Artiodactyla

The artiodactyls (Artiodactyla, from the Greek άρτιος (ártios), "par" and δάκτυλος ( dáktylos), "finger") or even-toed ungulates are an order of ungulate mammals whose limbs end in an even number of toes, of which rest on the soil at least two (except cetaceans, which are aquatic and have fin-like extremities); the most developed fingers are the third and fourth and, except for the hippos, they are the only ones that rest on the ground. The other three fingers are present, absent, vestigial, or backward pointing. On the other hand, perissodactyls or odd-toed ungulates support their weight on one (an odd number) of the five fingers: the third. Another difference between the two orders is that artiodactyls digest plant cellulose in one or more stomach chambers and not in the intestines, as occurs in the odd-toed ungulates.

Cetaceans evolved from artiodactyls, so modern taxonomic classification combines them under the name of Cetartiodactyla. Terrestrial artiodactyls inhabit all continents except Antarctica, although those found in Oceania have been introduced by humans. While aquatic artiodactyls (cetaceans) are found in all the world's oceans. This makes artiodactyls the group of mammals and vertebrates that has managed to colonize the entire planet, including all aquatic and terrestrial habitats.

The current artiodactyls (not counting the fossil taxa) include about 235 species of terrestrial artiodactyls divided into 10 families, among which are camels, llamas, pigs, cows or bulls, goats, sheep, hippos, antelopes, deer, giraffe, buffalo, wild boar, etc. and around 88 species of cetaceans or aquatic artiodactyls in 12 families, examples are whales, dolphins, killer whales, sperm whales, porpoises, etc. Many of these species are of great dietary, economic, and cultural importance to humans.

Traditionally Artiodactyla included terrestrial artiodactyls. However, if cetaceans are excluded, Artiodactyla is a paraphyletic taxon (which does not contain all its descendants), so the new taxonomic classifications prefer to include cetaceans so that Artiodactyla is monophyletic.

Evolutionary history

Cladogram showing Artiodactyla's position.

Like many groups of modern mammals, artiodactyls appeared during the Paleogene period. The oldest fossils of artiodactyls date from the beginning of the Eocene (56 million years ago), however other studies that include analyzes using the molecular clock technique, estimate the origin of artiodactyls at the end of the Cretaceous (67 million years ago). years). In appearance, its first members were reminiscent of modern mouse deer: small and short-legged. Even so, artiodactyls were far from dominant at the time; perissodactyls (the group to which horses and rhinos belong) were far more successful and numerous. Artiodactyls then survived in generally marginal habitats, and it was presumably around this time that they developed their complex digestive systems, which allowed them to survive on poorly nutritious food.

By the late Eocene (~46 million years), the four modern suborders had already developed: Suina (the pig group), Tylopoda (the camel group), and Ruminantia (the goat and goat group). bovines), together with the Whippomorpha (the group of hippos and cetaceans).

The appearance of grasses during the Eocene, and their subsequent expansion during the Miocene (about 20 million years ago), allowed a noticeable change; grasses are very difficult to digest, and terrestrial artiodactyls, with their highly developed stomachs, were better adapted for this new harsh, nutrient-poor diet, soon replacing perissodactyls as the dominant terrestrial herbivores. In this period, cetaceans had fully developed aquatic life.

Cladogram

Evolutionary relationships of artiodactyls by suborder:

Phylogeny of current groups

The phylogenetic relationships between the subgroups would be as follows:

Classification

This classification is based on the analysis of Spaulding et al., 2009 and the current families recognized in the work Mammal Species of the World published in 2005. Currently, cetaceans and artiodactyls have been placed in Cetartiodactyla as sister groups, although DNA analysis has shown that cetaceans in fact evolved directly from Artiodactyla, so they should be listed in this order. Cetartiodactyla is a synonym of Artiodactyla. The most recent theory of the origins of the family Hippopotamidae suggests that hippos and whales share a common semi-aquatic ancestor that diverged from other artiodactyls around 60 million years ago. This hypothetical ancestral group probably split into two branches about 60 million years ago. 54 million years ago. One of these branches would evolve in cetaceans, possibly beginning with the protowhale Pakicetus 52 million years ago, along with other primitive whales known collectively as the Archaeoceti, which would eventually develop adaptations towards the completely aquatic life that modern cetaceans have.

