CPSP118G Spring Semester: Earth, Life & Time Colloquium
A brief climb through the Tree of Animal Life
Animals in the broadest sense
- Sponges, the simplest animals have distinct cell types but don't really have proper tissues and certainly not organs. One cell type is indistinguishable from choanoflagellates, the closest relatives of animals. Have amazing powers of regeneration. Organizationally, they are living sieves, filtering microscopic food particles from water.
- Cnidaria and Ctenophora:
represent the next simplest grade of organization, with distinct tissues and organs. Still the gut is a simple pocket with an opening that can function as mouth or anus depending.
- Bilateria: Animals with:
- bilateral symmetry
- a flow-through gut in which we can distinguish a mouth and anus.
- The evolution of a coelom, or body cavity, made many types of life strategies possible because its possessors could truly move in a purposeful manner.
- Among such animals, we see two major branches identified by distinct patterns of embryology:
In all, the blastula invaginates to form a gastrula with an inner cavity communicating with the outside through an opening called the blastopore. In protostomous, the blastopore becomes the mouth. In deuterostomous it becomes the anus.
"Protostomes" briefly noted:
Protostomia contains the vast majority of animal diversity, including many familiar animals. Among them are two major groups, Ecdysozoa and Lophotrochozoa.
There is a trend in these active animals toward the evolution of special sense organs (eyes, antennae) and toward concentrating them at the front end along with an expansion of the nervous system to deal with the information they provide (heads).
And yet, these structures originate independently. Consider some close relatives of the major groups: Tardigrades for ecdysozoans, and phoronids for lophotrochozoans.
- Ecdysozoa includes Arthropods among others.
- Lophotrochozoa includes molluscs and annelids.
- Annelids - Segmented worms. But note, segmentation is ancesteral.
Deuterostome survey: The deuterostomes contain Chordata (us), Echinodermata, and some minor groups. Of all of the major natural groups of animals, echinoderms are our closest relatives, and yet they are our weird relatives.
- Echinoderm wierdness includes:
- Five-fold symmetry
- Water vascular system. Coelomic cavity that communicates with the outside, is full of sea water that is used as "hydraulic fluid" to power tube-feet that facilitate locomotion/feeding
- Like early echinoderms, acorn worms and their close relatives filter particles from the water, but do it differently. They filter water through a basket-like pharynx lined with slits.
- Adult sea squirts
are practically all pharynx, however as larvae they resemble tadpoles with segmented muscles and a stiffening rod (notochord).
- Imagine a baby sea squirt that never grew up and you have Branchiostoma (aka Amphioxus and Lancelet). It still has a large pharynx, but otherwise looks like a tiny headless fish.
Craniata - deuterostomes with heads
Craniates inherit the notochord and segmented muscles of earlier chordates, but are redically different in some ways:.
Osteichthyan survey: The diversity of "fish" breaks down into two groups:
- Actinopterygii: Ray-finned fish. Most living fish
- Sarcopterygii: Lobe-finned fish. Land vertebrates and a few aquatic weirdos.
- Two proper types of aquatic lobe-fin fish remain today:
- The rest are "land-fish" i.e. terrestrial vertebrates. What transformations are required to be a land vertebrate?
- The "lobe" of the coelacanth is supported by a skeleton that corresponds to the arm and leg skeleton of land vertebrates.
- Limbs that support the body. A simple embryological transformation accounts for the development of fingers and toes.
- Sensory modalities that work on land.
- The ability to breathe on land without dehyrdating.
- Skin that prevents dessication.
- A neck
- Fossil intermediates.
- Eusthenopteron - a fully aquatic sarcopterygian.
- Panderichthyes - with reduced tail and dorsal fins.
- Tiktaalik (Southpark's retarded fish frog) - pectoral fins with distinct wrist joints.
- Acanthostega - Fore and hind limbs with fingers and toes.
- Ichthyostega: Clearly capable of coming out of the water, albeit clumsily. Gill cover reduced, suggesting that it could hand out on land for a while without dessicating.
- Eryops: Clearly capable of spending considerable time on land as adult.
- Modern amphibians: Many highly adapted to life on land as adults, but most prefer moist environments. Almost all must find bodies of water to mate.
- Invention of internal fertilization of the egg as we know it:
- Proliferation of land vertebrates in two major groups:
- Synapsida: Mammals and their fossil relatives.
- Dominated land faunas before the age of dinosaurs and after, but not during
- Some fossil representatives.
- Living mammals
- Sauropsida: Reptiles.
- Dominated land faunas during the age of dinosaurs and an enduring presence since
- Major groups.
Last modified: 2 March 2007