Origin and Diversification of the Vertebrates

Who are the living vertebrates?

- Jawless fish: hagfish and lamprey - Fish with jaws & cartilage skeletons: sharks and rays - Fish with jaws & bony skeletons: all other fish (tuna, flounder, bass, etc.) - Amphibians: frogs and salamanders: cold-blooded, lay eggs in water - Reptiles: turtles, snakes and lizards: cold-blooded, lay eggs on land - Birds: warm-blooded, feathers, lay eggs - Mammals: warm-blooded, hair, eggs & live birth, nurse young.

Vertebrate Characteristics

- Backbone- Spinal cord- Heads- Tails- Heart- "Gill" Slits- Segmented muscle on body wall

- Internal skeletons of hydroxyapatite:- Jaws- Two pairs of appendages

Origin and Diversification of Vertebrates

EchinodermsUnique characters: 5-fold symmetry in adults, water-vascular system, a uniquely constructed calcite skeleton. Shared novelties: Embryonic traits (Radial pattern of embryonic cleavage, Deuterostome, Mesoderm formed by pouching); Skin-based nerve network; Bilateral, cilia-covered larvae.Range: Cambrian - Recent

Hemichordata (Acorn Worms)

Unique characters: Acorn worms are large(up to 2 m), burrowing worm- like filter-feeders with a long muscular proboscis and a fleshy collar.

Shared novelties: Adults are bilaterally symmetric; Closed circulatory system; Paired openings in the throat.

Range: Cambrian - Recent

Urochordata (Sea squirts)

Shared novelties: Notochord; Hollow nerve cord along back; Tail; Endostyle, an organ used for filter-feeding.

Range: No fossil record

Unique characters: Tunicates are small, box-like filter-feeding animals that live either alone or in colonies cemented to the sea floor.

Cephalochordata (Lancelates)Unique characters: Branchiostoma, also known as the lancelet, is a small, free- living fish-like animal that lives among sand grains and filter feeds. Shared novelties: Segmented muscle on upper body wall.

Range: Cambrian - Recent

Cephalochordates of the Burgess Shale - Pikaia

Unique characters: Branchiostoma, also known as the lancelet, is a small, free- living fish-like animal that lives among sand grains and filter feeds. Shared novelties: Segmented muscles on upper body wall.Range: Cambrian (Pikaia from the Burgess Shale) - Recent

The Importance of Swimming

The notochord, a stiff rod of connective tissue, provides internal support that permits efficient side-to-side motion for swimming. When muscles contract, the organism bends, rather than compressing like an accordian. The tail and hollow nerve cord, coupled with a closed circulatory system, are all probably related to this more active swimming life style.

Craniates: Class Myxini (Hagfish)

The most primitive known "vertebrate"

Unique characters: Scavengers and carnivores that actively feed by rasping at prey with a bony tongue. They tie themselves into a knot to lever a chunk out of prey. They can coat themselves with mucous for defense. They contain no vertebrae and no bone. Shared novelties: A head (cartilage brain case); Sense organs on the head (weak eyes); A true heart; True gills for efficient oxygen retrieval from water; Cartilage gill supports to hold up these flimsy sheets.Range: Carboniferous-Recent

Craniates: Heterostracans: The first truly abundant fishes

Unique characters: Jawless, armored body with scales on the tail. The tail was the main source of propulsion. Bottom feeding hunters and detritus feeders. Still no vertebrae.

Shared novelties: Improved sense organs (better balance & vision, lateral line system for motion detection and probably electroreception (used to hunt); Bone on the outer skull but not on the braincase. Range: Cambrian - Devonian

Why Bones of Calcium Phosphate? - Less soluble than calcite, not as subject to dissolution by metabolic acids.- Store of an important nutrient, phosphate.- May serve as insulation for electroreceptors.- Protection.

