Chapter 13 Paleozoic Life History Vertebrates and Plants Tetrapod Footprint Discovery Tetrapod trackway at Valentia Island Ireland These fossilized fooprints which are more than 365 million years old are evidence of one of the earliest four legged animals on land Photo courtesy of Ken Higgs U College Cork Ireland Tetrapod Footprint Discovery The discovery in 1992 of fossilized Devonian tetrapod footprints more than 365 million years old has forced paleontologists to rethink how and when animals emerged onto land The newly discovered trackway has helped shed light on the early evolution of tetrapods the name is from the Greek tetra meaning four and podos meaning foot Based on the footprints it is estimated that the creature was longer than 3 ft and had fairly large back legs Tetrapod Wader Furthermore instead of walking on dry land this animal was probably walking or wading around in a shallow tropical stream filled with aquatic vegetation and predatory fish This hypothesis is based on the fact that the trackway showed no evidence of a tail being dragged behind it Unfortunately there are no bones associated with the tracks to help in reconstructing what this primitive tetrapod looked like Why Limbs One of the intriguing questions paleontologists ask is why did limbs evolve in the first place It probably wasn t for walking on land In fact many scientists think aquatic limbs made it easier to move around in streams lakes or swamps that were choked with water plants or other debris The scant fossil evidence also seems to support this hypothesis Unable to Walk on Land Fossils of Acanthostega a tetrapod found in 360 million year old rocks from Greenland reveals an animal with limbs but one clearly unable to walk on land Paleontologist Jenny Clack who recovered hundreds of specimens of Acanthostega points out that Acanthostega s limbs were not strong enough to support its weight on land and its ribcage was too small for the necessary muscles needed to hold its body off the ground Acanthostega Had Gills and Lungs In addition Acanthostega had gills and lungs meaning it could survive on land but was more suited for the water Clack believes that Acanthostega used its limbs to maneuver around in swampy plant filled waters where swimming would be difficult and limbs would be an advantage Unanswered Questions Fragmentary fossils from other tetrapods living at about the same time as Acanthostega suggest that some of these early tetrapods may have spent more time on dry land than in the water At this time there are many more unanswered questions about the evolution of the earliest tetrapods than there are answers However this is what makes the study of prehistoric life so interesting and exciting Vertebrates and Plants Previously we examined the Paleozoic history of invertebrates beginning with the acquisition of hard parts and concluding with the massive Permian extinctions that claimed about 90 of all invertebrates and more than 65 of all amphibians and reptiles In this section we examine the Paleozoic evolutionary history of vertebrates and plants Transition from Water to Land One of the striking parallels between plants and animals is the fact that in passing from water to land both plants and animals had to solve the same basic problems For both groups the method of reproduction was the major barrier to expansion into the various terrestrial environments With the evolution of the seed in plants and the amniote egg in animals this limitation was removed and both groups were able to expand into all the terrestrial habitats Vertebrate Evolution A chordate Phylum Chordata is an animal that has at least during part of its life cycle a notochord a dorsal hollow nerve cord and gill slits Vertebrates which are animals with backbones are simply a subphylum of chordates Characteristics of Chordates The structure of the lancelet Amphioxus illustrates the three characteristics of a chordate a notochord a dorsal hollow nerve cord and gill slits Phylum Chordata The ancestors and early members of the phylum Chordata were soft bodied organisms that left few fossils so little is known of the early evolutionary history of the chordates or vertebrates Surprisingly a close relationship exists between echinoderms and chordates They may even have shared a common ancestor because the development of the embryo is the same in both groups and differs completely from other invertebrates A Very Old Chordate Yunnanozoon lividum is one of the oldest known chordates Found in 525 Myr old rocks in Yunnan province China 5 cm long animal Spiral Versus Radial Cleavage Echinoderms and chordates have similar embryonic development In the arrangement of cells resulting from spiral cleavage a at the left cells in successive rows are nested between each other In the arrangement of cells resulting from radial cleavage b at the right cells in successive rows are directly above each other This arrangement exists in both chordates and echinoderms Echinoderms and Chordates Both echinoderms and chordates have similar biochemistry of muscle activity blood proteins and larval stages The evolutionary pathway to vertebrates thus appears to have taken place much earlier and more rapidly than many scientists have long thought Hypothesis for Chordate Origin Based on fossil evidence and recent advances in molecular biology vertebrates may have evolved shortly after an ancestral chordate acquired a second set of genes the ancestor probably resembled Yunnanozoon According to this hypothesis a random mutation produced a duplicate set of genes allowing the ancestral vertebrate animal to evolve entirely new body structures that proved to be evolutionarily advantageous Not all scientists accept this hypothesis and the evolution of vertebrates is still hotly debated Fish The most primitive vertebrates are fish and some of the oldest fish remains are found in Upper Cambrian rocks All known Cambrian and Ordovician fossil fish have been found in shallow nearshore marine deposits while the earliest nonmarine fish remains have been found in Silurian strata This does not prove that fish originated in the oceans but it does lend strong support to the idea Fragment of Primitive Fish A fragment of a plate from Anatolepis cf A Heintzi from the Upper Cambrian marine Deadwood Formation of Wyoming Anatolepis is one of the oldest known fish a primitive member of the class Agnatha jawless fish Ostracoderms Bony Skinned Fish As a group fish range from the Late