Morphometric Discriminant Analysis of isolated chondrichthyan scales for palaeoecological inferences: the Middle Triassic of the Iberian Chain (Spain) as a case of study

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...

Dermal denticle assemblages in coral reef sediments correlate with conventional shark surveys

“…We described the crown, peaks and ridges using a dissecting microscope (following Dillon et al, ; Table ). We then visually categorized the denticles into previously recognized functional morphotypes: drag reduction , ridged abrasion strength , abrasion strength , generalized functions and defense (Dillon et al, ; Ferrón et al, ; Reif, ). To verify these classifications, a multinomial logistic regression was trained with our denticle reference collection (Dillon et al, ) and used to predict the functional morphotype of each denticle based on the measurements taken (see Supplementary Methods; Tables ).…”

Dermal denticles as a tool to reconstruct shark communities

“…Denticle morphology can also vary with sex (Crooks et al 2013) and ontogeny (Reif 1985a). Only in a few cases can isolated denticles be identified beyond the family level (Reif 1985a, Mello et al 2013, Ferrón et al 2014. Conversely, denticle morphology appears to be more closely linked to the ecological guild of the shark species to which it belongs as well as to the specific function it plays on the shark's body (Reif 1978, 1985b, Raschi & Musick 1986, Raschi & Tabit 1992.…”

“…In fact, the proportion of fragmented denticles was higher in modern (18.3%) than fossil (3.3%) sediments, which may be because modern denticles are likely to be exposed for a longer period of time prior to burial due to the slow-down of coral reef accretion. Alternatively, if present, fossilized shark teeth may provide supplemental insight into the presence of pelagic sharks in the case that their drag reduction denticles are not well-preserved (Ferrón et al 2014). …”

“…Character selection was based on proposed functional significance (e.g. Reif & Dinkelacker 1982), previous studies (Tway 1979, Raschi & Musick 1986, Salini et al 2007, Ferrón et al 2014, and observed variation in denticle morphology. Character data was or dinated using principal component analysis (PCA; R Core Team 2014), and each categorical character was included in the ordination as multiple isolated dichotomous variables.…”

Preface. Palaeodiversity and Palaeoecology of Iberian Ecosystems. New insights into the Phanaerozoic biotas from Spain and Portugal

“…Therefore, the study of the structure of both past and recent communities and their response to biotic and abiotic factors is essential to understand future environmental changes. A good example focused on the Middle Triassic chondrichthyan paleocommunity from the Iberian Ranges is presented by Ferrón et al (2014 chondrichthyan shark communities of several Iberian localities, not only concluding that they were dominated by benthic sharks, but also correlating the changes in these communities with important environmental changes -sea level fluctuations-that affected the Iberian Basin. As a result of the environmental changes, the clade Dinosauria began to dominate the Mesozoic terrestrial ecosystems, and started their reign as one of the most successful groups in the history of Earth.…”