The First True Vertebrates (no jaws): Lampreys

Unique characters: The adult is a parasitic bloodsucker. It is jawless, but its mouth has many hooks for latching onto prey, then they use the tongue to bore through the side of the prey. No bone on the body - an evolutionary reversal. Shared novelties: Vertebrae surrounding notochord (made of cartilage); Dorsal and anal fins; Endostyle turns into thyroid.Range: Carboniferous - Recent

Osteostracans: (extinct armored jawless fishesUnique characters: Similar to heterostracans, with a bony head shield, scales on the tail, propulsion from the tail, well developed sense organs, and elaborate plumbing for gill system. They were active swimmers. Many were bottom feeders.

Shared novelties: Paired pectoral fins (source of forelimbs); Braincase covered in bone.

Range: Silurian - Devonian

Invertebrate-Vertebrate Evolutionary Transition

Overview - Recent studies in vertebrate evolution (e.g., based on studies in developmental genetics and paleontology) suggest that evolution of the vertebrate brain may have had a surprisingly early start in invertebrate ancestors, long before the mineralized skeleton that makes vertebrates so distinctive- The true innovation that launched the lineage of fish and other vertebrates seems to have been new kinds of embryonic tissue, which could form new sensory organs- This allowed protovertebrates, such as Haikouella and Myllokunmingia, to embark on a new way to make a living- as predators

Craniates that may be important transitional fossils: Haikouella and Myllokunmingia

Amphioxus- One way to track vertebrates’ evolutionary history is to analyze their closest living relativeMolecular and anatomical research both indicate that this is Amphioxus

- Paleontologists have long suspected that vertebrate diverged from a lancelet-like relative sometime in the Cambrian period - Meanwhile, molecular studies of gene similarities between lancelets and today's vertebrates suggest that the vertebrate lineage goes all the way back to 750 million years ago!

Brain and Bone- Until very recently, the earliest undisputed vertebrates were a mere 475 million years old.- These small, jawless fish (heterostracans?) with bodies completely covered in bony plates of armor are thought to have fed on sea-floor invertebrates and to have used their armor to defend against predators. - Fossils retaining the imprint of the brain reveal that these fish had already evolved many of the major features of modern vertebrate brains, such as divisions into forebrain, midbrain, and hindbrain.- If these armored fishes represent the earliest vertebrates, they suggest that brains and bone evolved together. - But, with no obvious intermediates among either ancient or living creatures, biologists were hard put to explain the origins of the vertebrate skeleton and nervous system. - In 1983, Northcutt and Gans argued that the key to vertebrate evolution was the invention of a head, which in turn was made possible by the evolution of a new kind of embryonic cell.

- Various regions of the three germ layers develop into the rudiments of organs during the process of organogenesis- A number of kinds of morphogenetic changes, including folding, splitting, condensation (clustering) of cells etc. occur within the layered embryonic tissues and represent the first evidence of organ building

An Overview of Early Development of the Vertebrate Embryo

Gastrulation in a Vertebrate Embryo

Neural Tube Formation in a Vertebrate Embryo

- The organs that first begin to take shape in the embryos of vertebrates are the neural tube and notochord- The notochord is formed from condensation of the dorsal mesoderm just above the archenteron, and the neural tube originates as a plate of dorsal ectoderm just above the developing notochord- The neural plate soon undergoes folding, actually rolling itself into the neural tube, which will eventually become the CNS

- Unique to vertebrate embryos, a band of cells called the neural crest cells developsalong the border where the neural tube pinches off from the ectoderm- Cells of the neural crest later migrate to various parts of the embryo, forming pigment cells of the skin, some of the bones and muscles of the skull, etc.

Early Embyogenesis cont.

- Neural crest supposedly gave vertebrates the flexibility to build a new kind of body, one that included the complex sense organs, big brains, and powerful pumping throats seen for the first time in lampreys and fossil jawless fish. - Along with the new body plan came an ecological shift, as vertebrates evolved from small, passive filter feeders to large, active predators that darted about hunting their prey. - In short, developmental changes produced new structures and presented the opportunity to the animal to start doing something else- This developmental revolution may have also sparked the origin of bone. - Neural crest cells build the electroreceptors that line the bodies of fish; once these receptors evolved, the researchers theorized, neural crest started building mineralized bone around them to insulate them from the rest of the body. - Later, the bone spread out to form a protective coat of armor, as seen in the early bony fish.

