BIOL 108 Early Vertebrate Evolution PDF

Hierarchical Clades Topic 27: Clade Vertebrata Molecular data place Urochordata as sister taxon to the vertebrates (even though Cephalochordata look more like vertebrates) BIOL 108 Winter 2024 © 2024 Neil Harris Vertebrates Fig 34.2 Phylogeny of living chordates 1 Early chordate evolution BIOL 108 W...

Hierarchical Clades Topic 27: Clade Vertebrata Molecular data place Urochordata as sister taxon to the vertebrates (even though Cephalochordata look more like vertebrates) BIOL 108 Winter 2024 © 2024 Neil Harris Vertebrates Fig 34.2 Phylogeny of living chordates 1 Early chordate evolution BIOL 108 Winter 2024 © 2024 Neil Harris Ancestral chordates likely resembled lancelets, with characteristics of the chordate body plan, such as a notochord, dorsal nerve cord, pharyngeal slits, and a post-anal tail, − The expression of the same Hox genes involved in vertebrate brain development is observed during the development of the lancelet’s simple nerve cord tip. Fig 34.4 The lancelet Branchiostoma, a cephalochordate Genome sequencing suggests a whole-genome duplication occurred early in chordate evolution. − Duplicate genes evolve new functions, contributing to the development of traits and structures in chordates. − Diversification of gene families, e.g. Hox. Fig 34.6 Expression of developmental genes in lancelets and vertebrates 2 Clade Vertebrata BIOL 108 Winter 2024 © 2024 Neil Harris During the Cambrian period, a lineage of invertebrate chordates evolved into the first vertebrates. − Vertebrates are chordates that have a vertebral column (backbone), which provides structural support and protects the nerve cord. − The evolution of skeletal and complex nervous systems in vertebrates provided advantages in capturing food and avoiding predators, leading to their widespread success and diversification. Extant vertebrates inhabit a wide range of environments, including marine, freshwater, and terrestrial habitats. − ~69k extant vertebrate species, including some of the largest animals ever to have lived, such as sauropod dinosaurs and blue whales. 3 Vertebrate characteristics Shared, derived traits Vertebrate shared, derived traits enabled greater complexity that supported larger body size and more active lifestyles: 1. Vertebrates have a vertebral column (backbone). − Skeletal elements (cartilage or bone) enclose and protect the nerve cord. − The vertebral column replaces the function of the notochord.  Enhances rigidity and serves as attachment points for muscles and skeletal elements like ribs  Facilitates improved food capture and predator evasion. BIOL 108 Winter 2024 © 2024 Neil Harris 2. Elaborate braincase enclosing the brain. − Endoskeleton development was associated with pronounced cephalization. − The head consists of the brain, sense organs (such as paired eyes), and a cranium (skull) without jaws.  Evolution of the cranium allows for the anterior expansion of the nerve cord into a complex brain and nervous system. Enables coordination of more sophisticated movement and feeding behaviours.  The cranium does not completely encase the brain in early vertebrate lineages. 4 Vertebrate characteristics Shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris 3. Vertebrates have two or more sets of Hox genes, likely originating from whole-genome duplication. − Invertebrate chordates (lancelets and tunicates) have only one set of Hox genes. − Hox genes regulate the embryonic body plan along the head-tail axis, determining segment structures. 4. Neural crest cells develop along the neural tube edge in vertebrate embryos. − Neural crest cells migrate within the embryo, contributing to structures such as teeth, certain skull bones/cartilage, and diverse types of neurons (nerves). Fig 34.7 The neural crest, embryonic source of many unique vertebrate traits 5 Vertebrate characteristics Shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris 5. Dorsal, anal, and tail fins stiffened by fin rays along the centerline. − Fins are membrane extensions supported by cartilaginous or bony spines (rays) that provide balance and propulsion during swimming. − Tetrapods (terrestrial vertebrates) have lost fins. 6. Vertebrates possess a modified, complex circulatory system. Dorsal fin of a chub (Leuciscus cephalus) stiffened by fin rays (WC) − Closed circulatory systems, including a heart with at least two chambers.  Invertebrate chordates either have no heart (Cephalochordates) or one-chamber hearts (Urochordates). − O2-transporting hemoglobin in red blood cells is oxygenated via gills or lungs. − Kidneys remove metabolic waste products from the blood. − A complex circulatory system supports higher metabolic rates and more muscularity compared to lancelets and tunicates. 