The Origin and Evolution of Vertebrates
Chapter 34: The Origin and Evolution of Vertebrates
Introduction
Chordates possess a notochord and a dorsal, hollow nerve cord.
The phylum Chordata belongs to a clade known as Deuterostomia and includes all vertebrates along with two subphyla of invertebrate chordates: Urochordata and Cephalochordata.
There are over 60,000 species of vertebrates categorized in the Subphylum Vertebrata, which encompasses the largest and heaviest animals on Earth, while the smallest vertebrates are less than 1 cm long.
Derived Characters of Chordates
All chordates are characterized by a set of four derived features:
Notochord: A longitudinal, flexible rod located between the digestive tube and nerve cord providing flexible skeletal support.
Dorsal, hollow nerve cord: Formed from an ectoderm plate rolled into a neural tube situated dorsal to the notochord; it develops into the central nervous system.
Pharyngeal slits or clefts: openings in the pharynx allowing water passage; these slits enable water to exit from the mouth while bypassing the digestive tract.
Muscular, post-anal tail: A tail situated behind the anus that features skeletal elements and muscles, aiding in propulsion.
Subphyla of Chordates
Subphylum Cephalochordata: Lancelets
Lancelets are marine organisms that perform suspension feeding and swim using a simplified mechanism akin to that of fishes.
Adult lancelets can grow up to 6 cm and maintain key characteristics of the chordate body plan.
Subphylum Urochordata: Tunicates
Tunicates are more closely related to vertebrates than lancelets, with chordate features being most evident during their larval stage.
During this stage, they swim using tail muscles and notochord, subsequently settling on substrates for metamorphosis.
Vertebrates: Definition and Evolution
Vertebrates are defined as chordates possessing a backbone, allowing greater efficiency in food capture and predator evasion.
Living vertebrates feature two or more sets of Hox genes (compared to one in lancelets and tunicates), facilitating innovations in nervous systems and skeletal structures.
The neural crest, a defining characteristic of vertebrates, forms at the margins of the closing neural tube and contributes to the development of skull elements, teeth, cartilage, neurons, and sensory structures.
Hagfishes and Lampreys
Hagfishes (Class Myxini) and lampreys (Class Petromyzontida) represent the only extant vertebrates that lack jaws and possess rudimentary vertebrae:
Hagfishes are jawless, retaining a notochord as a flexible cartilage rod, with features including a cartilaginous skull and sensory organs.
Lampreys also lack jaws, possess reduced vertebrae, and a cartilaginous skeleton, inhabiting both marine and freshwater environments:
Larvae live in streams, feeding for several years; some species are free-living, while others are parasitic, migrating to oceans or lakes to consume fish blood and tissues.
Early Vertebrate Evolution
Fossils from the Cambrian period (approximately 530 million years ago) mark the transition towards vertebrates:
Haikouella: primitive suspension feeders measuring ~3 cm; they have defined brains and eyes but lack skulls and ear organs.
Myllokunmingia: the first chordate exhibiting ear and eye capsules, indicating a skull presence, yet devoid of vertebrae.
The development of a head allowed more complex movements and feeding behaviors.
Conodonts: early vertebrates from around 500 million years ago; possessing large eyes and intricate sets of barbed hooks for prey capture.
Derived Characters of Gnathostomes
Gnathostomes (meaning "jaw mouth") represent vertebrates with hinged jaws, which likely evolved from skeletal rods supporting pharyngeal slits. This group includes sharks, ray-finned fishes, lobe-finned fishes, amphibians, reptiles (including birds), and mammals.
Notable features of gnathostomes include:
Duplication of the genome, hence possessing four sets of Hox genes.
Increased size of the forebrain.
Enhanced olfactory and visual senses.
Lateral line system in aquatic species, consisting of vibration-sensitive organs along the body sides.
By 420 million years ago, jawed vertebrates had diversified into Chondrichthyans, ray-finned fishes, and lobe-finned fishes.
Class Chondrichthyes: Sharks, Rays, and Their Relatives
Chondrichthyans possess a skeleton largely made of cartilage, often supplemented with calcium.
Remnants of bone, formed post their diversification as gnathostomes, are observable in scales, tooth bases, and some vertebrae.
Sharks are streamlined for swimming but lack maneuverability:
The dorsal fin acts as a stabilizer while paired pectoral and pelvic fins support limited maneuvering.
Sharks maintain buoyancy through oil stored in their liver, although they must continuously swim to prevent sinking and to facilitate water flow through their gills for oxygen exchange.
Feeding and Reproduction in Chondrichthyans
The majority of sharks are carnivorous, equipped with multiple rows of sharp teeth designed for tearing flesh, while the largest sharks and rays are filter feeders.
Rays and skates exhibit adaptations for bottom-dwelling lifestyles with enlarged pectoral fins acting like wings, and they possess crushing jaws for feeding on mollusks and crustaceans.
Modes of reproduction in sharks include:
Oviparous: Eggs hatch externally from the mother’s body.
Ovoviviparous: Eggs are retained in the oviduct, hatching inside the uterus.
Viviparous: Young develop within the uterus, nourished via the yolk sac, nutrient-rich fluids, or cannibalizing other eggs.
Ray-Finned and Lobe-Finned Fishes
Osteichthyans, predominantly ray-finned fishes, possess a bony endoskeleton:
Gills are shielded by a bony operculum, allowing water to flow over the gills facilitated by muscle contractions.
Many fishes have a swim bladder — an air sac used for buoyancy.
Most species exhibit external fertilization and are oviparous, although some demonstrate internal fertilization and live births.
Lobe-Finned Fishes (Sarcopterygii)
Evolved in the Silurian period, characterized by pectoral and pelvic fins fortified with rod-shaped bones encased in thick muscle, facilitating movement across substrates; only three lineages persist:
Coelacanths (Actinistia), long believed extinct, were rediscovered in 1938.
Lungfishes (Dipnoi), alive in southern Africa and Australia.
Tetrapods, which are now thought to have descended from the Dipnoan lineage.
Derived Characters of Tetrapods
Terrestrial life prompted numerous modifications in the tetrapod body plan, which includes:
Four limbs with digits, a neck for independent head movement, the fusion of the pelvic girdle to the backbone, the absence of gills (in most), and auditory adaptations for airborne sound detection.
Tiktaalik, a fossil depicting traits bridging fish and tetrapods, showcases both:
Fish characteristics: fins, gills, scales.
Tetrapod qualities: ribs for lung support, neck and shoulder adaptations for head movement, limbs with tetrapod-like bone patterns, and robust pelvic structures.
Class Amphibia
Amphibians are categorized into three clades:
Urodela (e.g., salamanders, approximately 550 species): some remain aquatic as larvae and adults, while others are terrestrial.
Anura (e.g., frogs and toads, approximately 5,420 species): adults are tailless with powerful hind limbs for on-land locomotion.
Apoda (e.g., caecilians, approximately 170 species): legless and partially blind, they evolved from ancestors with legs, adapted for burrowing.
Paedomorphosis: a common phenomenon in aquatic species where larval features persist into adulthood.
Evolution of Amniotes
Amniotes: tetrapods possessing a terrestrially adapted egg, encompassing reptiles (including birds) and mammals, marked by the amniotic egg, which contains four extraembryonic membranes (amnion, chorion, yolk sac, and allantois), facilitating embryo protection and reducing reliance on water:
Amnion: a fluid-filled sac providing cushioning and support.
Other membranes serve in gas exchange, nutrient transfer, and waste management.
Most reptilian and some mammalian amniotic eggs possess shells that slow dehydration, whereas mammalian young typically develop within the body.
Reptiles
Reptiles primarily consist of squamates (i.e., lizards and snakes, with 10,425 species) and birds (around 10,000 species).
Shared derived characteristics among reptiles include:
Keratin-containing scales protecting skin from desiccation and abrasions.
Oviparous reproduction with shelled eggs laid on land, necessitating internal fertilization.
Ectothermic behavior predominates in most reptiles, regulating body temperature through environmental heat absorption.
Birds are exceptions, being endothermic, able to regulate body temperatures through metabolic processes.
Origin and Evolutionary Radiation of Reptiles
The earliest recognized reptiles emerged about 310 million years ago, classified as diapsids, characterized by dual holes on the skull sides for jaw muscle attachment and comprising:
Turtles
Lepidosaurs: encompassing tuataras and squamates, including lizards and snakes.
Archosaurs: which include crocodilians, extinct pterosaurs, and dinosaurs.
Pterosaurs, first appearing in the late Triassic, held the status of the original flying tetrapods and held various ecological roles prior to the rise of birds.
Dinosaurs displayed an extraordinary variance in size and morphology:
Herbivores exhibited a range of defensive adaptations, while theropods, including Tyrannosaurus rex, were agile bipedal predators.
