Sobrarbesiren
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''Sobrarbesiren'' (meaning "siren from
Sobrarbe Sobrarbe is a comarca of Aragon, Spain. It is located in the north of Huesca province, making up part of the autonomous community of Aragon. Many of its people speak the Aragonese language locally known as ''fabla''. Sobrarbe is a mountainous ...
") is a
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of extinct
sirenia The Sirenia (), commonly referred to as sea cows or sirenians, are an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The extant Sirenia comprise two distinct famili ...
n that lived in the
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
, about 47 million years ago. The type and only species is ''S. cardieli'', known from a multitude of specimens from the Spanish
Pyrenees The Pyrenees are a mountain range straddling the border of France and Spain. They extend nearly from their union with the Cantabrian Mountains to Cap de Creus on the Mediterranean coast, reaching a maximum elevation of at the peak of Aneto. ...
. ''Sobrarbesiren'' was a medium-sized animal, long and still retaining both pairs of limbs. Although initially thought to be amphibious, later studies instead suggest that they would have been fully aquatic and been selective sea grass browsers. Unlike modern dugongs and manatees, they likely lacked a tail fluke, although it would have appeared horizontally flattened.


Discovery and naming

''Sobrarbesiren'' was discovered in the Castejón de Sobrarbe-41 locality ( Sobrarbe Formation) in the
Spanish Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many countries in the Americas **Spanish cuisine **Spanish history **Spanish culture ...
Pyrenees The Pyrenees are a mountain range straddling the border of France and Spain. They extend nearly from their union with the Cantabrian Mountains to Cap de Creus on the Mediterranean coast, reaching a maximum elevation of at the peak of Aneto. ...
, specifically the
Huesca province Huesca (; ), officially Huesca/Uesca, is a province of northeastern Spain, in northern Aragon. The capital is Huesca. Positioned just south of the central Pyrenees, Huesca borders France and the French departments of Haute-Garonne, Pyrénées-A ...
. The locality dates to the
Lutetian The Lutetian is, in the geologic timescale, a stage (stratigraphy), stage or age (geology), age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it ...
stage of the
Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...
and preserved 300 individual siren fossils thought to belong to at least six individuals of different growth stages. The
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
specimen is complete skull of a juvenile animal housed at the
Museo de Ciencias Naturales de la Universidad de Zaragoza Museo may refer to: * ''Museum'' (2018 film), Mexican drama heist film *Museo station Museo is a Naples Metro station on Line 1. It opened on 5 April 2001 as the eastern terminus of the section of the line between Vanvitelli and Museo. On 27 Ma ...
. Later discoveries recovered several additional specimens, including remains of young adults. ''Sobrarbesiren'' represents the first record of sirenians in western Europe as well as the first and only well preserved instance of a quadrupedal siren on the continent. Prior to its discovery, only more derived members of Dugongidae have been known from adequate remains, while any more basal fossils were only present in the form of fragments. The name ''Sobrarbesiren'' derives from the type locality, while the species name is a patronym, styled after Jesús Cardiel Lalueza who first discovered the site.


