Tethytheria (marine mammals)

 

The name Tethytheria was coined by the American paleontologist Malcolm C. McKenna in 1975 to group extant sirenians (order Sirenia) and elephants (order Pro-boscidea) and extinct desmostylians (order Desmostylia). Tethytheria means “beast of the Tethys,” with the Tethys Ocean being a kind of a proto-Mediterranean sea. which joined Atlantic and Indian oceans up until the early to mid-Tertiary.

This concept revived an old idea put by the French naturalist Henri Ducrotay de Blainville in the 19th century. Blainville indeed was the first in 1836 to classify together elephants and sirenians only, under the name “les gravigrades.” i.e., beasts with a heavy gait, a name first used by him for elephants only. Blainville did not formalize this name scientifically, and Gravigrada was then used for a group of extinct ground sloths. Earlier, Linnaeus in his “Systema Naturae” of 1758 classified elephants and sirenians together under the name Bruta, but also with sloths and anteaters, with a very weak argument (the lack of front teeth). Blainville was impressed by the fact that manatees and elephants shared a distinct dental replacement, called “horizontal tooth displacement,” a character that now is considered to be a convergence. In elephants and manatees the tooth row is never complete: the anterior molars are lost before posterior ones erupt so that posterior molars take the place of anterior molars, moving anteriorly. In manatees this pattern is associated with polydonty, i.e., the existence of supernumerary molars. In elephants the number of molars is not altered but the three deciduous premolars and the three permanent molars erupt successively during life; in other words, the last molar (M3) is never associated to the first one. Among sirenians this character evolved only in manatees, and it appeared among proboscideans only in Neogene taxa.

I. Characters of Tethytheres

Various characters understood as synapomorphies support a sister group relation between Sirenia and Proboscidea. A few characters based on soft anatomy can only be seen in extant taxa: a bifid apex of the heart, with each ventricle being separated from the other at the tip (a convergence with some large cetaceans), a single pair of pectoral mammae (also seen on other mammalian taxa, but not in those more or less related to tethytheres). Others (osteological and dental characters) can be checked in fossils and define the Tethytheria at their ancestral node. They are a processus zygomaticus of the squamosal expanded laterallv, an anterior orbit above the premolars with infraorbital canal under the orbit (Fig. 1). the reduction of the mastoid process, and bilophodont bunolophodont molars. This last character can be explained relatively easily. On the upper molars a posterior crest (loph) is made by three cusps, which are in line (metacone, and metaconule close to hypocone, a condition needed for development of a transverse loph called metaloph); on the lower molars the four main cusps are arranged in two crest-like rows, one anterior and one posterior.

II. Proboscideans

The order Proboscidea is represented by two extant species only, which are the biggest terrestrial mammals today: Elephas maximus, the Asian elephant, and Loxodonta africana, the African elephant. Approximately 160 fossil species are recognized since the late Paleocene. It is very likely that the earliest proboscideans were amphibious, an adaptation probably inherited from the hypothetical ancestral species of tethytheres.

Eocene proboscidean genera such as Moeritherium (discovered in Egypt, Mali. Libya, and Algeria) and Nwnidotherium (discovered in Algeria) were described as having amphibious characters. The low cranium of Moeritherium (Fig. 1A). with sirenian proportions, has a tubular shape with no postorbital constriction, high anterior orbits, and high auditory meatus, traits associated with amphibious habits, with the long body and short limbs (Fig. 2). The nasal fossa is not pushed backward, suggesting that no trunk was present. Moeritherium was of the size of a pig. Numidotheriuin was slightly taller (tapir sized) with a shorter body. The cranium was higher but still with anterior orbits and high auditory meatus (Fig. IB). Post-cranial bones display distinctive features unknown in Moeritherium and in elephant-like proboscideans. The shoulder and hip joints are permanently abducted (or semisprawl-ing), more reminiscent of the semisupinated position of aquatic mammals, such as sirenians, desmostylians, and pinnipeds. This contrasts markedly with the subunguligrade gait of modern proboscideans. Extant sirenians and elephants lack tubular perspiratory glands and this may be a character inherited from an amphibious (but not fully aquatic) common ancestor.

Crania of Eocene proboscideans showing a tethythere feature despite different overall morphologies: the orbit is located above the anterior premolar. A more derived proboscidean feature is the opening of the orbit in the maxilla and not the jugal. (A) Moeritherium lyonsi from the Eocene of Libya. Scale: 3 cm. (B) Numidotherium koholense from the Eocene of Algeria. Scale: 5 cm. Draivings by Dominique Visset.

Figure 1 Crania of Eocene proboscideans showing a tethythere feature despite different overall morphologies: the orbit is located above the anterior premolar. A more derived proboscidean feature is the opening of the orbit in the maxilla and not the jugal. (A) Moeritherium lyonsi from the Eocene of Libya. Scale: 3 cm. (B) Numidotherium koholense from the Eocene of Algeria. Scale: 5 cm. Draivings by Dominique Visset.

