Vertebrates which have fur, secrete milk for their young, and whose jaw joint is specifically formed by the articulation of the dentary element of the jaw with the squamosal part of the temporal bone of the skull.
This latter is, of course, the criterion by which paleontologists distinguish mammals from near-mammals and non-mammals. Generally speaking, mammals possess teeth of several different shapes – ie, incisors, canines, premolars and molars; but some mammals lose all teeth or reduce them to vestiges (armadillos, pangolins, aardvarks, sloths), others modify them to fantastic new structures (baleen whales), and still others return to having teeth of uniform morphology (dolphins, killer whales).
No matter how modified, however, mammals have only one row of teeth along the margins of the jaws, and they replace only the anterior part of the dental arcade only one time.
The teeth develop in, and erupt through the gums out of, sockets called alveoli. Primitively, the limbs of mammals terminate in five digits of about equal length arranged in a fan shape.
From this original design many modifications have been tried, including loss of some digits, lengthening or shortening of one or more digits, fusion of digits to form a single stout digit, change from claws to either fingernails or hoofs, or change of the whole limb from a paw into a flipper or a wing. With few exceptions, mammals have exactly 7 cervical vertebrae (whales and dolphins, and also giant ground sloths, have less than 7 vertebrae). Each half of the mandible (the jawbone) in mammals is made up of but one, single element, the dentary bone (in reptiles, amphibians, fish, and birds the mandible is formed either of more than one bony element, or is formed from a different bone than the dentary). The pelvis of mammals is likewise relatively simple, each half being composed of only three elements, the ischium, ilium, and pubis (the pelvis in other vertebrates tends to have additional elements).
Importantly, mammals possess a sacrum composed of five or more vertebrae which fuse to make a rod above the pelvis.
Birds and frogs also show fusions in this area, but they are far more extensive and serve to prevent flexion of the pelvis on the lumbar vertebrae.
In mammals, up-and-down flexion of the spine is a crucial and characteristic element of locomotion, in contrast to the characteristically side-to-side, sinusoidal motions of the vertebral chain in reptiles, amphibians, and fishes (you can tell a fish from a whale or dolphin by the orientation of its tail fins: the fish’s tail fin is vertical, because in order to swim, he “wags his tail” from side to side.
Marine mammals, by contrast, have their flukes oriented horizontally, because the main swimming motion is “humping” or up-and-down action of the tail). Every student of the horse should be able to name and know the characteristics of all five of the classes of vertebrates: mammals, birds, reptiles, amphibians, and fishes.
By contrasting mammals with the other four, we derive a richer picture of the unique nature of all mammals, and of the horse in particular. ORDER Perissodactyla Herbivorous or omnivorous mammals that retain a caecal digestive system.
Perissodactyls have single, obliquely-ridged tibial tarsal bones (astragali), in contrast to the other major order of herbivorous mammals, the Artiodactyla, which have double, parallel-ridged astragali. The term “Perissodactyl” comes from Greek words meaning “digits arranged around (peri-) a central toe (-dactyl)” – and this is indeed the foot-symmetry of all animals belonging to this Order.
The central digit, designated by Roman numeral III, is the largest and bears most of the limb’s weight, even when the animal retains three, four, or five toes.
This too is in contrast to the Artiodactyla, in which the main part of the weight is shared by closely-appressed or fused digits III and IV. There is a strong tendency to flatten the radius and to closely appress it to the ulna, so as to inhibit or prevent supination of the manus (turning the forefoot inward or upward).
In the hind limb, the tibia becomes reduced in size and loses its articulation below with the tarsus.
A deep pair of grooves on the distal end of the tibia, which are obliquely oriented to match the ridges on the astragalus, forms a strong, tight, and stable articulation and likewise prevents the hind foot from turning inward. All Perissodactyls have 18 thoracic vertebrae (and thus 18 pairs of ribs), although the number of lumbar vertebrae may be as high as 8. Perissodactyls have bunodont (cusped) or lophodont (ridged) teeth.
In the upper dentition, the teeth have the cusps or ridges are so arranged that when the tooth is somewhat worn, they make a shape that looks like the Greek letter “pi”.
Worn lower teeth look like the small letter “m”. — Equines are Equids adapted for eating grass.
Thus, they have hypsodont teeth with cementum, a bone-like material, to support and strengthen each tooth.
They have stout, wedge-shaped skulls in which both the maxilla and dentary are deep to accommodate the tall-crowned teeth. The superior teeth of Equines have two fossettes.
