The Origin of Phyla and the Cambrian Explosion of Life

Geology 230 Fossils and Evolution Cambrian Life

• The first animals evolved about 60 my before the start of the Cambrian. These are the Ediacaran fossils of the latest Proterozoic. • None of these animals had hard parts. • Base of the Cambrian defined by first animals with hard parts. Life at the end of the Proterozoic Life at the end of the Proterozoic Cambrian Life

• Early Cambrian fossils consist mostly of small little shells that are later followed by trilobites and brachiopods. Small little shells: sclerites on soft-bodied animals

Cambrian trilobites cruising on Saturday night Typical Cambrian trilobites Modern horseshoe crabs look similar to trilobites, but they are not closely related. Example of a “living fossil.” Trilobites are extinct. A living Inarticulate Brachiopod. Very common in the Cambrian. Modern Inarticulate Brachiopods in their burrows Modern Inarticulate Brachiopods for dinner

The Cambrian “Explosion” of Life

• What is the Cambrian “Explosion”? • Is it a true explosion of phyla, or was there a “slow fuse” back into the Proterozoic? • Why did so many new phyla appear at this time? Hox genes hold the answer. • Why have no new phyla appeared since this time? MicroRNA holds the answer. The Tree of Life

www.evogeneao.com/tree.html Cambrian Explosion, radiation of triploblasts (3 tissue layers)

Diploblasts (2 tissue layers) Diploblastic Animals: Triploblastic Animals: Two Tissue Layers Three Tissue Layers

Mesoderm in blue (jelly) Deuterostomes (mouth is second opening during development)

Protostomes (mouth is first opening during development) Ecdysozoa

Lophotrochozoa Protostomes (mouth is first opening during development)

Deuterostomes (mouth is second opening during development)

Prothero, 2007 Prothero, 2007 Hox genes determine the head to tail anatomy of animals. Vertebrates have 4 sets, but their Hox genes are almost identical to those of insects. Mutations in Hox genes create new body plans or phyla. Videos

Genetic Tool Kit: Hox genes http://www.pbs.org/wgbh/evolution/library/03/4/l_034_04.html

The Cambrian Explosion http://www.pbs.org/wgbh/evolution/library/03/4/l_034_02.html HOX Genes – arranged in order of expression from anterior to posterior Molecular Paleobiology Metazoan Phylogeny Based on the Fossil Record and Molecular Clocks (see Fig. 10.11 in Zimmer) Explosion of Phyla Triploblasts

Slow fuse

Kevin Peterson, Dartmouth

MicroRNA genes: Their evolutionary significance has only been recently understood. They come from “redundant” DNA. They are responsible for developmental precision. MicroRNAs are continually acquired and fixed in animal genomes with the most morphologically complex animals having the greatest number of microRNAs.

jellyfish Diploblasts flatworm earthworm Protostomes millipede

arthropods Triploblasts

Sea urchin

Deuterostomes Sea squirt lancelet fish Vertebrates frog Tetrapods dog monkey

Time, Ma Kevin Peterson, Dartmouth MicroRNA constrains variability during development so that morphological innovations are constrained as developmental precision increases allowing more complex animals. Note that the microRNA differences between phyla were 95% complete by the end of the

Cambrian, and 90% for classes by the Ordovician.

of homologous homologous microRNA of

Cambrian Ordovician

Kevin Peterson, Dartmouth The Burgess Shale of British Columbia: record of the Cambrian Explosion Mt. Stephen in Yoho National Park, Canada Geologists at the Burgess Shale quarry Trilobites! Charles Walcott, Smithsonian Institution Geologist, at the Burgess Shale Quarry, ~ 1910 Paleontologist collecting a slab of fossils Trilobites with preserved legs and antennae The strange animals of the Middle Cambrian Burgess Shale Opabinia and Amwiskia, representatives of two extinct phyla Opabinia Burgess Shale worm Ottoia A spiny “worm,” Wiwaxia Hallucigenia, a spiny lobopod Hallucigenia Specimens of lobopods Living and fossil lobopods Early crustacean from the Burgess Shale, Canadapsis The first sea scorpion on the attack! Marella, extinct class of arthropods Marella as Cambrian road kill (or a squished bug?) Yohoia, an extinct class of arthropods Anomalocaris, the largest predator of the Cambrian. Is it an extinct phylum, or a proto-arthropod? Pikaia

Anomalocaris in hot pursuit of Marella

Pikaia, an early chordate Pikaia, a chordate from the Burgess Shale Yunnanozoan, a chordate from the early Cambrian of China