The Skull of Quadruped and Bipedal Vertebrates. Djillali Hadjouis

cut out so as to be able to recover the ivory teeth in their totality.

      This practice could only have been carried out by hunters or scavengers in two periods: either the individuals were contemporaries or they were post-Neanderthals. In any case, they recovered the tusks from the carcass of the slaughtered or naturally dead animal to transform ivory materials or for any kind of development.

Figure 1.2. Lower view of a mammoth skull (Mammuthus primigenius), found in the gravel of Bonneuil Port, showing the replacement of the jugal teeth (© Hadjouis)

1.6. A particular tooth eruption

      The most evolved Proboscidians of the genera Elephas and Mammuthus adopt/adopted a horizontal type of dental eruption of the jugal teeth, different from the other groups of the Mammalian class whose vertical eruption first concerns the deciduous teeth, replaced by the permanent teeth. In the woolly mammoth (M. primigenius), whose life expectancy was around 60 years, a second tooth eruption would not have been sufficient during its lifetime and all the teeth would have been worn down to the root, causing a final fall of the last stumps. Only the upper incisors called tusks saw two permanent and milky dentitions. To meet the biological longevity of the Elephantidae, a dental replacement cycle exists whose eruptive mechanism will be renewed six times. The mammoth’s extinction coincided with the total stop of this cycle.

When the premolar and molar teeth were drilled, the mesial (anterior) blades of each jugal tooth appeared first, whose tooth wear was not balanced with the distal (posterior) blade teeth, because they were still embedded in the alveolus. This mechanism of pushing from the back to the front, especially in the case of M3s, caused the anterior teeth to fall out, regardless of the dental row (premolars or molars), as they erupted until the end of a cycle. In the end, it was the last molar (M3) that occupied the alveolar spaces of both jaws (Figure 1.3).

Figure 1.3. Replacement of jugal teeth in the mammoth. Premolars and first molars were expelled from the back to the front, keeping only the M3 in each half jaw (© Anthony)

      2

      Equidae

      2.1. The horse (Equus caballus)

      Order Perissodactyla Owen, 1848.

      Family Equidae Gray, 1821.

      Genus Equus Linnaeus, 1758.

      Species Equus caballus (the horse).

      2.1.1. Chronological, geographical and morphological indications of species

      The current species belonging to the genus Equus Linnaeus sensu lato are divided into groups of horses (E. przewalski, E. ferus and E. caballus), zebras (E. zebra, E. burchelli and E. grevyi), hemiones (E. hemionus and E. kiang) and donkeys (E. africanus and E. asinus) (Groves 1986; Guérin 1994).

      The dental formula is: 3/3 I, 0-1/0-1 C, 3/3-4 P, 3/3 M.

      The domestic horse has short ears, a strong mane and long hair implanted along the entire length of the tail. These characteristics change according to the great equine diversity. Two types of domestic horses are recognizable: the light and fast “blood” horses and the heavy and powerful horses suitable for various service work. Among these varieties are: the Arabian thoroughbred, the English thoroughbred, the Frederiksborger horse, the Oldenburg horse, the Lipizzaner and breeds of ponies (Northern pony, Iceland pony and Shetland pony) (Huass and Petter 1900).

      In addition to the dental characteristics, the base of horses’ and donkeys’ skulls can be distinguished by an occipital bone that protrudes over the dorsal face of the head in horses. The upper cranium forms a protruding external occipital protuberance. In donkeys, the more pronounced protrusion of the external occipital protuberance initiates an overjet curve on the nuchal face (Barone 1976). On the face, there are also specific peculiarities between these two species (the two frontal bones forming a flat surface between the orbits, with a large and strong zygomatic process in horses). The horse’s parietal bone is characterized by a temporal line which joins its opposite by a sagittal ridge, non-existent in the young. The sagittal suture is fused late and the synostosis that begins from the rear at about 10 months is not completed until about 3 years of age (Barone 1976). Mandibular synchondrosis is fused from the age of 6 to 8 months.

However, the character that seems more important according to Lesbre, Petit and Barone (in Barone 1976) is that of the skull’s angle of flexion on the face which is more pronounced in donkeys than in horses. The shortening of the spheno-basilar axis has resulted in greater craniofacial flexion in donkeys. The very much open basion-prosthion geometric triangle in horses reflects the large extension of the craniodental and craniospinal fields (Figure 2.1).

Figure 2.1. Horse skull viewed from the left. The basion-prosthion geometrical triangle translates the extension of the craniodental and craniospinal fields. The perpendicular line of the triangle passes through M3 (© Hadjouis)

      The hybrid forms between the horse and the donkey give two infertile varieties: the mule, which is the product of the donkey and the mare, and the bardot, which is the product of the stallion and the donkey.

The Family Equidae made its first appearance in the North American Eocene. Two genera were recognized in Eurasia and Africa. The first, Christol’s Hipparion, represents the last lineage of the tridactyl species. It arrived at the beginning of the Upper Miocene and survived in Africa until the Middle Pleistocene. Four species were defined: Hipparion crissum Déperet; Hipparion fissurae Crusafond and Sondaar; Hipparion rocinantis Hernandez Pacheco and Hipparion crusafonti Villalta. These were fairly large forms, with folded upper jugals, moderately hypsodont and isolated protocones (Eisenmann 1977; Eisenmann and Sondaar 1989; Guérin 1996b). The second, Equus Linnaeus, appeared in the Plio-Pleistocene with Stenon’s horse. The latter Equus stenonis was characterized by primitive forms of the jugal teeth: jugal teeth superior to the narrow, simple and rounded parastyle and mesostyle, the protocone was short with a slightly elongated mesial lobe. In the lower ones, the medial lingual groove was narrow and angular. Compared to caballine-type horses, the limb bones had a less elongated metatarsal than the metacarpal, and the peri-articular muscular reliefs were more prominent (Prat 1980).

      The genus Equus represents a large number of species that came to constitute the essence of the taxa evolving in Europe and France.

      During their evolution, the Equidae have acquired new adaptations according to their biotopes (increase in size, specialization of teeth adapted to graminivorous regimes, monodactyl locomotor adaptation after being tridactyl, etc.). Among these major skeletal transformations, horses of the late Tertiary and early Quaternary adapted to open environments (prairie, steppe and savannah) with paleoenvironmental particularities. This is how we find running