The Hind Limbs of Sobrarbesiren cardieli (Eocene, Northeastern Spain) and New Insights into the Locomotion Capabilities of the Quadrupedal Sirenians

Abstract

In the transition from a terrestrial to an aquatic environment, sirenian marine mammals reduced and lost their hindlimbs and developed a horizontal tail, the main propulsive organ in extant sirenians. Quadrupedal forms are only known from the Eocene and are represented by three different taxa: the amphibious prorastomids, the aquatic quadrupedal protosirenids and Sobrarbesiren cardieli, a four-legged sirenian from the middle Eocene of Spain, considered the sister taxon of the aquatic Dugongidae. This ecological shift was naturally associated with adaptations, among others, of the skeleton. However, sirenian hindlimb bones have been poorly studied because of the scarce material available in the fossil record. Here we describe in detail the hindlimb bones of Sobrarbesiren, analyzing their functional morphology and comparing them with other basal sirenians and cetaceans. Sobrarbesiren had strong control of its hindlimbs, which were capable of a great variety of movements. Based on the presence of a strong sacroiliac articulation, we propose that it swam by dorsoventral pelvic undulation combined with pelvic paddling analogous to some protocetid archaeocete whales. We also conducted the first microanatomical analysis of hindlimb bones of an Eocene sirenian. Data reveal extreme inner compactness in the Sobrarbesiren innominate bone and femur, with the first description of 2 osteosclerosis in an amniote innominate bone combined with the highest degree of osteosclerosis observed in amniote femora. The results confirm that the microanatomical changes precede the external morphological changes in such ecological transitions. The process of adaptation of sirenians to an aquatic life was thus a more complex process than previously thought.

The Miocene La Venta Biome (Colombia): A century of research and future perspectives

“…Sirenians evolved from terrestrial ancestors and colonized both freshwater and shallow marine environments in tropical and subtropical waters (Domning 2001). The oldest fossil records of sirenians are prorastomids from the middle Eocene of Jamaica, Senegal, and other sirenians of uncertain affinities from the early-middle Eocene of Tunisia and middle Eocene of Spain (Savage et al 1994;Domming 2001;Hautier et al 2012, Benoit et al 2013Díaz-Berenguer et al 2018). Sirenia includes the extant families Dugongidae and Trichechidae, and their fossil relatives (Domning 2000(Domning , 2018Velez-Juarbe & Wood 2019;Díaz-Berenguer et al 2020).…”

“…The oldest fossil records of sirenians are prorastomids from the middle Eocene of Jamaica, Senegal, and other sirenians of uncertain affinities from the early-middle Eocene of Tunisia and middle Eocene of Spain (Savage et al 1994; Domming 2001; Hautier et al 2012, Benoit et al 2013; Díaz-Berenguer et al 2018). Sirenia includes the extant families Dugongidae and Trichechidae, and their fossil relatives (Domning 2000, 2018; Velez-Juarbe & Wood 2019; Díaz-Berenguer et al 2020). The only sirenian reported for La Venta is the trichechid Potamosiren magdalenensis Reinhart 1951 (Reinhart 1951; Kellogg 1965; Domning 1982, 1997; Suarez et al 2021).…”

Neurocranial bones are key to untangling the sea cow evolutionary tree: osteology of the skull of Sobrarbesiren cardieli (Mammalia: Pan-Sirenia)

“…Nevertheless, Díaz-Berenguer et al (2018) proposed that some differences exist in the morphology of this bone among Eocene taxa when they described the Spanish taxon Sobrarbesiren cardieli Díaz-Berenguer et al, 2018. Sobrarbesiren is a medium-sized quadrupedal sirenian from the middle Eocene (Lutetian) of the southern Pyrenees (Huesca Province, Spain), known from cranial and postcranial elements including skulls, vertebrae, ribs and limb bones of several individuals at different ontogenetic stages. This fossil collection has given us a good idea of the overall appearance of this ancient animal (Díaz-Berenguer et al, 2018, 2020. Sobrarbesiren has been proposed as the sister taxon of Dugongidae and Trichechidae (Díaz-Berenguer et al, 2018) or as a member of the basal group Pan-Sirenia (Velez-Juarbe & Wood, 2018).…”