Each fossette is filled, or largely filled, with cementum. Except for the first premolar, which is greatly reduced in size, the premolar and molar teeth of equines are large, square in cross-sectional shape, and tightly appressed to each other to form “cheek batteries” for the efficient grinding of grass blades. There is a strong tendency among equines to retract the nasal notch, ie to “undercut” the nasal bone, even to an extreme degree (vis., Hippidium and Onohippidion).
In such forms, deep pits or “facial fossae” tend to be present on the face, the complex as a whole indicating the presence of a small trunk or a strongly prehensile upper lip longer than that in the living horse.
Where there is little or no retraction of the nasal notch (Neohipparion, Nannippus), the face will be smooth, lacking facial fossae.
Forms that have smooth faces tend to have the most complex and most highly hypsodont teeth. Equines are universally unguligrade, having hoofs rather than claws and never locomoting with the hock, carpus, or “ankles” (the joint between the distal end of the cannon bone and the pasterns) touching the ground.
Generally speaking, they are are tall and have proportionally long distal limb elements.
They are adapted for straight-line flight over firm substrates in open terrain. Equines characteristically develop joints between the “wings” of the sacrum and the transverse processes of the last lumbar vertebra.
They also have articulations between the transverse processes of the last several lumbar vertebrae.
Moreover, the joints between the accessory articular processes in the lumbar vertebrae of equines are vertically-oriented, and they are shaped to articulate like dovetail joints.
These “inter-transverse” and “dovetail” articulations almost totally inhibit rotation and lateral flexion among the lumbar and lumbo-sacral joints, while promoting up-and-down coiling of the lumbo-sacral joint and loin-span.
This is in sharp contrast to Artiodactyls, which retain long, relatively loosely-articulated lumbars which permit twisting and a greater degree of lateral flexion. INFRAFAMILY Protohippini Protohippine equines are distinguished by possessing high-crowned teeth that are nevertheless comparatively simple in structure (the genus Equus has the most complex teeth within the Protohippine lineage). An important distinguishing characteristic of the Protohippine dentition which is easy to see is that the protocone loop of enamel is joined to the hypoloph, not set off (as it is in Hipparionines) as a separate circlet. Two kinds of skull and skeletal structure may be found within the Protohippini: one is the “normal” or mesomorphic build we associate with the genus Equus.
These animals have flat or slightly-rounded backs, nasal notches only moderately retracted, moderately prehensile upper lips, and smooth faces.
The other type re-echoes the “chalicomorph” or “okapi-like” body design in having forelimbs longer than hind limbs, a sloping back, deeply-retracted nasal notch, and deep facial fossae for the attachment of the muscles to move a long, strongly prehensile upper lip or short trunk.
Pliohippus is an outstanding example of this, and study of Pliohippus should remind students never to fall back into the old 19th-century “evolutionary progression”, proposed by O.C.
Marsh, that supposedly led from Protohippus through Pliohippus to Equus.
Equus is the descendant of Protohippus and Dinohippus, both flat-backed and smooth-faced genera, not Pliohippus. Not surprisingly, to go along with the two different body-styles found within the Protohippini, there are two sorts of dentition.
Equus and earlier members of its direct lineage have relatively straight teeth, with the angles of the “tables” or occlusal surfaces of the cheek batteries set at from 7 to 10 degrees of slope.
Pliohippus and other “okapi-like” forms have upper teeth with a lot of curve to them, and thus table angles ranging from 10 to 25 degrees of slope.
These teeth have exceptionally heavy outer styles and buttresses, and very simple lower teeth; such a design is less for dealing with grass than with “chop”, ie twigs, leaves, and bark rather than grass.
The supposition is that Pliohippus and similar forms were eating a diet similar to a deer’s. The other infrafamiliar taxon is the Hipparionini.
Students of the horse should take every opportunity, by visiting Museums of Natural History, to familiarize themselves with these animals as well.
Hipparionines are typically small, narrow-bodied, lightweight, agile and dainty.
While Protohippines tend to be large and heavy, Hipparionines are relatively small, some even becoming dwarfs no larger than the African dik-dik.
All Hipparionines have extremely hypsodont teeth with highly complex structure, good for processing dry forage.
These animals were, evidently, filling the ecological niche now exclusively occupied by antelopes. GENUS Equus
Read more about Flexion : Birds and frogs also show fusions in this area but….: