Vertebrae Evolution & Diversity

Invertebrate Chordates & The Origin of Vertebrates The vertebrates are part of a phylogenetic branch of the animal kingdom consisting of deuterostomes. The deuterostome branch has two main modern phyla: the chordates and the enchinoderms.

Four anatomical features characterize the phylum Chordata

Chordates are named for a skeletal structure, the notochord, present in all chordate embyros. The notochord is a longitudinal, flexible rod located between the digestive tube and the nerve cord. Composed of large, fluid-filled cells encased in fairly stiff, fibrous tissue, it provides skeletal support through most of the length of the animal. In most vertebrates a more complex, jointed skeleton develops, and the adult only retains remnants of the embryonic notochord, the gelatinous material of the disks between the vertebrae of humans.

The nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube located dorsal to the notochord. The result is a dorsal, hollow nerve cord unique to chordates. The nerve cord of a chordate embryo develops into the central nervous system: the brain and spinal cord. Other animal phyla have solid nerve cords, usually ventrally located.

In the embryos of all chordates, a tube that eventually develops into the digestive tract extends between the future mouth and the future anus. The region posterior to the mouth is the pharynx, which has pairs of pouches in chordate embryos. The pharyngeal pouches of various vertebrates develop into a diversity of other structures, including gills and components of auditory organs.

Most chordates have a tail extending posterior to the anus Most chordates have a tail extending posterior to the anus. The chordate tail contains skeletal elements and muscles and provides much of the propulsive force in many aquatic species. Most nonchordates have a digestive tract that extends nearly the whole length of the body.

Invertebrate chordates provide clues to the origin of vertebrates

Subphylum Urochordata Urochordatesare commonly called tunicates. Most tunicates are sessile marine animals, but others are planktonic or colonial. Adult tunicates scarcely resembles a chordate, it doesn’t display a trace of notochord, nerve cord or tail. All four chordate trademarks are manifest in the larval form of some groups of tunicates.

Subphylum Cephalochordata Known as lancelets because of their bladelike shape, cephalochordates closely resemble the idealized chordate. The notochord; dorsal, hollow nerve cord; numerous gill slits; and postanal tail all persist into the adult stage. Lancelets are small animals that live in the sand at the bottom of the sea in coastal regions.

The Relationship Between Invertebrate Chordates & Vertebrates Molecular evidence suggests that cephalochordates are the vertebrates’ closest relatives, and urochordates are their next closest relatives.

According to one hypothesis, there are two stages in the evolution of vertebrates from invertebrates: In the first stage, an ancestral cephalochordate evolved from a form resembling a modern urochordate larva; in the second stage, a vertebrate evolved from a cephalochordate. The first stage may have been preceded by paedogenesis, the precocious development of sexual maturity in a larva.

Introduction To The Vertebrates

Neural crest, pronounced cephalization, a vertebral column, and a closed circulatory system characterize the subphylum Vertebrata In vertebrates, a group of embryonic cells called the neural crest forms near the dorsal margins of the closing neural tube. The neural crest contributes to the formation of certain skeletal elements such as some of the bones and cartilage of the cranium, and many other structures that distinguish vertebrates from other chordates.

An overview of vertebrate diversity Gnathostomes have jaws and also have two sets of paired appendages. Among these jawed vertebrates, there are various classes of aquatic animals that we generally call “fishes”: the cartilaginous fishes and three classes of bony fishes. In the case of fishes, the two sets of paired appendages are fins that function in swimming.

All the other gnathostomes are tetrapods, in which the two sets of paired appendages are modified as legs that can support the animal on land. The tetrapods include the amphibians along with the clade identified as amniotes. The amniotes are named for the amniotic egg.

Jawless Vertebrates

Some extinct jawless vertebrates had ossified teeth & bony armor A diversity of taxa informally called ostracoderms thrived from about 450 to 375 million years ago. Most species were small, less than 50 cm in length. Most lacked paired fins and apparently were bottom dwellers, but there were also some more active species with paired fins.

