112lect35 - GY 112 lecture notes D. Haywick (2006) 1 GY 112...

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GY 112 lecture notes D. Haywick (2006) 1 GY 112 Lecture Notes Evolution of the Vertebrates Lecture Goals : A) Fish to Amphibians B) Amphibians to Reptiles C) Reptiles to Mammals Textbook reference: Levin 7 th edition (2003) Chapters 10, 11, 14; Levin 8 th edition (2006) Chapters 12, 13, 16 A) Fish to Amphibians Before we get started on the evolution of the vertebrates, it’s perhaps best to restate how the vertebrates are classified. The vertebrates are a subphylum ( vertebrata ) of the phylum chordate . The chordates are a large group of animals that are bilaterally symmetrical and which possess a notochord with a hollow neural tubeway. The vertebrates are animals that possess a backbone complete with vertebrae (hence the taxonometric classification), a well-defined head and sense organs. The vertebrates include fish, amphibians, reptiles and mammals. They also include you and me. The first chordates probably evolved before the Cambrian explosion. Indeed, there is the possibility that some of the Ediacaran beasties were chordates without hard notochords/backbones (see a previous web lecture about these curious beasties). The first true vertebrates were the fish and as stated in an earlier web lecture, these animals first appeared in the Cambrian. They exploded in terms of diversity in the Devonian (5 distinct fish classes developed). This is the point where today’s lecture will begin. Basic vertebrate evolution
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GY 112 lecture notes D. Haywick (2006) 2 The first appearance of the amphibians was a major evolutionary event that occurred in the Devonian. Why this occurred is really simple to envision. Terrestrial environments were becoming increasing attractive places to live. There were plenty of yummy plants and insects, and water was abundant all across the continents. Don’t forget that the Devonian, Mississippian and especially the Pennsylvanian periods were swamp-rich intervals of time. There were no animals on land at this time. Picture yourself a Devonian-aged fish living in a very hostile ocean -- there are evil BIG things all trying to eat you before you eat them. The land was largely free of anything that would eat you, but there was plenty of food for you to eat, that is if you could handle plants and insects. The reason that amphibians arose in the Devonian is because the environment had changed enough to promote evolution, or if you prefer a bad cliché from a semi-bad movie, “if you build it, they will come.” How the amphibians first evolved is a bit more speculative, but unlike some of the evolutionary steps that we have previously considered in this class (e.g., the arise of the ediacarans, or the Cambrian explosion) there is a lot of evidence suggesting that amphibians evolved from one group of the fish, specifically species assigned to the class Osteichthyes . As I’m sure everyone remembers (ha!), the Osteichthyes are the bony fish and dominate today’s oceans and freshwater bodies. These fish are sub-classified according to a lot of morphological criteria, including the nature of the fins. Some boney
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112lect35 - GY 112 lecture notes D. Haywick (2006) 1 GY 112...

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