The Skull of Quadruped and Bipedal Vertebrates. Djillali Hadjouis

V^e siècle-XVII^e siècle, published in French by ISTE Editions in 2018, was perhaps the trigger for the new set of books I am directing. Part of the “Biology” collection, entitled “Comparative Anatomy and Posture of Animals and Humans”, the set brings together works dealing with all anatomical regions, either the holistic concept, integrating the architectural ensemble of the quadruped or bipedal vertebrate body and its postures, or a particular region (skull, teeth, limbs, spine, pelvis, etc.), or a particular joint (occipito-cervical hinge, shoulder joint, hip joint, knee joint, etc.).

      The first book published in this series (June 2020) is by Dr. Cyrille Cazeau and is entitled Foot Surgery Viewed Through the Prism of Comparative Anatomy: From Normal to Useful. In addition to these, other works are to follow.

I.2. Introduction to the craniofacial and dental taxonomic terminology of vertebrates and dimorphism

      The skull of Vertebrates and in particular that of Mammals envelops the encephalon while protecting it, while the face is made up of two articulated jaws. One, welded to the skull and immobile, is called the maxilla, and the other, the mandible, articulated to the skull through the temporo-mandibular joint, is mobile in order to produce all the necessary mechanical and dynamic movements (mastication, grip). Two fundamental developmental processes are to be considered in the formation of the skull. The ossification of these two ontogenic processes relies on the chondrocranium and desmocranium. In the first, ossification is achieved by a substitution process (enchondral ossification); in the second, membrane bones develop directly in the connective tissue (chondral ossification). The latter make up the majority of the bony scales of the cerebral skull and facial skull, while the bones formed from a cartilaginous outline are those located at the basi-cranial level (occipital, sphenoid, temporal with the petrous bone, ethmoid, hyoid bone (Kahle et al. 1980)). It will be seen later that in cranial malformations causing asymmetries, spheno-occipital synchondrosis (SOS) will be at the center of any ontogenic interpretation of an imbalance of this type, since its disjunction will be the cause or effect of the problem.

      Studies on the skull, as on the rest of the skeleton, are nowadays analyzed from several angles, given the improvement of analytical tools, particularly through medical imaging. However, we must not neglect the fundamentals of anatomy and its twin sister, comparative anatomy, which in addition to the normal development of individuals, a great intraspecific variability is recorded in human species as in other Vertebrates, not to mention sexual dimorphism.

      The paleontological analysis of the skull of Hominidae and quadruped Mammals involves a series of taxonomic, morphometric, geometric, cladistic, biomechanic and radiographic studies, grouped or isolated in order to recognize the different morphotypes, evolutionary mechanisms, ancestral origin of taxa, developmental stages or intra- and interspecific variability of each family, genus or species. The subspecies is itself considered as a geographic variety. All these studies are not necessarily carried out in the same research institution and may sometimes be unrelated to each other due to different objectives. Thus, research conducted in the medical world, particularly in dentofacial orthopedics, orthodontics, rheumatology or neurology, brings its share of knowledge to evolutionary sciences and the anatomy of fossil species, which sometimes prove to be very complementary. Throughout this work, a superficial or in-depth overview of these different research parameters will be provided, each time with examples supported by detailed illustrations.

      Due to the radiation of certain Mammals, such as the Bovidae Family, the intra- and interspecific variation is great and manifests itself as much by polymorphism as by sexual dimorphism. Paradoxically, in some species of closely related Ungulates, such as small, large or medium-sized antelopes, difficulties appear during the taxonomic separation of the bony remains. This is particularly the case for certain dentures, cranial portions or limb bones belonging to gazelle species, but it is also the case for wildebeest (Connochaetes) females with male hartebeest (Alcelaphus). However, the most cited example in the zoological and paleontological literature is clearly that of the three or four genera (if we add Bubalus), Bos, Bison and Syncerus represented by the species of aurochs (Bos primigenius), bison (Bison priscus) and buffalo (Syncerus). Many authors have multiplied the methods of generic and specific distinction of these species with their imposing size, only the cranial remains, in this case the horn cores (orientation, curvature, flattening, etc.) and the metapodial segments provide fairly significant elements of diagnosis.

      In caballine Equidae fossils, here again only the presence of canines in males sometimes allows a sexual diagnosis and this is not often the case because of a poor conservation of the symphysis elements in archeological excavations.

In Carnivores or in certain Great Apes, it is rightly the canine (or fang), clearly larger in males, which makes sexual determination possible, while the cranial vault of the majority of Primates is provided with a sagittal crest in males. Fossil Hominidae, on the other hand, represents a clear dimorphism whose cranial, pelvic and appendicular characters offer better elements of distinction. Another scenario is emerging for the current representatives of the human species where studies suggest that bony sexual dimorphism decreases greatly between men and women.

      Cranial and in particular dental terminology has followed the methods of several authors: Gentry and Gentry (1978a) for Antelope jugal teeth, Heintz (1970) for Cervidae and Bovidae, Eisenmann (1980, 1981) for Equidae and Kurten for Carnivores (1963).

      The different chapters discussed on the skull are based on current and fossil species of Hominidae and certain groups of four-legged Mammals. A large number of species studied by the author are based in particular on the cranial and dental forms of herbivorous and carnivorous species endemic to Eurasia, the Mediterranean Basin and Africa.

PART 1 The Skull of Fossil and Present-day Quadruped Vertebrates: Craniofacial Structure and Postural Balance

      1

      Proboscideans: The Mammoth (Mammuthus primigenius)

      Super-Order Proboscidea.

      Order Elephantoidea.

      Family Elephantidae.

      Genus Mammuthus Brooks, 1828.

      Species Mammuthus primigenius Blumenbach, 1799 (woolly mammoth).

1.1. Chronological, geographical and morphological indications of the species

      In the species of the Elephantidae Family, the orbits are very advanced on the antero-posterior axis of the skull and open in front of the jugal teeth; these are anatomical characteristics of animals without snouts. In general, in Mammals, the orbits open above the last molars (Lecointre and Le Guyader 2001).

The nose and upper lip are replaced by a flexible tube used for breathing, drinking, picking things up, etc. The lower canines are lost and the upper incisors are transformed into tusks that are continuously developing (Figure 1.1).

The species belongs to one of the three related genera regrouping mammoths and elephants (Mammuthus, Elephas and Loxodonta) included in the Family Elephantidae and classified in the Super-Order Proboscidea. These trunk bearers have provided no less than 170 fossil species, the oldest dating back 55 million years (Eocene). The first Elephantidae appeared in Africa 7 million years ago, the first mammoths 3–4 million years ago. Great confusion reigned for a long time among specialists for the classification of the mammoth and its origin. Today everyone is unanimous in classifying the first mammoths in Africa.

      The arrival of mammoths in Europe and Asia took place around