6 Basal vertebrates BIOL 108 Winter 2024 © 2024 Neil Harris Fossil evidence shows that the earliest vertebrates lacked jaws. − Today, only two lineages of jawless vertebrates remain, the hagfishes and lampreys.  Both hagfishes and lampreys lack jaws and a backbone in adulthood. The presence of rudimentary vertebrae in embryonic or mature forms, and molecular phylogenetic analyses confirm their vertebrate classification.  They belong to the clade Cyclostomes (“round mouths”) Cyclostomes − Hagfishes and lampreys are sister taxa in a clade of living jawless vertebrates, clade cyclostomes (“round mouths”).  Extant cyclostomes look rather like eels: long, flexible, tubular bodies, but no paired lateral fins. Vertebrates with jaws make up a much larger clade, the gnathostomes. Fig 34.2 Phylogeny of living chordates 7 Cyclostomes Hagfishes BIOL 108 Winter 2024 © 2024 Neil Harris Hagfishes (class Myxini) are jawless vertebrates that have a cartilaginous cranium, greatly reduced vertebrae, and a flexible rod of cartilage derived from the embryonic notochord. − Eel-shaped, slime-producing marine fish (20-30 extant species). − Hagfish have a small brain, eyes, and a mouth composed of tooth-like keratin projections that protract and retract to pull in food. Sixgill hagfish (Eptatretus hexatrema) (WC) Cartilage cranium Fibrous sheath surrounds brain Cartilaginous rod 8 Cyclostomes Hagfishes BIOL 108 Winter 2024 © 2024 Neil Harris Hagfishes are marine; most are bottom-dwelling scavengers. − Feed on decaying corpses of larger animals, using the rasping action of tooth-like projections to strip flesh from carrion. − Bury themselves in a corpse and absorb the carcass’s nutrients through their skin. Fig 34.8 A hagfish Hagfish are notable for exuding copious fibrous slime as a defense mechanism. − Slime absorbs water and rapidly swells to choke predators: https://youtu.be/lZq4Dme7wi4 9 Cyclostomes Lampreys BIOL 108 Winter 2024 © 2024 Neil Harris WC Lampreys (class Petromyzontida) have a cartilaginous skeleton. Fig 34.9 A sea lamprey − The lamprey’s cartilaginous skeleton includes a more elaborate cranium (without jaws), a gill basket, and rudimentary vertebral elements.  Adults have a notochord surrounded by cartilaginous segments that partially arch over the nerve cord forming primitive vertebral elements. − Lampreys have no paired lateral fins but do have dorsal and tail fins. − Larval lampreys exhibit lancelet-like behaviours (cephalochordates).  Suspension-feeders that bury themselves in stream substrates. nerve cord notochord cartilaginous tube segment with dorsal spines Lampreys have a more elaborate cartilaginous cranium, including a cartilaginous “branchial basket” surrounding the gills slits Lamprey larvae 10 Cyclostomes Lampreys BIOL 108 Winter 2024 © 2024 Neil Harris glfc.org Lampreys inhabit marine and freshwater habitats (~35 species). Many lamprey species parasitize fish by clamping onto them with a funnel-like sucking mouth. glfc.org  They scrape skin using a rasping tongue (keratin spines) and suck body fluids.  Lampreys can hold onto rocks using their oral sucker when not feeding (petro = rock, myzo = suck). − Sea lampreys have become a significant invasive parasite in the North American Great Lakes.  Great Lakes Fishery Commission http://www.glfc.org/sea-lamprey.php glfc.org WC Lake trout with lamprey parasites Salmon with an open wound caused by a sea lamprey 11 Phylogeny of living chordates BIOL 108 Winter 2024 © 2024 Neil Harris Vertebrates Cyclostomes Gnathostomes Fig 34.2 Phylogeny of living chordates 12 Early vertebrate evolution BIOL 108 Winter 2024 © 2024 Neil Harris Fossils from the Cambrian explosion document the transition between chordates and vertebrates. − The most primitive of these fossils, such as tiny (3 cm) Haikouella, had a well-formed brain, eyes, and segmented muscles, but not a cranium or vertebral column. − Other Cambrian chordates, such as Myllokunmingia, had parts of a cranium, but no vertebral column. Fig 34.10 Fossil of early Cambrian chordate (Haikouella) from Haikou district (China) Myllokunmingia reconstruction Fig 34.11 Fossil of chordate Myllokunmingia from the Cambrian 13 Early vertebrate evolution Conodonts were among the earliest vertebrates in the fossil record, dating from 200–500 mya. − Conodonts had cartilaginous cranium and vertebral column. − Jawless, but possessed mineralized skeletal elements in their mouth and pharynx.  