AMIMAL GROUPS AND NAMES
Like all classification schemes, it represents current thinking, which is always liable to change. Groups are “nested” to show how they are related; informal groups with no distinct biological identity are bounded by dotted lines. Some of the species totals, particularly for the invertebrates, are based on estimates; as with new discoveries, these can be subject to rapid change.
The Animal Kingdom is divided into more than 30 phyla. Some of the smallest phyla contain less than 100 species, while at the other extreme the arthropod phylum contains as least 1.2 million, making these arguably the most successful animals of all time. Between these 2 extremes are the chordates – animals that usually have a backbone. Although not front runners in terms of species, chordates are exceptional in many ways. They show an extraordinary range in dimensions, from fish the size of a fingernail to majestic 100-tonne whales. They can be found on land, at sea, and in the air, where they outrun, out-swim, or out-fly all other forms of life. Since antiquity, they have been much observed, which is why many names for animal groups have their origin in those times.
Like species names, group names often look obscure but they have a language and logic of their own. Many of them are named after particular body parts that make a group distinct. Chordates, for example, get their name from the notochord – a strengthening rod that runs down their back, for part or all of their lives. Arthropods get their name from their joint-bearing legs, one of the features that accounts for their success. Among chordates, frogs and toads make up the order Anura, which simply means “without tails”, while elephants make up the order Proboscidea, after their long probosci,or nose.
Sometimes, ancient folklore also plays a part in names. For nightjars and frogmouths, in the avian order Caprimulgiformes, got their name from the belief that nightjars sucked milk from goats. Dugongs and manatees are more fanciful still. They belong to the order Sirenia, from their supposed resemblance to the three sirens of Greek mythology – mythical figures who seduced sailors into shipwreck on the rocks.
AN ONGOING PROCESS
Classification changes all the time as more is discovered about animals and their evolutionary past. Some changes affect life at its highest levels: today, for example, it is generally accepted that there are 5 kingdoms of life, but in the recent past there were 6. Changes to lower-level classification happen every day. Through cladistics and DNA analysis, much more information about the relationships between species is becoming available, and as a result, groups at different taxonomic levels are often moved, split, or joined together.
Seals, for example, were formerly considered to be a separate group of mammals, the order Pinnipedia. Today, most zoologists classify them as a family within the order Carnivora, together with terrestrial carnivores such as cats, dogs, and bears. One widely accepted change not reflected here is the creation of the order Cetartiodactyla, which shows the close relationships between even-toed hoofed animals and the cetaceans. With moves like these, the animals themselves do not change. What does change is our understanding of them: how they have evolved, and which animals are their closest relatives.
Mammals are chordates characterized by having fur or hair and raising their young ones on milk. There are over 5,000 living species, placed in 29 orders, ranging in size from a single species to over 2,000. Reproductively, mammals form 3 groups: monotremes, marsupials (Metatheria), and placental mammals (Eutheria). Monotremes lay eggs, while marsupials and placentals give birth to live young.
Marsupial young are born at an early stage, and complete their development in a pouch. In placentals, the young are nourished inside their mother’s body, so they are at a more advanced stage at birth. Even so, parental care in many species may last at least a year.
Birds are the only members of the phylum Chordata that possess feathers. They use them to keep warm and, in most cases, to fly. Birds are closely related to reptiles and form a single clade with them and the now-extinct dinosaurs. In recent years, comparison of DNA from different species of birds has led to many changes in avian classification, with the system used in this book separating the class into 40 orders.
Some bird orders are very small: the ostrich order, for example, contains just 2 species, while that containing the mouse-birds has only 6. By comparison, the order containing the Passeriformes, or perching birds, contains more species (over 60000) than all the other orders combined, and it includes all the world’s songbirds. Within this huge rent order, there is considerable disagreement about how many families of passerine birds there are. Some ornithologists consider that there are just 60 families, but many of the larger ornithological societies are moving towards a system that recognizes more than 130 families.
Chordates with scaly skin, or reptiles, were the first 4-legged animals to be fully at home on dry land. This is because their skin is waterproof. Snakes and lizards make up over 90 percent of living reptile species, although the largest species are tortoises and crocodiles. Most reptiles lay eggs, but a small minority give birth to live young.
Frogs and toads makeup the largest order of amphibians, and show the widest range of adaptations for terrestrial life. Newts and salamanders most closely resemble ancestral amphibians; caecilians are an aberrant relatively little-known group.
Despite similarities, fish are a varied collection of animals with different evolutionary histories. Lampreys and hag-fish have no jaws. Bony and cartilaginous fish have jaws, skulls, and skeletons, but their anatomy is very different, and their lifestyles distinct. Today bony fish make up by far the largest class across aquatic habitats. The major subclass of this group contains so many orders and species that it is dealt with at suborder level.
Invertebrates are classified in about 30 phyla, which vary considerably in size. In this book, major phyla are treated separately, Almost all these minor contain marine animals or ones that live damp habitats.
Invertebrate chordates are animals that share some characteristics with vertebrates but lack a bony skeleton. There are 2 subphyla — tunicates (the majority of species), and lancelets. Tunicates have a swimming tadpole stage but are bag-like as adults; lancelets are mobile and in their internal anatomy bear strong resemblances to vertebrates.