"Phylum" - what is it, definition of the term
In biological taxonomy, a major division positioned between kingdom and class assembles organisms that exhibit a shared fundamental body architecture; this rank groups species by core developmental and morphological characteristics that set them apart from other divisions, as illustrated by rodents such as rats and mice, which belong to the same high‑level category defined by a single pair of continuously growing incisors and a comparable skeletal plan.
Detailed information
The taxonomic rank that groups organisms above the level of class and below kingdom represents a major biological division. This division includes all animals possessing a dorsal nerve cord, a notochord, pharyngeal openings, an endostyle or thyroid gland, and a post‑anal tail at some stage of development. These features define the chordate lineage, to which rats and mice belong.
Key anatomical traits of chordates:
- Dorsal hollow nerve cord that becomes the spinal cord
- Flexible notochord serving as a primitive axial support
- Paired pharyngeal slits, which in vertebrates develop into gill arches or associated structures
- Endostyle/thyroid gland for iodine metabolism
- Tail extending beyond the anus in embryonic forms
During embryogenesis the notochord induces formation of the vertebral column, which replaces the notochord in most adult members. The neural tube derived from the dorsal nerve cord differentiates into the central nervous system. Segmentation of somites yields the musculature and dermal skeleton.
The chordate division encompasses several classes, among them:
- Mammalia – hair, mammary glands, three‑bone middle ear
- Aves – feathers, beak, powered flight
- Reptilia – scaly skin, ectothermy
- Amphibia – dual life stages, permeable skin
- Chondrichthyes – cartilaginous skeleton, placoid scales
- Osteichthyes – bony skeleton, diverse forms
Rats and mice are placed within:
- Class: Mammalia
- Order: Rodentia
- Family: Muridae
- Genus: Rattus (rats) and Mus (mice)
These species share the typical mammalian traits of endothermy, hair, and lactation, while exhibiting rodent‑specific adaptations such as continuously growing incisors and a gnawing dentition.
The chordate lineage first appears in the Cambrian fossil record (~525 million years ago). Molecular data suggest divergence of the vertebrate branch, which includes mammals, around 500 million years ago, with the murid lineage separating from other rodents approximately 12 million years ago. This evolutionary timeline underlies the anatomical and genetic continuity observed in modern rats and mice.