Description

''Sobrarbesiren'' was a medium-sized sirenian that retained all four limbs in a well developed state, unlike later sirenians that would eventually lose their hindlimbs entirely. Estimates suggest that it reached a length of . The rostrum shows a significant downturn of 32°, which is notably more deflected than in prorastomids, but far from as developed as the deflection in later sirenians. The symphysis of the
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...
is high but laterally flattened with a tapering nasal process that contacts the
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
,
nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...
and
frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...
, as in other sirenians. The nares are enlarged and retracted and overhung by a triangular process formed by both nasal bones. The posterior end of the nasal bones is rounded with a small incursion of the frontal bone, which otherwise wraps around the nasals on either side. The
lacrimal bone The lacrimal bones are two small and fragile bones of the facial skeleton; they are roughly the size of the little fingernail and situated at the front part of the medial wall of the orbit. They each have two surfaces and four borders. Several bon ...
s are only partially preserved and entirely surrounded by the maxilla and frontal, however in spite of the preservation a small foramina can be observed. The frontal shows no bosses and the parietal does not form a proper
sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are excepti ...
, instead possessing a temporal crest formed by the
parietal bones The parietal bones ( ) are two bones in the skull which, when joined at a fibrous joint known as a cranial suture, form the sides and roof of the neurocranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four bord ...
that fuse just before the coronal suture. The back of the head is marked by three foramina arranged in a triangular pattern just before the nuchal crest. Like in other basal sirenians, ''Sobrarbesiren'' possesses an alisphenoid canal. The upper jaw of ''Sobrarbesiren'' preserved two
incisors Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...
, the first and the third, showing no signs of the second incisor whatsoever. Like in later dugongids, the first incisor forms a tusk at the anterior end of the premaxilla, while the third incisor of ''Sobrarbesiren'' is situated much further back, just before the suture between premaxilla and maxilla. The following canines and
premolars The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mout ...
were not preserved in situ, but the alveoli they would have fit into show that they only had a single root. However the fifth and final premolar is still present in the holotype skull. Although still relatively small compared to the molars that follow it, the presence of a permanent premolar is considered to be a
plesiomorphic In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, an ...
state in sirenians. The premolar shows a single central cusp surrounded by much lower outer (labial) and inner (lingual) cuspules. An anterior cingulum with its own cuspules is also present. The
molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...
are significantly larger than the fifth premolar, longer than wide (especially prominent in the last two teeth),
bilophodont The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone toot ...
and three-rooted. Of the molars lophs the protoloph is more developed thanks to the higher trigon. The lophs of each tooth are separated by a valley that opens outwards but is closed off towards the inside of the mouth. The vertebral column is only incompletely known, but the preserved neural spines do not show the same horizontal projections of prorastomid sirenians, nor the split neural spine seen in protosirenids. The
atlas An atlas is a collection of maps; it is typically a bundle of world map, maps of Earth or of a continent or region of Earth. Advances in astronomy have also resulted in atlases of the celestial sphere or of other planets. Atlases have traditio ...
is wide and relatively shallow, while the
axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...
is robust with a swollen neural spine. The genus' neural canal is teardrop-shaped, a condition similar to derived sirenians, while the neural spines themselves are much higher than in dugongids, which indicates the presence of
ligament A ligament is a type of fibrous connective tissue in the body that connects bones to other bones. It also connects flight feathers to bones, in dinosaurs and birds. All 30,000 species of amniotes (land animals with internal bones) have liga ...
that may have supported the head. The vertebrae of the sacram, of which there are at least two (compared to one in protosirenids and dugongids), are unfused with short transverse processes. The caudal vertebrae are not well enough preserved nor is that part of the skeleton complete enough to determine whether or not ''Sobrarbesiren'' had a tail fluke, however it was likely horizontally flattened. The ribs are thick and dense ( pachyosteosclerotic) as well as flat and broad. The
scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...
is stout and the
humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...
robust, while the
ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...
is straight and unfused to the absent
radius In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
(however this is likely connected to the age of the specimen). The pubis, ilium and
ischium The ischium (; : is ...
are fused to form an
innominate bone The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the ilium, isch ...
which is long and narrow. The symphysis of the pubic elements of the innominate is long and rugose and the two pubes likely had a bony connection to one another. The
sacroiliac joint The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis, which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. The ...
between the sacrum and the ilium is bony and thus stronger than what is preserved for protosirenids. The socket for the
femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...
is well developed and the femur itself resembles that of ''Pezosiren'' and protosirenids, however with a hemispherical head. Despite still being fully developed, the hindlimbs of ''Sobrarbesiren'' are already notably shorter than the forelimbs, with the ratio of femur length to humerus length being 1.42–1.5. However this number is uncertain, as the fossil humerus and femurs do not belong to specimens of the exact same age, rendering the exact ratio unknown.


Phylogeny

Phylogenetic analysis conducted by Díaz-Berenguer and colleagues recovered ''Sobrarbesiren'' as a basal sirenian just outside of Dugongidae and more derived than prorastomids (which were found to be
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
by them) and protosirenids. This they note to be strange, as ''Sobrarbesiren'' shows anatomy that is much better adapted to terrestrial locomotion than in what the authors found to be more basal taxa in their phylogeny. A later analysis made greater use of the neurocranial anatomy of ''Sobrarbesiren'', both utilizing previously known specimens as well as newly discovered remains. Use of these features helped better resolve basal members of Sirenia. Under implied weighting, one of their analysis recovered ''Sobrarbesiren'' as a member of a new clade including ''
Prototherium ''Prototherium'' is a genus of extinct sirenian related to the dugong. It is known from middle (Bartonian) and upper Eocene deposits in Italy and Spain. Type species is P. veronenses (Zigno, 1887) Three species are now included in the genus, bu ...
'' and ''
Eotheroides ''Eotheroides'' is an extinct genus of Eocene sirenian. It is an early member of the family Dugongidae, which includes the extant dugong. Fossils have been found from Egypt, India, and Madagascar. ''Eotheroides'' was first described by Richard O ...
'', which nests in a position more basal than protosirenids, unlike in prior studies. However the authors hesitate to name the new clade, as they found poor support for this grouping, with another of their analyses instead finding the genus to nest in a large polytomy.