The earliest known proboscidean is Phosphatherium escuil-liei, a small species (the size of a fox) described in 1996 from the phosphates of the Ouled Abdoun basin, Morocco, an area previously known to yield marine fauna, especially shark teeth. The only fossils known for the genus are two partial maxillae with premolars and molars. Its teeth are relatively generalized, making it difficult to identify these teeth to proboscideans if it were not for the knowledge of Eocene and Oligocene taxa found elsewhere in North Africa: Moeritherium, Numidotherium, and Barytherium. The teeth of Phosphatherim are very similar to those of Numidotherium koholense from Algeria, a truly lophodont proboscidean. Hence, during the late Pa-leocene and Eocene, proboscideans underwent a small radiation, of lophodont and bunolophodont taxa. The largest lophodont taxon is Barytherium from Egypt and Libya, which was similar in size to a living elephant. A lophodont lineage, the deinotheres, survived in the Neogene of Africa and Eurasia (up to 1.5 million years in Africa).

Reconstruction of the fossil proboscidean Moeritherium, late Eocene and early Oligocene of North Africa (Museum National d'Histoire Naturelle, Paris; photograph by P. Tassy). Proportions of skull, body, and legs are markedly different from those of elephants. Its gait was probably primitive, not the unusual gait of elephants, which lift two feet on one side at the same time.

Figure 2 Reconstruction of the fossil proboscidean Moeritherium, late Eocene and early Oligocene of North Africa (Museum National d’Histoire Naturelle, Paris; photograph by P. Tassy). Proportions of skull, body, and legs are markedly different from those of elephants. Its gait was probably primitive, not the unusual gait of elephants, which lift two feet on one side at the same time.

Anthracobunids are puzzling tethytheres from the Eocene of Pakistan and India, often closely compared to Proboscidea. Most of the known material consists in portions of maxillae, mandibles, and isolated teeth. Teeth are bunodont and more primitive than those of the bunolophodont contemporaneous African proboscideans. Postcranial remains allocated to anthracobunids (scapula, astragalus) match with those of proboscideans, especially the astragalus, which is the serial type and has a medial tuberculum (however, because no astragalus is known in primitive sirenians the proboscidean characters of the astragalus could well be tethythere characters).

Although early differentiation of elephantoids took place in Africa, elephantoids dispersed into Eurasia in the early Miocene, and probably even earlier, when the Arabo-African plate connected Eurasia closing the eastern Tethys. Elephan-tiformes such as Phiomia are bunolophodont, with an ever growing pair of upper and lower incisors (tusks), a long face with retracted nasal fossa (presence of a trunk), and a long mandibular symphysis. They do not show the horizontal tooth displacement only known in Miocene (and later) taxa. Elephantoids differentiated into several main groups with diverse shapes of crania and upper and lower tusks. Modern elephants are rooted in one of them, usually called “tetralophodont gom-photheres.” with extra lophs on molars, shorter crania, and extended lower tusks, which are associated with a shorter mandibular symphysis. Stem elephants (Stegotetrabelodon) are known from the late Miocene of Africa and the Arabian Peninsula. The earliest elephants related to the extant genera Loxodonta and Elephas are contemporaneous in the fossil record, known as soon as the late Miocene (circa 6 millions years) in Uganda and Kenya.

III. Other Fossil Tethytheres

Extinct orders often classified as tethytheres together with Sirenia and Proboscidea are the Desmostylia and Em-brithopoda. They share with Sirenia and Proboscidea the anterior orbit and the expanded zygomatic process. They share with the proboscideans the loss of mastoid process (“amastoidy”). However, hands (and feet) are not serial, contrary to both Sirenia and Proboscidea. The horizontal tooth displacement appeared also during the evolution of Miocene desmostylians.

Embrithopods are known in the late Eocene and Oligocene sediments of Egypt, Turkey, and Romania. Despite their very different overall morphology, they share a few derived characters with proboscideans: the loss of hypoglossal foramen (the hypoglossal nerve leaves the cranium by way of the foramen metoticum), the processus ascendens of the palatine bone is reduced greatly in the orbitotemporal fossa, and the astragalus has a prominent medial tuberculum, although the foot is not serial.

Extant tethytheres are also characterized by an unusual ontogenetic character. During the inner ear ontogeny of elephants and sirenians, the perilymphatic foramen fails to subdivide into two foramina so that the perilymphatic duct persists until the adult stage. This trait, probably paedomorphic, is not present in several Paleogene taxa such as the most primitive known sirenian (Prorastomus from the Eocene of Jamaica) and the Eocene proboscidean Numidotherium from Algeria. As a consequence, it is probably a convergence that appeared among sirenians and proboscideans. Moreover, this character is also known in embrithopods (the genus Arsinoitherium from Egypt), which probably is another convergence.

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