Chapter 13 – Paleozoic Vertebrates & Plants From the Sea to the Land, the development of Vertebrates, Insects, and Plants. First evidence of Chordates, organisms with a notochord, a dorsal hollow nerve cord,and gill slits – Early Cambrian - Found in 525 m.y. old rocks in Yunnan province, China “Chengjiang Fauna”. (See slide 3). Also Pikaia – Burgess Shale Fauna – British Columbia. Earliest fish remains are found in Late Cambrian rocks. All known Cambrian and Ordovician fossil fish have been found in shallow nearshore marine deposits.

Yunnanozoon lividum is one of the oldest known chordates Found in 525 Myr old rocks in Yunnan province, China 5 cm (2 inches) – long

A fragment of a plate from Anatolepis (A primitive member of the class Agnatha (jawless fish)), one of the oldest known fish from the Late Cambrian Deadwood Fm. of Wyoming.

The oldest fossil remains of nonmarine fish are from the Silurian Period, evidence that fish originated in the oceans. The oldest and most primitive of the class Agnatha are the ostracoderms (“bony skin”). These are armored jawless fish that first evolved during the Late Cambrian reached their zenith during the Silurian and Devonian and then became extinct. The majority of ostracoderms lived on the seafloor, probably browsing through the bottom sediment for small bits of food.

Geologic Ranges of Major Fish Groups

an ostracoderm (Hemicyclaspis) –a placoderm (Bothriolepis) an acanthodian (Parexus) a ray-finned fish (Cheirolepis) The Devonian Period - the “Age of the Fish”, all major fish groups present during this time.

The evolution of jaws was a major evolutionary advantage among primitive vertebrates. Jawless ancestors could only feed on detritus, jawed fish could chew food and become active predators opening new ecological niches. The vertebrate jaw is an excellent example of evolutionary opportunism Movable jaw– evolution of the first three gill arches. Movable jaw may have likely initially provided for more oxygen movement through gills.

First jawed fish Early Silurian rocks belong to the acanthodians, a group of enigmatic fish charac- terized by large spines, scales covering much of the body, jaws, teeth, and reduced body armor. May have included ancestors of bony and cartilaginous fish groups. The acanthodians were most abundant during the Devonian. Late Silurian jawed fish also included the placoderms (armored, “plate-skinned”) present in both fresh water and marine sediments. Example: Late Devonian Dunkleosteus, 12 meters long, armored head, shoulders, jaw with razor sharp teeth, flexible tail – largest known fish of its time.

Class Chrondrichthyes – cartilaginous fish (sharks, rays, skates) & Class Osteichthyes – bony fishes (bass, trout, etc.) appeared during the Middle Devonian. Bony fishes include “ray- finned” (above) and “lobe-finned” (below). Muscles of lobe-fined extend into fins = more flexibility. Lobe-fined fish include lungfish and crossopterygians. Modern lungfish are found in S. America, Africa, and Australia

Development of lungs allowed fish to burrow into mud and survive seasonal and drought period drying of ponds and lakes = greater chances of survival. Crossopterygians that remained in fresh water likely ancestors to amphibians. Stubby lobes evolved into stubby legs for navigation of shallow, debris-clogged waterways. Initially, legs may not have been strong enough to support animals on land.

Diagram showing similarities between Crossop- terygians (below) and an early Amphibian (above).

Earliest tetrapod (four- footed) trackway – 365 m.y. (Devonian) – Ireland. Likely in shallow stream- bed (or lake), no sign of tail being dragged. No bones are preserved of this creature. When these creatures arrived on land, what was there to eat?

Oldest evidence of land plants during the Ordovician Period. Insects, millipedes, spiders, & even snails invaded the land before amphibians. Scorpions and flightless insects present in Devonian fossil record. Leaving the water = amphibians did not have to compete with fish for food. With no land competition = adaptive radiation - became common during Carboniferous and Early Permian. Likely remained close to water, in swampy areas. One common group – labyrinthodonts (up to 2 meters in length) survived changing climate into Early Triassic Period.

Amphibians remained close to water to prevent dessication of skin and their gelatinous eggs. The development of the amniotic, shelled egg in reptiles allowed them to move away from water. Amphibians and reptiles differ in skull structure, jawbones, ear location, and limb and vertebral construction. Fossil evidence suggests reptiles evolved from labyrinthodont ancestors by the Late Mississippian. The oldest known reptile, Westlothiana, - Late Mississippian-age rocks in Scotland. Early Pennsylvanian reptiles found in Nova Scotia. (see slide 15).

