Ecomorphological inferences in early vertebrates: reconstructingDunkleosteus terrelli(Arthrodira, Placodermi) caudal fin from palaeoecological data

Abstract

Our knowledge about the body morphology of many extinct early vertebrates is very limited, especially in regard to their post-thoracic region. The prompt disarticulation of the dermo-skeletal elements due to taphonomic processes and the lack of a well-ossified endoskeleton in a large number of groups hinder the preservation of complete specimens. Previous reconstructions of most early vertebrates known from partial remains have been wholly based on phylogenetically closely related taxa. However, body design of fishes is determined, to a large extent, by their swimming mode and feeding niche, making it possible to recognise different morphological traits that have evolved several times in non-closely related groups with similar lifestyles. Based on this well-known ecomorphological correlation, here we propose a useful comparative framework established on extant taxa for predicting some anatomical aspects in extinct aquatic vertebrates from palaeoecological data and vice versa. For this, we have assessed the relationship between the locomotory patterns and the morphological variability of the caudal region in extant sharks by means of geometric morphometrics and allometric regression analysis. Multivariate analyses reveal a strong morphological convergence in non-closely related shark species that share similar modes of life, enabling the characterization of the caudal fin morphology of different ecological subgroups. In addition, interspecific positive allometry, affecting mainly the caudal fin span, has been detected. This phenomenon seems to be stronger in sharks with more pelagic habits, supporting its role as a compensation mechanism for the loss of hydrodynamic lift associated with the increase in body size, as previously suggested for many other living and extinct aquatic vertebrates. The quantification of shape change per unit size in each ecological subgroup has allowed us to establish a basis for inferring not only qualitative aspects of the caudal fin morphology of extinct early vertebrates but also to predict absolute values of other variables such as the fin span or the hypocercal and heterocercal angles. The application of this ecomorphological approach to the specific case of Dunkleosteus terrelli has led to a new reconstruction of this emblematic placoderm. Our proposal suggests a caudal fin with a well-developed ventral lobe, narrow peduncle and wide span, in contrast to classical reconstructions founded on the phylogenetic proximity with much smaller placoderms known from complete specimens. Interestingly, this prediction gains support with the recent discovery of fin distal elements (ceratotrichia) in a well preserved D. terrelli, which suggests a possible greater morphological variability in placoderm caudal fins than previously thought.

Negative allometric growth during ontogeny in the large pelagic filter-feeding basking shark

“…Nevertheless, no information on liver volume is available for juvenile specimens, and only one measure for subadult basking sharks [11.9% in a 375 cm specimen (Kruska 2004)]. From this available literature, it is obvious that the huge liver volume of basking sharks (Gleiss et al 2017) can be highly variable (Lingham-Soliar 2005;Sims 2008) and seemingly fluctuating, with specimens caught in areas of low plankton density having a reduced liver volume (Fairfax 1998). This high variability of liver volume to total body mass reported for basking sharks supports the view that the liver holds nutritional reserve, and hence, its volume may potentially strongly fluctuate in adult basking sharks depending on availability of their patchily distributed food source.…”

Body dimensions of the extinct giant shark Otodus megalodon: a 2D reconstruction

“…Our geometric morphometrics approach followed similar methodology to Ferrón et al 56 , which used allometric regression analyses of shark palaeoecological data to infer the caudal fin morphology of † Dunkleosteus terrelli . We defined a series of landmarks of type 1, 2 and 3 (head: N = 10; pectoral fin: N = 15; dorsal fin: N = 20; caudal fin: N = 25; total body: N = 68) that were digitised using tpsDig2 software 57 (Supplementary Fig.…”

Studying the present, understanding the past

“…La información disponible sobre la morfología del cuerpo de los primeros vertebrados es limitada, principalmente, en cuanto a la región post-torácica. Esto se debe a la rápida desarticulación del esqueleto dérmico y a la falta de un endoesqueleto bien osificado (Ferrón et al, 2017). Dunkleosteus terrelli (Devónico Superior) es conocido únicamente por algunas placas del escudo cefálico desarticuladas y algunos restos articulados de aletas pectorales incompletas (Carr, 2010;Carr et al, 2010).…”

“…Sin embargo, el plan corporal de los peces (la forma de su cuerpo y aletas) está determinado en gran medida por su estilo de vida, es decir, por aspectos como el nicho alimentario o su tipo de natación; haciendo posible reconocer diferentes ecomorfotipos (Lindsey, 1978;Webb, 1984). Esta correlación ecomorfológica ha sido empleada por Ferrón et al (2017) para establecer un marco comparativo basado en tiburones actuales mediante el cual predecir algunos aspectos anatómicos de vertebrados acuáticos extintos a partir de datos paleoecológicos y viceversa. El estilo de vida y la posición de D. terrelli en la cadena trófica son conocidos por datos sedimentológicos, tafonómicos, paleogeográficos y biomecánicos, de manera que es considerado un nadador y depredador activo en el vértice de la cadena trófica (Carr, 1995;Anderson & Westneat, 2009).…”

“…Aquí presentamos cuatro trabajos basados en los principios de la actuopaleontología y mediante los cuales podremos comprobar la importancia de su uso en los estudios paleontológicos. Estos están centrados en el estudio del microdesgaste dental en elasmobranquios (Paredes-Aliaga et al, 2019), el estudio morfológico del húmero en aves (Costa-Pérez et al, 2019), el estudio de la morfología caudal del placodermo Dunkleosteus terrelli (Ferrón et al, 2017) y el estudio de las comunidades de artrópodos actuales atrapadas en resina (Solórzano Kraemer et al, 2018). Poniendo la mira en futuros trabajos y en las posibilidades que ofrecen las nuevas técnicas usadas en paleontología y los estudios multidisciplinares, la actuopaleontología se plantea como una disciplina indispensable.…”