Supportng Research Involving Lancelets

- Lancelets don't have a true neural crest, but they do have cells in the same position as neural crest cells, and they express some of the same genes that neural crest cells express before they begin to migrate. - These cells also migrate, but only as a sheet moving on the surface of an embryo, not as small clusters traveling inside it; they haven't managed to break loose and wander- These observations suggest that one innovation of vertebrates was the wandering neural crest; it opened up the potential to get a potentially complex vertebrate head."- Interestingly, the swollen bud on the front end of the lancelet nerve cord bears a striking similarity to the vertebrate brain. - Thus, the same genes that organize major regions of the forebrain, midbrain, and hindbrain of vertebrates express themselves in a corresponding pattern in this small cluster of cells in the lancelet's nerve cord.

Slicing the Lancelet Brain

- A detailed examination of the neuroanatomy of lancelets by Lacalli indicates that the nervous nerve cord is divided like a vertebrate brain. - In the regions of the lancelet nerve cord where forebrain and midbrain genes are being expressed, the neuronal structure matches that of the vertebrate forebrain and midbrain. -Lacalli claims that clusters of neurons in the lancelet brain seem to perform the same functions as their vertebrate counterparts--even though in the lancelet these clusters may be made up of only a handful of neurons.

Lancelet Brain cont.

- Lacalli alos claims that lancelets have a rudimentary limbic system – a cluster of nerve cells in the lower part of the brain that interact with the cerebral cortex- He has found lancelet neurons whose structure and organization resemble those of vertebrate limbic neurons and that are located in the corresponding parts of the midbrain and forebrain. - He suggests that the common ancestor of vertebrates and lancelets used its protolimbic system to switch between its handful of behaviors, such as swimming and feeding.

- The very beginning of the limbic system is to be found in Lacalli's] work; he shows that there is the precursor of the hypothalamus, a crucial part of the limbic system.

The Advent of Predators

- Although there are tremendous similarities between lancelets and vertebrates, many anatomists believe that the head of vertebrates represents a huge evolutionary and developmental step. - And it’s the differences in vertebrate and lancelet brains that become critical for understanding vertebrate evolution- For example, lancelets apparently have no sense of smell- One of the parts of the vertebrate brain that's missing from the lancelet nerve cord is the telencephalon- Gans and Northcutt's suggest that early vertebrates shifted from filter-feeding to predation, and a key innovation in this process might well have been the beginning of a nose. - A lancelet doesn't need to sniff out its prey, but as the early vertebrates became predators, smell became an asset- They would also benefit from eyes to see prey and sophisticated control of their bodies to chase prey down.

Evidence of the Filter Feeding-Predatory Shift- The discovery of Haikouella - In some ways these fossils look like lancelets, but they also have a few key vertebrate traits unnecessary for filter feeders, such as eyes and muscle blocks. - These clues suggest that Haikouella is poised at the transition from invertebrate to vertebrate, closer to vertebrates than even the lancelet.- That makes another feature of their anatomy significant: The fossil nerve cord has an even larger swelling than does that of the lancelet; it appears that they do have a brain.

- If so, this fossil discovery pushes the origin of a vertebrate-like brain back to more than 530 million years ago.

The Evolution of Bone

- In a recent paper in Biological Reviews of the Cambridge Philosophical Society, Donoghue, Forey, and Aldridge create a new evolutionary tree for vertebrates that for the first time incorporates a mysterious group of animals called conodonts. - These creatures left behind vast numbers of enigmatic little fossils in the shapes of cones and thorns, ranging in age from 510 million to 220 million years old. - Over the years "conodonts have been attributed to almost every major phylum you can think ofFinally in the 1980s new fossils began to emerge with the conodont elements lodged in soft tissue.- Now researchers envision conodonts as eel-shaped predators with a pair of giant eyes and a gaping mouth filled with the tooth-like, bony conodont elements, which are made of dentine and other ingredients of the vertebrate skeleton.