Reconstruction and skeleton of Hylonomus lyelli from the Pennsylvanian Period –Fossils of this animal have been collected from sediments that filled tree stumps –Hylonomus lyelli was about 30 cm long Besides reproduction, other reptile advantages = better jaws, teeth, and faster mobility.

Suggested development of Reptiles during Late Paleozoic/Early Mesozoic. Dominant reptile family group – Early Permian. Descendants included carnivores and herbivores. Reptiles with some mammal features. 90% of reptile genera by end of Paleozoic.

Dicynodon Moschops Therapsids may have been endothermic and may have had a covering of fur. Late Permian reconstruction – southern Africa.

Other “mammal” features of Theraspids include: Fewer bones in the skull due to fusion of many of the small skull bones. Enlargement of the lower jawbone & differ- entiation of the teeth for various functions such as nipping, tearing, and chewing food. And a more vertical position of the legs for greater flexibility as opposed to the sideways sprawling legs in primitive reptiles. Endothermism (“warm-blooded”) = greater climate and seasonal flexibility.

Plant evolution – plants faced similar problems of vertebrates when leaving the water. Dessication – Support – Effects of Gravity Earliest evidence of land plants – Middle to Late Ordovician spores & other debris. Major evolution took place during Devonian Period. Higher land plants – Non-vascular and Vascular. Nonvascular – bryophytes, e.g., liverworts, mosses, fungi Vascular – have tissue system of specialized cells for the delivery of water and nutrients. Appearance likely before Middle Silurian.

A likely algae – vascular link - Primitive seedless vascular plants such as ferns: –resemble green algae in their pigmentation, –important metabolic enzymes, –and type of reproductive cycle. Vascular tissue provide support for the plant. Development of cutin, an organic compound in plant tissues offers resistance to UV radiation, oxidation, and entry of parasites. Roots developed to bring soil water and nutrients to the plant. Leaves developed to provide more surface area for photosynthesis. Middle Silurian Cooksonia – Wales, Ireland, oldest known vascular plants.

Cooksonia features: upright, branched stems, a resistant cuticle, spores typical of vascular plants These plants probably lived in moist environments such as mud flats. This specimen is 1.49 cm long. No true roots, rhizome (buried part of stem) delivered nutrients.

During the Paleozoic, seedless vascular plants evolved many of the major structural features characteristic of modern plants such as leaves, roots, and secondary growth. Evolution at different times = mosaic evolution. Plant adaptive radiation took place in Late Silurian – Early Devonian. Devonian Period (408 to 360 m.y.) Early Devonian landscape was dominated by relatively small, low-growing, bog-dwelling types of plants. By Late Devonian there were forests of large tree-size plants up to 10 m tall. Also during Late Devonian, first seeds appeared, allowed plants to move away from areas of standing water.

Reconstruction of Early Devonian landscape. Dawsonites / Protolepidodendron \ - Bucheria –showing some of the earliest land plants

Non-flowering gymnosperm plants have male & female cones, pollination causes female cone to develop with seeds. Example: Pine trees. The step before the development of the gymno- sperms was the Early Devonian development of heterospory plants – two diff. sizes of spores. Larger spore – female-gamete bearing plant. Smaller spore – male-gamete bearing plant. Seedless vascular plants dominated coal swamps. Became highly diversified during Pennsylvanian Period (slide 25). Gymnosperms colonized non-swampy uplands.

Reconstruction of Pennsylvanian Coal Swamp

Important Pennsylvanian seedless vascular plant major types - lycopsids and sphenopsids. Lycopsids - Lepidodendron and Sigillaria – up to 30 meters tall. Similar to today’s palm trees. Sphenopsids – jointed trunks, root systems with stems – 6 meters tall. Horsetails, scouring rushes modern examples. Other trees - cordaites, a group of tall gymnosperm trees that grew up to 50 m and probably formed vast forests. Glossopteris - temperate, non- swamp dwelling plant with fossils present on modern day southern hemisphere continents. Gymnosperms survived Late Permian declines, spread during Early Mesozoic.