• Order Artiodactyla/Cetartiodactyla
  • Suborden Tylopoda
    • † Anoplotheriidae
    • † FamilyChoeropotamidae
    • † FamilyCainotheriidae
    • † FamilyMerycoidodontidae
    • † FamiliaAgriochoeridae
    • Camelidae Family: camels and flames (six species)
    • † FamilyOromerycidae
    • † FamiliaXiphodontidae
  • Suborden Suina
    • Suidae family: pigs, wild boars and facoqueros (19 species)
    • Tayassuidae family: pecaris (four species)
    • † FamilySanitheriidae
  • Suborden Ruminantia
    • Infraorden Pecora
      • Bovidae family: antelopes, goats, sheep, cows or bulls, buffaloes and bison (135 species)
      • Antilocapridae family: berrendos (a species)
      • Giraffidae family: giraffes and okapis (two species)
      • Moschidae family: almizcleros deer (seven species)
      • † FamilyLeptomerycidae
      • Cervidae family: deer and reindeer (49 species)
      • † Gelocidae family
      • † FamilyClimacoceratidae
      • †Palaeomerycidae family
      • † FamilyHoplitomerycidae
    • Infraorden Tragulina
      • † FamilyAmphimerycidae
      • † FamilyProdremotheriidae
      • Tragulidae family: swallowed or mouse deer (ten species)
      • † FamilyProtoceratidae
      • †Hypertragulidae
      • † FamilyPraetragulidae
      • † FamilyArchaeomerycidae
      • † Lophiomerycidae
  • Suborden Whippomorpha
    • †Andrewsarchus
    • † FamilyEntelodontidae
    • † FamiliaRaoellidae
    • † FamiliaDichobunidae
    • † Helohyidae
    • † FamilyCebochoeridae
    • Infraorden Hippopopotamorpha
      • †Anthracotheriidae
      • Hippopotamidae family: hippos (two species)
    • Infraorden Cetacea: (included recently)
      • Parvorden Mysticeti
        • Balaenidae family: free whales (four species)
        • Balaenopteridae family: rorcuals and yubartas (nine species)
        • Eschrichtiidae family: grey whales (1 species)
        • Neobalaenidae family: Pygmy whales (1 species)
      • Parvorden Odontoceti
        • Delphinidae family: ocean dolphins and orcas (34 species)
        • Iniidae and Platanistidae family: river dolphins (six species)
        • Phocoenidae family: marsopas (six species)
        • Physeteridae family: common cachalotes (1 species)
        • Kogiidae family: Pygmy and dwarf cachalotes (two species)
        • Ziphiidae family: zifios (21 species)
        • Monodontidae family: belugas and narvales (two species)
      • Parvorden †Archaeoceti
        • † FamilyPakicetidae
        • † FamilyAmbulocetidae
        • † FamilyRemingtonocetidae
        • † FamilyProtocetidae
        • † FamilyBasilosauridae

Characteristics of artiodactyls

Terrestrial Artiodactyls

Common range.

Toes of a pig.

Toes of a drugstore.

In terrestrial artiodactyls, the axis of the feet passes between the third and fourth toes, which are similar to and longer than the second and fifth toes, which are almost always reduced or even atrophied. The number of fingers in extant species is even (2 or 4), except in tayasuids, whose hind legs have only three functional fingers. The toes are covered by hooves. In most cases, the metacarpals and metatarsals of the long toes are welded together to form a single element, called the shank. The second and fifth fingers are reduced to thin bony structures attached to the previous ones or they can disappear completely. Both the ulna and the fibula are greatly reduced, while the radius and tibia are highly developed.

A very typical character of many terrestrial artiodactyls (cervids, antilocaprids, bovids and giraffids) is the presence of horns located symmetrically on the sides of the forehead, near the eye sockets; they grow on processes of the frontal bones and may be covered by skin (giraffes and deer) or by a keratin sheath (antelopes, goats, cows).

The dentition of these artiodactyls is highly specialized; the upper incisors of the extant species have atrophied, the premolars are not molarized, and the molars present an efficient grinding surface formed by longitudinal ridges; the result is similar to that of perissodactyls (horses) but has been achieved in very different ways, in a clear case of evolutionary convergence.

The stomach is complex in all artiodactyls. In the simplest cases, such as pigs and hippos, there is a single pouch next to the opening of the esophagus; in the case of ruminants there are four chambers: rumen, mesh, omasum and abomasum. It is an adaptation that allows them to break down the cellulose present in plants thanks to the presence of symbiotic microorganisms. The abomasum secretes powerful acids, it is like the stomach of any other mammal.

Artiodactyls have a complex system of scent glands on the head, between the fingers, in the inguinal region, and less commonly around the anus. These glands are involved in signaling territory and in their social and sexual life, which is often complex.

Aquatic Artiodactyls

Cetaceans according to class classifications evolved from terrestrial arthyodactyls.

Having evolved from terrestrial artiodactyls, cetaceans have had to develop anatomical and physiological adaptations in order to have a fully aquatic life:

• the body is fusiform and has taken a hydrodynamic form similar to that of a fish;
• has developed a dorsal fin on the back, formed by connective tissue;
• the previous legs have become pectoral fins and have taken shape of rowing;
• the end of the tail is flat and consists of two lobes;
• the rear legs are absent and there are only small bones hidden inside the body;
• they present a breath at the top of the head;
• hairs disappear completely after the first months of life;
• Atrial pavilions are absent;
• external genitals are hidden inside pockets.