Ostracoderm means “shelled skin,” a reference to the armor of bony plates that encased these animals. The plates may represent an early evolutionary stage of ossification, the hardening of connective tissue that occurs when specialized cells secrete calcium and phosphate, which precipitate as calcium phosphate, a hard mineral salt. Even earlier evidence of ossification is found in fossils of ancient vertebrates called conodonts, which date back as far as 510 million years. These vertebrates are named for their cone- shaped toothlike structures, which are ossified.

Fishes and Amphibians

Vertebrate jaws evolved from skeletal supports of the pharyngeal slits The remaining gill slits, no longer required for suspension feeding, remained as the major site of respiratory gas exchange with the external environment. New adaptations usually evolve by the modification of existing structures. Evolution is limited by the raw material with which it must work.

Class Chondrichthyes: Sharks & rays have cartilaginous skeletons The vertebrates of the class Chondrichthyes are called cartilaginous fishes because they have relatively flexible endoskeletons made of cartilage rather than bone. Jaws and paired fins are well developed in the cartilaginous fishes. The cartilaginous skeleton is a derived characteristic because it evolved secondarily.

The largest sharks and rays are suspension feeders that feed on plankton. Most sharks, however, are carnivores that swallow their prey whole or use their powerful jaws and sharp teeth to tear flesh from animals too large to swallow in one piece. Shark teeth probably evolved from the jagged scales that cover the abrasive skin. The digestive tract of many sharks is proportionately shorter than the digestive tube of other vertebrates. Within the shark intestine is a spiral valve, a corkscrew- shaped ridge that increases the surface area and prolongs the passage of food along the short digestive tract.

Sharks have sharp vision but cannot distinguish colors Sharks have sharp vision but cannot distinguish colors. The nostrils function only for olfaction (smelling), not breathing. Along with eyes and nostrils, the shark head also has a pair of regions in the skin that can detect electrical fields generated by the muscle contractions of nearby animals. Shark eggs are fertilized internally. Some species of sharks are oviparous, which means they lay eggs that hatch outside the mother’s body. Other species are ovoviviparous, which means they retain the fertilized eggs in the oviduct; they are nourished by the egg yolk and then they hatch within the uterus. A few species are viviparous; the young develop within the uterus, nourished prior to birth by nutrients received from the mother’s blood through a placenta.

Osteichthyes: The extant classes of bony fishes are the ray-finned fishes, the lobe-finned fishes, & the lungfishes Nearly all bony fishes have an ossified endoskeleton with a hard matrix of calcium phosphate. Glands in the skin of a bony fish secrete a mucus that gives the animal its characteristic sliminess, and adaptation that reduces drag during swimming. Bony fishes breath by drawing water over 4 or 5 pairs of gills located in chambers covered by the operculum. Another adaption of most bony fishes is the swim bladder. Most species are oviparous, however, internal fertilization and birthing characterize other species.

Nearly all the families of fishes familiar to us are ray-finned fishes Nearly all the families of fishes familiar to us are ray-finned fishes. Examples are bass, trout, perch, tuna and herring. The fins, supported mainly by long flexible rays, are modified for maneuvering, defense, and other functions.

Lobe-finned fishes have muscular pectoral and pelvic fins supported by extensions of the bony skeleton. Many lobe-fins were large, apparently bottom dwellers.

Three genera of lungfishes live today in the Southern Hemisphere Three genera of lungfishes live today in the Southern Hemisphere. They generally inhabit stagnant ponds and swamps, surfacing to gulp air into lungs connected to the pharynx of the digestive tract. Lungfishes also have gills, which are the main organs for gas exchange. When ponds shrink during the dry season, some lungfishes can burrow in the mud and aestivate.

Tetrapods evolved from specialized fishes that inhabited shallow water Amphibians were the first tetrapods to spend a substantial portion of their time on land. As the earliest tetrapods, amphibians benefited from an abundance of food and relatively little competition.

