The evolution of the modern day Equus Callabus is said to be one of the most complete examples of evolutionary fossil remains. Dating back 65 million years ago during the Eocene epoch, the first recorded fossils of the species called Hydrocotherium meaning ‘rabbit like animal’ were discovered as the first descendant of the modern-day horse. Hydrocotherium adapted and splintered into varying groups across the Americas and the Eurasian Steppes. Climatic and varying conditions over time would change their phenotypes and gradually their genotype.
This would be reflected through the mutation of their DNA and the development of skeletal structures throughout the body and skull. The skull developed a longer deeper jaw to form high crowned cement teeth (hypodonty) capable of eating and breaking down grass instead of tree or shrub like vegetation previously eaten. And the lateralisation of the eyes for almost 360 degrees of vision around the horse would aid the detection of predators.
One of the most important developments to the skeleton, due to microevolution would result in the merging and fusing the 2nd, 4th and 5th toes, into the main 3rd toe. This created a single hoof and longer limb which became super-efficient at flexing upwards and closer to the mass centre with muscle and tendons capable of storing and releasing energy through pendulum effect propulsion of levers increasing speed and length of stride. The overall size of the horse also increased lowering the number of predators able to capture and kill the horse.
Further mutations effecting the digestive system created a hind gut fermentation system better equipped at digesting grass and a respiratory system designed to enable great speed and muscle capabilities.
Populations with speed, size and awareness would survive their new environment grazing in the open, the development of a more advance neocortex also became evident.
By the Pliocene epoch Dinohippus would give rise to Equus Callabus and variations such as the ass and zebra.
The equine skeleton evolved to be perfectly adapted for speed and agility and is categorised in two parts. The axial skeleton which consists of the skull, vertebral column, ribs and sternum. And the appendicular skeleton which is the thoracic and pelvic limbs.
The skeleton is the foundation upon which all other biomechanical, structural, and internal systems are supported and protected. It is the framework for the entire body and supports muscle, tendon and ligaments enabling the body to move and the flexion/extension of joints. The skeleton plays a part in the storage and production of red blood cells and calcium.
The structure of bones consists of an external hard compact layer surrounding the bone marrow found within. The bone marrow is surrounded by the endosteum and is supplied nutrients via arteries. Bone marrow is responsible for the production of stem cells and blood cells vital for life. For example, red blood cells which carry oxygen around the body and white blood cells crucial to the effective management of the immune responses in the body.
The Skeleton is the basis for the horse’s conformation dictating shape and biomechanical movement through the arrangement of bones which connect creating varying angles.
The bone structures are linked together with ligaments, tendons, muscle, and cartilage.
Where bones meet, they form joints allowing movement. The movement is dependent on the type of joint, with differing joints producing the type and amount of movement.
Synovial Joints
The greatest movement comes from synovial joints like the carpal or stifle joint.
Condylar ends of two bones covered with articular cartilage come together and are surrounded by a fibrous joint capsule, synovial membrane, and collateral ligaments.
Within the joint capsule the membrane secrets synovial fluid which contains Hyaluronate acid. Hyaluronate acid is the component in synovial fluid responsible for enabling smooth lubricated articulation between two surfaces. The collateral ligaments provide stabilisation and support to the joint. The hyaline cartilage which covers the articular surfaces of the bone condyle, also assists with lubrication. As the joint is compressed through weight barring it expels and reabsorbs some synovial fluid.
When disease or injury strikes the levels of hyaluronate acid is often depleted resulting in damaging concussion and stressing of the joint. If this happens the hyaline cartilage often suffers creating wear within the joint.
The synovial joint is the mostly like to be injured or effected by disease due to the complexity of its design, with poor conformation, workload of an individual horse or poor-quality riding/working surfaces contributing to the trauma.
Cartilaginous Joints
Cartilaginous joints are not as mobile as synovial joints and are sometime immovable depending on their location. Cartilaginous joints consist of fibrous and/or hyaline cartilage constructed of collagen fibres. An example of a cartilaginous joint would be between each vertebra.
Fibrous Joints
Fibrous joints are immovable and can be found between bones such as the symphysis of the pelvis or between the bones of the skull. These fibrous joints ossify with age and become fixed.
Synovial joints are further categorized as…
Hinge joints
e.g. interphalangeal, knee, elbow and hock.
Hinge joints creating flexion (decrease the angle) and extension (increase the angle). Hyperextension damages the hinge joint causing it to move beyond its normal range of motion.
Sliding joints
e.g. intercarpal joints including
radial, intermediate, ulnar, 3rd, 4th carpal bones and
mid carpal and
carpometacarpal joints of the knee.
Ball and Socket
e.g. acetabulum of the pelvis Coxofemoral joint.
Pivotal joint
e.g. Atlantoaxial joint between the atlas and axis cervical vertebrae.
Ellipsoid Joints or condyloid joints
e.g radiocarpal or antebrachiocarpal joint of the knee.
Allowing back and forth and side to side movement of the joint.
Two type of movement biaxial
e.g. the Human wrist.
An important part of the equine skeleton is the correct formation and development of bone growth,
For example, long bones such as the canon (3rd metacarpal/metatarsal) consist of the Diaphysis, the main shaft of the bone and the Epiphysis, the head, tuberosity or condylar end of the bone for articulation. Between these both proximally and distally is the metaphyseal growth plate (also called the physis). Within the growth plate as the bone lengthens a process called endochondral ossification gradually turns soft cartilage to bone once maturity is reached.
The equine skeletal growth plates close and mature at differing rates with the most distal bones maturing first progressing proximally. Recent research by Dr Deb Bennet suggests that the proximal growth plate of the cannon bone for instance is fused at approximately eighteen months whereas the Radius, Humerus and Scapula are approximately three to three and half years of age. However, the spinal and cervical vertebrae are said not be fused until a horse is up to five and a half years of age and possibly longer if the horse is a tall and rangy warm blood. Bones can be categorised by their shape e.g. long, short, flat or irregular all bones having a growth plate at each end. Bony structures such as the pelvis having multiple growth plates.