Paleobiology


Locomotion

Several characteristics of ''Sobrarbesiren'' have initially been used to suggest that it was still capable of supporting its own weight on land and could have moved quadrupedally. The tall neural spines could have helped keep the head upright while the pubis and sacroiliac joints would have made quadrupedal locomotion possible. It was initially argued that this differentiated ''Sobrarbesiren'' from the protosirenids, which were likely incapable of supporting themselves and were restricted to movement more like that of modern
pinnipeds Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the walr ...
. However despite these anatomical features, ''Sobrarbesiren'' was still clearly adapted to life in the water as shown by the anatomy of the sacral vertebrae, which differs from those of the more terrestrial ''Pezosiren''. Taking into account all of its other adaptations to aquatic life (dense and broad ribs, retracted nares, short hindlimbs, unfused sacrum), Dìaz-Berenguer and colleagues initially hypothesize that ''Sobrarbesiren'' was an amphibious animal, living both on land and in the water. They further suggested that the animal likely fed in the water, but may have traveled between different bodies of water, which matches the environment it was found in being an intertidal floodplain. However, a later paper by Díaz-Berenguer examined the hindlimbs and pelvis of ''Sobrarbesiren'' closer, comparing them to those of other basal sirenians, archaeocete whales and conducting microanatomical analysis on them. Their research concluded that the hips would have allowed for a wide variety of hindlimb movements while the sacral vertebrae allowed for lateral movement. The authors suggest that while swimming it could have moved by undulating its pelvis up and down while additionally paddling with its feet, similar to river otters. It is also possible that the hindlimbs of ''Sobrarbesiren'' complimented the flattened tail in its movement, as indicated by a torsion found on the femur. However, unlike the original description of the taxa, here the authors doubt that the animal would have been able to effectively move on land. They point towards the advanced
osteosclerosis Osteosclerosis is a disorder characterized by abnormal hardening of bone and an elevation in bone density. It may predominantly affect the medullary portion and/or cortex of bone. Plain radiographs are a valuable tool for detecting and classif ...
of the innominate and femur, which is much greater than what is seen in pinnipeds and more similar to what is known from protocetid whales. This could have rendered the bones too brittle to be used for terrestrial locomotion. They conclude that the internal changes of the bone would have preceded the external adaptations, explaining why they superficially appear sufficient for use out of water. Despite this, they note that the range of motion still suggests that the animal actively used its feet for swimming.


Diet and environment

The degree to which the rostrums of sirenians deflect varies between genera and is generally connected to their diet. This is a particular adaptation to feeding on aquatic plants on the seafloor, with some of the most extreme cases displaying a downturn of up to 70°, like in the seagrass-specialized dugongs. The downturn of ''Sobrarbesiren'' is with 32° within the range of modern
manatees Manatees (, family Trichechidae, genus ''Trichechus'') are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living spe ...
, which range in deflection between 25 and 50°. Unlike the highly specialized dugongs, manatees have a broader diet feeding on plants growing in fresh and saltwater. Such a diet has previously been proposed for basal groups such as prorastomids, which are known coastal river deposits. This is however contrasted by isotope analysis, which suggests that even in these basal groups sea grass made up an important part of their diet. Additionally to the downturn of the rostrum, the width may also play a role in sirenian feeding habits. Domning proposes that narrow, laterally flattened mandibular symphysis and rostrum are indicative of selective feeding. While the mandible of ''Sobrarbesiren'' is unknown, the rostrum is compressed and would match this interpretation. Combining isotopic and morphological data, ''Sobrarbesiren'' is inferred to have been a selective browser primarily feeding on sea grass and occasionally on floating plant matter. The Castejón de Sobrarbe-41 locality is thought to have been a channel within the tidal zone of the
Bay of Biscay The Bay of Biscay ( ) is a gulf of the northeast Atlantic Ocean located south of the Celtic Sea. It lies along the western coast of France from Point Penmarc'h to the Spanish border, and along the northern coast of Spain, extending westward ...
, a deep marine gulf located between the
Iberian Peninsula The Iberian Peninsula ( ), also known as Iberia, is a peninsula in south-western Europe. Mostly separated from the rest of the European landmass by the Pyrenees, it includes the territories of peninsular Spain and Continental Portugal, comprisin ...
and mainland Europe during the Eocene. The fact that the bones of ''Sobrarbesiren'' are well preserved but disarticulated and widely dispersed has been interpreted by Díaz-Berenguer and colleagues to mean that the channel containing the bones was filled by sediment in a single event, burying the bones instantly. Fossil foraminifera show that sea grasses could be found in the Ainsa Basin during the Eocene.


References

{{Taxonbar, from=Q111038562 Tethytheria Eocene sirenians Prehistoric placental genera Fossil taxa described in 2018 Eocene mammals of Europe