These fossilized dental elements are common in the fossil record. Fig 34.12 A conodont BIOL 108 Winter 2024 © 2024 Neil Harris Other groups of jawless vertebrates were armoured with defensive plates made of dermal bone on their skin. − All early vertebrate groups, including conodonts and armoured jawless vertebrates, went extinct by the end of the Devonian period, following the evolution of jawed fish. Fig 34.13 Jawless armoured vertebrates 14 Clade Gnathostomes BIOL 108 Winter 2024 © 2024 Neil Harris Gnathostomes are a clade of vertebrates that have jaws. − gnath = jaw, stoma = mouth − ~99% of all living vertebrates are gnathostomes. − Gnathostomes include sharks and their relatives, ray-finned fishes, lobefinned fishes, amphibians, reptiles (incl. birds), and mammals. Cyclostomes Gnathostomes Fig 34.2 Phylogeny of living chordates 15 Clade Gnathostomes Shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris 1. Opposing jaws that open/close the mouth forcefully to capture and process diverse foods. − Gnathostome jaws are hypothesized to have evolved from skeletal supports of the pharyngeal slits.  Two pairs of branchial elements (hinged skeletal rods supporting the gill arches) evolved to open and close the mouth to more effectively pump water over the gills.  Natural selection likely favoured larger and wider mouths, enabling the capture of larger prey Figure 34.14 Possible step in the evolution of jawbones − Posterior branchial elements evolved as specialized supports for gas exchange (gill slits). 16 Clade Gnathostomes Shared, derived traits 2. Mineralization of skeleton. − Gnathostomes exhibit a mineralized skeleton, including mineralized teeth 3. Two pairs of lateral appendages, such as fins or legs. − Extinct jawless vertebrates had 0 or 1 pair of lateral fins (extant cyclostomes have no paired lateral fins). opb.org BIOL 108 Winter 2024 © 2024 Neil Harris Chinook salmon paired pectoral and pelvic fins 4. Genome duplication, including duplication of Hox genes. − Resulted in four sets of Hox genes. 5. Enlarged forebrain. − Gnathostomes have an enlarged forebrain, which is associated with enhanced smell and vision. 6. Lateral line system. − Aquatic gnathostomes possess a lateral line system that detects vibrations in the water. − The lateral line system is lost in terrestrial gnathostomes. Paired Paired 17 Origin of jaws and mineralization of the skeleton BIOL 108 Winter 2024 © 2024 Neil Harris The earliest vertebrates likely had cartilaginous skeletons. − Cartilage is a flexible tissue that provides support but lacks the rigidity of bone. Some vertebrate groups evolved mineralization of their skeletons (with calcium phosphate), replacing or augmenting cartilage with bone. − The evolution of bone provided advantages such as increased structural support, protection for internal organs, and the ability to support larger body sizes. − Opposing jaws and mineralization of the skeleton likely evolved simultaneously, with changes in jaw structure coinciding with the development of mineralized jaw elements.  Natural selection favoured mineralization in the jaw region, enhancing strength and durability, improving the effectiveness of jaws as predatory structures.  Conodonts were among the earliest vertebrates to evolve mineralized skeletal elements in their mouth and pharynx. − Mineralization of the axial skeleton, including the vertebral column and associated structures, occurred later, providing better support for the body and enhancing locomotion. Fig 34.12 A conodont 18 Bones vs. cartilage Two types of bones: 1. The endoskeleton is derived from preformed cartilage. − In gnathostomes, the cartilage undergoes ossification (becomes bony) with calcium phosphate as the embryo develops.  In humans, ossification is incomplete at birth and continues until ~25 years of age. − Cartilage is a “flexible connective tissue with an abundance of collagenous fibers embedded in chondroitin sulfate” (Campbell Biology).  Collagen is the main extracellular structural protein connecting tissues in animals.  Chondroitin sulfate is a protein-sugar polymer (proteoglycan). BIOL 108 Winter 2024 © 2024 Neil Harris 2. The dermal skeletal structures are derived from intramembranous ossification (not from preformed cartilage). − Formed external armour in early armoured fishes. − Retained in living vertebrates, such as bony fin rays in fish, facial and pectoral bones, and teeth. Juvenile spotted gar at 22 days stained for cartilage (blue) and bone (red) WC 19 Dunkleosteus, a 10 m extinct placoderm fish Early gnathostome fossils BIOL 108 Winter 2024 © 2024 Neil Harris The earliest gnathostome fossils from the Silurian-Devonian periods reveal a diverse array of extinct armoured vertebrate fish, including placoderms (“plate-skinned”) and acanthodians (“spiny sharks”). Fig 34.15 Fossil of early gnathostome, Bothriolepis canadensis − Most extinct by the end of the Devonian period. − Placoderms evolved ~450 mya and included giant predators (e.g. Dunkleosteus), and smaller (