature of the chondrichthyan en- resented by three different morphologies: square-shaped with doskeleton, the fossil record of this group consists mainly of concave facets (Morphology 5), bristle-shaped (Morphology disarticulated remains such as teeth, scales and fin spines. 6) and thorn-shaped (Morphology 7). Scales with ridges and Within these elements teeth are by far the most informative lateral cusps well developed but shorter than the principal in terms of taxonomy, systematics or autecology. Most of cusp fulfill generalized functions and are found in almost all the extinct species of Chondrichthyes are described on the sharks (Morphology 8). Other functions related to hatching basis of isolated teeth (see for example monographs of Cap- and feeding has been documented for shark scales (Grover, petta, 1987, 2012; Ginter et al., 2010 and references therein). 1974; Reif, 1974; Southall and Sims, 2003), but they are very In contrast, isolated chondrichthyan scales that commonly atypical and have not been found among our material. occur together with disarticulated teeth, provide limited In this study we firstly characterize the morphology of the taxonomic information (especially Mesozoic and Cenozoic five functional types proposed by Reif (1985a) using tradi- taxa, see Reif, 1985a; Karatajute-Talimaa, 1998; Leidner tional morphometrics and discriminant analysis. Secondly and Thies, 1999; Thies and Leidner, 2011) for three main we assign isolated placoid scales from synchronous levels of reasons: the presence of a high morphological diversity two stratigraphical sections (Middle Triassic) of the Iberian of scales in different regions of the body, which has been Chain (Spain), to these “functional” morphologies. The dif- documented in both extant and fossil specimens (see Reif, ferential abundance of the functional types allow us to evalu- 1973, 1974, 1985a); the evolution of some scales of similar, ate the structure of the chondrychtyan paleocommunity in or even, identical morphologies in distantly related species terms of the relative dominance of more pelagic or benthic (Muñoz-Chápuli, 1985); and the occurrence of ontogenetic sharks and the properties of the physical environment they variability (Reif, 1973, 1978). As a consequence, palaeon- inhabited, such as the dominant substrate type. tological studies of disarticulated remains of chondrichthy- ans have focused on teeth and very little attention has been 2. Material provenance paid to scales. However we propose that, despite the lack of taxonomic and systematic significance, isolated scales The Middle Triassic carbonates in facies Muschelkalk could provide useful information regarding the palaeoeco- (Anisian-Ladinian in age) of the eastern Iberian Chains, is logical and palaeoenvironmental conditions. Our proposal represented by two trasngressive-regressive cycles showed is based on the classic works of Reif (1982; 1985a) where by two carbonate units (ramps) mainly constituted by epi- the relationship between the shape and function of scales of continental shallow marine environments. The Cañete Fm. extant sharks was pointed out. Reif (1982, 1985a) differenti- (López-Gómez and Arche, 1992) corresponds to the upper ated scales into five different functions: abrasion resistance, carbonate unit and represents the second and main marine defense, drag reduction, bioluminescence and generalized transgression of the Middle Triassic in the southeast area of functions, and indentified eight characteristic morphologies the Iberian Chains. Litologically this unit can be subdivided associated with these functional types. Abrasion resistant in two parts: a lower one, strongly dolomitized and consti- scales are found in demersal sharks, which inhabit rocky tuted by grey massive dolomites; and an upper part, less or coralline substrates and, generally, in small body regions dolomitized and mainly composed by thin-bedded bioclas- that are often subject to abrasion in all other sharks, such as tic limestones, planar stromatolictic lamination, tempestite the mouth area. Scales of this type are knob-like and smooth and marls levels constituting shallowing-upwards sequences (Morphology 1) or strongly ornamented (Morphology 2). (shallow inner ramp-tidal flat). The studied
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