A potential mass extinction event, possibly due to an asteroid impact, may have occurred; however, decline patterns manifest millions of years prior to the Cretaceous period.
Turtles and Their Adaptations
Turtles (specifically 351 species) are most closely related to crocodilians and birds, even though they have lost characteristic diapsid skull holes.
Their boxlike shells evolved progressively, fusing upper and lower shields to the vertebral column, clavicles, and ribs.
The tuatara represents a rare surviving lineage within Lepidosaurs, found geographically confined to New Zealand.
Snakes, descendant of legged lizards, have various adaptations for prey capture and consumption, including:
Chemical sensors, specialized heat-detecting organs, flicking tongues for odor detection, venom, and sensitivity to vibrations.
Crocodilians
Crocodilians (including alligators and crocodiles) belong to a lineage traceable back to the late Triassic period:
Ancestors transitioned from small terrestrial quadrupeds to larger aquatic forms.
The 24 living species primarily inhabit warm regions.
Birds
Approximately 10,000 species of birds belong to the archosaurs, characterized by extensive adaptations promoting flight efficiency:
Physical features including absence of a urinary bladder, reduced gonads, and a single ovary in females help in weight reduction.
Beak adaptations replacing teeth, with food processing occurring in the gizzard.
Air-filled bones with honeycombed scans foster lighter structures.
The evolution of feathers made of β-keratin preceded powered flight, serving potential functions such as insulation and courtship before evolving as adaptations for flight.
Archaeopteryx stands as the earliest avian fossil, displaying feathered wings along with primitive traits like teeth and claws.
Behavior and Physiology of Birds
Birds demonstrate intricacies in color vision, heightened visual acuity, and fine muscle control essential for flight:
Advantages of flight include foraging, escaping predators, and migrating across long distances.
Flight incurs high energy demands, necessitating efficient metabolism and heat retention strategies, such as insulative feathers and subcutaneous fat layers.
Well-developed respiratory and circulatory systems accommodate the elevated metabolic rates.
Complex behaviors are common, including courtship rituals and parental care practices.
Fertilization occurs internally through cloacal contact between male and female.
Hard-shelled eggs are maintained at optimal temperatures through parental brooding.
Living Birds
Multiple avian groups include flightless varieties:
Ratites: flightless birds adapted for terrestrial movement, e.g., ostrich, kiwi, cassowary.
Penguins: utilize strong pectoral muscles and modified wings to navigate aquatic environments, effectively "flying" underwater.
Flight adaptations result in morphological similarities among flying species, while distinguishing factors include size, shape, behavioral traits, and limb configurations.
Mammals
Mammals represent a distinct lineage of amniotes characterized by hair and the ability to produce milk:
Approximately 6,400 species are categorized as mammals, encompassing several unique traits:
Mammary glands allowing for lactation.
Hair and subcutaneous fat promoting thermoregulation.
Endothermic nature coupled with efficient metabolic systems.
Extensive parental care and varied dental adaptations pertaining to diet.
Three significant mammalian lineages emerged around 160 million years ago:
Monotremes: egg-laying mammals.
Marsupials: pouch-bearing mammals.
Eutherians: placental mammals.
Early Evolution of Mammals
Synapsids: amniotes including all mammals, display a hallmark of a single temporal fenestra behind each eye socket, with gradual jaw modifications over 100 million years contributing to ear development.
The first authentic mammals emerged during the Jurassic period (201–145 million years ago), coexisting with dinosaurs before diversifying markedly post the late Cretaceous extinction event, paving the way for varied ecological forms – predators, herbivores, as well as terrestrial and aquatic adaptations.
Monotremes
Monotremes consist of five species confined to Australia and New Guinea, noted for their egg-laying trait:
Females secrete milk from their abdominal glands, with offspring deriving sustenance through licking milk off fur.
Marsupials vs. Eutherians
Marsupials share specific derived traits exclusive from monotremes, notably:
Enhanced metabolic rates and the presence of nipples for lactation.
Offspring develop shortly after conception and rely on maternal nourishment in a pouch specified as the marsupium, which can be orientated towards the front or rear or may be entirely absent.
Eutherians (placental mammals) differ from marsupials by presenting:
A more intricate placenta alongside lengthened gestation periods, where the majority of embryonic development occurs within the uterus connected by the placenta.
Molecular evidence suggests the diversification of eutherians began approximately 100 million years ago, with morphological studies indicating a secondary event around 60 million years ago.