Kingdom: Animalia
The Kingdom Animalia is made up of multicellular eukaryotic organisms that are heterotrophic. Most animals have specialized cells which are grouped into tissues (with the exeption of sponges). These tissues are often organized into specialized organs and organ systems. The first animals are widely believed to have evolved during the Cambrian period 570-550 million years ago, though there is some debate as to whether earlier forms were simply not preserved as fossils and/or are to small to be identified (Perkins). Today, the Animal kingdom comprises of more than 30 Phyla, and in turn potentially more than 10 million individual species.
Phylum : Chordata
The phylum Chordata is comprised of organisms that possess a notochord, or a flexible rod that extends most of the length of the body, at some point in their development. This notochord acts as support for the body. Other common characteristics include bilateral symmetry, a segmented body, a closed blood system, a complete digestive system, and a bony or cartilaginous endoskeleton. All Chordates also develop with pharyngeal pouches at some point in their development (Myers). The first Chordates can be identified in the fossils of the Cambrian Explosion. The oldest fossil chordate is believed to be Cathaymyrus which was found in southern China and is believed to be related to Pikaia gracilens found in the Burgess shale deposits in Canada (Gee). Modern day chordates include fish, reptiles, birds and mammals, including humans (and horses!).
Class: Mammalia
Mammals first evolved during the early Triassic period from a group of reptiles like creatures known as synapsids. In their early development, mammals were small, nocturnal scavengers, and remained that way until the end of the Mezozoic Era and the Cretatious mass extinction. After this, during the Tertiary period (66 million years ago), mammals became the dominant group of vertebrate animals as they grew larger and developed specialized teeth and limbs. All mammals share 3 characteristics- they have 3 bones in the middle ear, they produce milk for the young in modified sweat glands (mammary glands) and at some point in their development all mammals have hair. By the time of the Pliocene, 5 million years ago, the well known mastadons and other large mammals reined supreme. During the Quaternary Period, 1.8 million yearsago, many modern mammals are represented, but in huge body size. These mammals included mammoths, sloths, bears, elk and bison. However, approximately 12,000 years ago the climate underwent several rapid changes which caused a sharp decline in the number of large animal species and led to the evolution of the more modest sized mammals we are familiar with today.