Conodonts

Conodonts are the enigmatic, microscopic and phosphatic remains of a group of primitive chordates. They are mainly tooth-like in shape and functioned as a food-gathering apparatus. They are extinct, having ranged from the Cambrian through the Triassic Periods of the Paleozoic Era.

Conodonts cont.

- This new information seemed to elevate conodonts to the status of chordate predators, but paleontologists have fought over exactly what sort of chordate they might be. - Donoghue et al. tried to resolve the debate with a massive study of both fossil and living creatures, analyzing 103 different traits in 17 different groups of chordates, ranging from lancelets to jawed vertebrates.- Their results show that after the vertebrate lineage split from lancelets, the first group to branch away were the hagfish; lampreys are only slightly less primitive. - Conodonts, surprisingly, turn out vertebrates, even closer to living jawed fish than to lampreys or hagfish.

Re-examining Vertebrate Phylogeny

Conodonts cont.

- Only after the rise of conodonts did the armored jawless fish, the ostracoderms, appear, and from one of their ranks, the jawed fish eventually evolved.- According to the new phylogeny, hagfish and lampreys offer a good representation of what the most ancient vertebrates were like: unarmored and without mineralized skeletons. - And conodonts represent the first appearance of a mineralized skeleton. - The conodont skeleton is believed to be the primitive vertebrate skeleton - Mineralization began not in the skin of fish but in the mouths of conodonts, and it presumably made them fiercer predators.

Invertebrate-Vertebrate Transition

- There are now fossil fish known from the Cambrian that are more advanced than hagfish, so it must have happened during the Cambrian explosion. - The transition appears to have happened in the ocean - all invertebrate next of kin, all non-vertebrate chordates, and the most primitive living vertebrates all come from the ocean. - Jawless, armored fish appeared in the Cambrian, were present but not diverse in the Ordovician, and flourished in the Silurian and Devonian. - However, other fishes soon began to appear

Placoderms: the most primitive jawed fish

Unique characters: Heavy armor on head and front of the trunk, scales on the tail, no teeth, just plates of bone for shearing food, heavy fish (probably slow swimmers). Shared novelties: Jaws; Paired pelvic fins (source of hindlimbs); Paired nasal openings.Range: Silurian - Carboniferous

Acanthodians: earliest known jawed fishes

Unique characters: Their fins were supported by erectable spines. Some filter fed, others had teeth. Highly manueverable swimmers propelled by their tails. Shared novelties: Teeth; Advanced jaw joint.

Range: Silurian - Permian

Chondrichthyes: Cartilaginous fish

Fossil rhinobatoid (guitarfish -- one of the earliest rays)

Unique characters: No bone except in their scales (an evolutionary reversal). Fin and tail structures suggest an active, highly efficient swimming for a predatory life style. Sharks give birth to live young.  This requires internal fertilization of eggs.  They link up when breeding by using claspers. Shared novelties: Regular pattern of tooth replacementRange: Silurian - Recent

The Origin of Jaws and Teeth Jawless fishes have gills and these flimsy sheets of tissue are supported by gill arches. Many fish actively pump water past the gills using muscles acting on the gill arches. One hypothesis for the origin of jaws is that the pair of gill arches furthest in front were jointed and actively involved in pumping water. These jointed arches developed a pincher motion that was eventually used for holding prey. Support: the front gill arch in living sharks supports the jaws and attaches them to skull.

Osteichthyes: Vertebrates with skeletons made entirely of bone

Range: Silurian - Recent Shared novelties: Skeleton completely composed of bone, including skull, vertebral column, fins, and ribs; Swim bladder for buoyancy control.

Osteichthyes (Bony fish): Ray-finned fish

Shared Novelty uniting bony fish: Fins made of bony spines connected by poorly muscled webs.

Range: Silurian - Recent

Osteichthyes (Bony fish): Lobe-finned fish

Unique Characters: Torpeodo shaped body with heavy scales, unusual bone with many pores, perhaps for electroreceptive cells. Shared novelties: Paired pectoral and pelvic fins that are fleshy and muscular; Peculiar convoluted dentin and enamel.Range: Devonian - Recent

Overview of Vertebrate Phylogeny