Before we can study the history of life, we need to know some basic geological concepts and vocabulary.

Transcription

1 Biology Major Features of Vertebrate Evolution Dr. Robert Reisz, University of Toronto Introduction: Background in Geology Before we can study the history of life, we need to know some basic geological concepts and vocabulary. Catastrophism: The theory that profound geological changes in the past result from geological processes that were more rapid than those that occur today. Such processes were usually thought of as catastrophes. This was the first geological theory invented to explain the presence of large mountain chains and the succession of fossils in the rocks. Under this theory, life was thought to have been created and destroyed several times in the history of the earth. The extinctions and creations explained the presence of different fossils in different strata. Uniformitarianism: The theory that profound geological changes in the past result from slow but continuous processes. This means that we don t have to invoke major catastrophes in the past to explain the present distribution of rocks, mountain ranges, and continents. The succession of fossils can be explained by the evolution of life through time. Uniformitarianism is a more recent theory and is accepted by almost all scientists. Superposition: Layers of sand and mud settle and are compressed into sedimentary layers called strata. Therefore, the oldest strata are at the bottom, and the most recent ones are on top of a geological section. Superposition can be used to determine the relative age of fossils found in rocks. Absolute dating: Methods that allow to known the age of rocks in years (absolute age) rather than just telling us which rocks formed first. Absolute dating is based on the decay of radioactive isotopes. A good example is the K-Ar method. A radioactive isotope of K decays and becomes Ar. Half of the radioactive K nuclei decay in 14 billion years. The length of time required for half of the nuclei of a certain isotope to decay is called its half-life. After two half-lives (28 billion years in this case), 3/4 of the nuclei have decayed. Various radioactive isotopes have different half-lives, so by using all of them, we can date rocks of almost any age, provided that these rocks contain a sufficient amount of the right radioactive isotopes. Of course, there is always an error factor of about 3.5% (at best).

2 This provides an approximate time scale but it is not useful to determine the precise age of organisms. It is not good enough to know which Paleozoic strata are the oldest if they are separated only by a few million years (you will known what the Paleozoic is very soon). An other disadvantage of this method is that it can usually be done only with lava beds (K-Ar); the sediments themselves cannot be dated. This is a serious constraint, because ancient lava beds are rarely found between fossiliferous sedimentary rock layers. Relative dating: This is the most commonly used method of dating rocks because radioactive isotopes are relatively rare in rocks and the uncertainty of absolute dating methods does not allow to order very old rocks. Relative dating uses fossils of wide distribution and short existence and the principle of superposition to determine the order in which different rock layers formed. The strata containing the oldest fossils are the oldest (biostratigraphy). The strata found below are the oldest (superposition). Stratigraphy: The study of rock layers and their relative age. Biostratigraphy: The study of the age of rocks by their fossils. Fossil: Any trace of life that existed in the past (at least from the Pleistocene). This includes of course bones, shells and invertebrate exoskeletons, but it also include less common traces of ancient life such as wood, leaves, skin impressions, internal organs (Burgess shales), footprints, and burrows. Types of rocks: Igneous: It forms from solidified magma or lava. It usually contains no fossils. Sedimentary: It forms by the disintegration of other rocks. Particles of various sizes are packed together, usually at the bottom of oceans, seas, rivers and lakes and solidify over long periods of time. There are several types of sedimentary rocks. They are usually classified by the size of the particles of which the rock is made.

3 Table 1.1. Types of sedimentary rocks. Type of sedimentary rock Mean diameter of particles Conglomerate At least 2 mm Sandstone 1/16 mm to 2 mm Siltstone 1/256 mm to 1/16 Shale Less than 1/256 mm Sedimentary rocks are often fossiliferous. The fine-grained ones yield usually well preserved fossils with a lot of information. Very small structures can be seen. Some soft structures can be preserved in shales (ex. Burgess shales in British Columbia). Sedimentary rocks made of coarse particles yield poorly preserved fossils, with a lower resolution (only large structures can be seen). Metamorphic: It forms by the alteration of rocks (which may be igneous, sedimentary or metamorphic) by pressure and heat. It usually contains no fossils. The few fossils that may be found in them are usually poorly preserved. Geological timescale and the evolution of life: We will now study briefly the geological periods and the prevailing types of vertebrates that lived in the last 500 My. Give yourself two pages and start at the bottom of the second page, because the first period is usually placed at the bottom of geological tables, to follow the relative position of rocks (the oldest ones are at the bottom). Create four columns, three narrow ones (Era, Period, Beginning in million years) and a wide one (prevailing vertebrates). Several fo the names that I will mention will be unfamiliar to you, but you will soon know them. Table 1.2. Geological timescale and dominant vertebrates. Era Period Beginning in Prevailing Vertebrates million years Cenozoic Quaternary 2 Man s ancestors (Australopithecus and Homo erectus) and most modern genera are present. Tertiary 65 Mostly mammals, birds, lizards, snakes, crocodiles, turtles, frogs, urodeles, apodans, teleosts, sharks, rays. Mesozoic Cretaceous 130 Dinosaurs, birds, small, nocturnal mammals, teleosts, sharks, rays. Jurassic 205 Early dinosaurs, first birds, first

4 Paleozoic Precambrian (Proterozoic) small, nocturnal mammals, first salamanders and first apodans, teleosts, sharks, first rays. Triassic 250 Early archosaurs, first dinosaurs, therapsids, first relative of frogs, first teleosts, sharks. Permian 300 Earliest archosaurs, therapsids, labyrinthodonts and lepospondyls, early ray-finned fishes, rhipidistians, lungfishes, sharks, last acanthodians. Pennsylvanian (sometimes included with the Mississippian in the Carboniferous) Mississippian (sometimes included with the Pennsylvanian in the Carboniferous) 330 Earliest amniotes, labyrinthodonts and lepospondyls, crossopterygians, lungfishes, acanthodians, sharks. 370 Labyrinthodonts and lepospondyls (early amphibians), crossopterygians, lungfishes, early actinopterygians, acanthodians, sharks, last placoderms. Devonian 420 First amphibians, crossopterygians, lungfishes, earliest certain actinopterygians, acanthodians, sharks, placoderms, last ostracoderms. Silurian 455 Isolated actinopterygian scales, first acanthodians, first placoderms (all gnathostomes) and ostracoderms. Ordovician 510 Early ostracoderms (jawless fishes). Cambrian 560 Isolated scales indicating potential vertebrates Unicellular animals, algae, bacteria.

5 Notes: There is now a tendency to replace the division of the Cenozoic into Tertiary and Quaternary by a division into Paleogene and Neogene. The Paleogene starts 65 My ago and ends about 24 My ago. The Paleogene includes the Paleocene, Eocene and Oligocene, while the Neogene includes the Miocene, Pliocene, Pleistocene and Holocene. This new division of the Cenozoic has the advantage of having two subdivisions of equivalent periods of time, whereas the older subdivisions did not. The Tertiary lasted about 63 My while the Quaternary lasted only about 2 My. Some people recognize a period of the Paleozoic before the Cambrian. This period, called the Ediacarian, is known from only a few localities, and no vertebrates have been found in them. Some people prefer to consider the Ediacarian as part of the Precambrian or Proterozoic.