What factors may have triggered some of the large diversification events in deuterostomes?
The large diversification of deuterostomes is similar to the diversification of protostomes. As stated in my post titled “Protostome Diversity,” these changes were triggered by transitions to land and their heterotrophic nature. The transition from marine echinodermates to vertebrates has brought about several key innovations.
The development of the pharyngeal gill slits, nerve cord, notochord and post-anal tail allowed for better feeding and movement.
Next, animals became distinguished by their cartilaginous or bony structures called the vertebrae and the presence of a cranium to protect the brain. These developments led into key innovations including jaws, limbs, feathers, amniotic eggs, placentae, and parental care. Jaws serve as an innovative way to ingest food, and can be modified for different organisms that feed on different energy sources. Limbs allowed for better movement when animals made the transition to land. Feathers and flight allowed some animals to travel in ways that aided their search for food and protected them from other predators. The amniotic egg, with and external membrane along with three internal membranes allowed animals to reproduce in terrestrial environments. The development of a placenta triggered viviparity to evolve, which is common in mammals and is another way in which terrestrial animals can reproduce efficiently. This, however, reduced the number of offspring that could be produced in each cycle. The behavioral trait of parental care evolved to ensure that those low numbers of offspring could survive into adulthood.