Class Amphibia: Salamanders, frogs, & caecilians are the 3 extant amphibian orders Today the amphibians are represented by a total of about 4800 species of salamanders (order Urodela), frogs (order Anura), and caecilians (order Apoda). Some urodeles are entirely aquatic, but others live on land as adults or throughout life.

Anurans are more specialized than urodeles for moving on land Anurans are more specialized than urodeles for moving on land. Frogs display a great variety of adaptations that help them avoid being eaten by larger predators. They exhibit color patterns that camouflage. The skin glands of frogs secrete distasteful, or even poisonous, mucus. Many poisonous species have bright coloration that apparently warns predators.

Apodans, the caecilians, are legless and nearly blind, and superficially resemble earthworms. The reduction of legs evolved secondarily from a legged ancestor.

Amphibian means “two lives”, a reference to the metamorphosis of many frogs. In spite of the name amphibian, however, many frogs do not go through the aquatic tadpole stage, and many amphibians do not live in a dualistic life. There are some strictly aquatic and strictly terrestrial frogs, salamanders, and caecilians. Most amphibians maintain close ties with water and are most abundant in damp habitats such as swamps and rain forests. They reply heavily on their moist skin to carry out gas exchange with the environments; some terrestrial species lack lungs and breathe exclusively through their skin and oral cavity.

Amniotes

Evolution of amniotic egg expanded the success of vertebrates on land The amnioteclade consists of the mammals, birds, and vertebrates we commonly call reptiles. The amniotic egg, a reproductive adaptation that enabled terrestrial vertebrates to complete their life cycles on land and sever their last ties with their aquatic origins. Amniotic eggs have a shell that retains water and can hen be laid in a dry place.

The shells of bird eggs are calcareous and inflexible, while shells of many reptile eggs are leathery and flexible. Most mammals have dispensed with the shell; instead, the embryo implants in the wall of the uterus and obtains nutrients from the mother.

Reptiles, birds, and mammals all have specialized membranes within the amniotic call called the extra-embryonic membranes. The extra-embryonic membranes aren’t part of the body of the developing animal, these structures function in gas exchange, waste storage, and the transfer of stored nutrients to the embryo. Amniotes also show other adaptations to terrestrial life, including waterproof skin and increasing use of the rib cage to ventilate the lungs.

A reptilian heritage is evident in all amniotes Reptilian Characteristics Scales containing the protein keratin waterproof the skin of a reptile, helping prevent dehydration in dry air. Reptiles can’t breathe through their keratinized dry skin, and they obtain all their oxygen with lungs.

Most reptiles lay shelled amniotic eggs on land Most reptiles lay shelled amniotic eggs on land. Fertilization must occur internally, before the shell is secreted as the egg passes through the reproductive tract. They are sometimes labeled “cold-blooded” animals because they do not use their metabolism extensively to control body temperature. Because they absorb external heat rather than generating much of their own, reptiles are said to be ectotherms.

Birds began as feathered reptiles Characteristic of Birds Almost every part of a typical bird’s anatomy is modified in some way that enhances flight. The bones have an internal structure that is honeycombed. Another adaptations reducing the weight of birds is the absence of some organs. Birds are endothermic. Senses must be acute.

Mammals diversified extensively in the wake of the Cretaceous extinctions Mammalian Characteristics All mammalian mother nourish their babies with milk. Hair is another mammalian characteristic. Mammals are endothermic and most have an active metabolism. Most are born rather than hatched. Have generally larger brains and many species are capable learners. Differentiation of teeth is another important mammalian trait.

Monotremes The platypuses and the echidnas are the only mammals that lay eggs. Monotremes have hair and produce milk. On the belly of the mother are specialized glands that secrete glands. After hatching, the baby sucks the milk from the fur of the mother, who has no nipples.

Marsupials Examples are opossums, kangaroos, bandicoots, and koalas. A marsupial is born very early in its development and completes its embryonic development while nursing. In most species, the nursing young are held within a maternal pouch called a marsupium.

Eutherian (Placental) Mammals Young eutherians complete their embryonic development within the uterus, joined to the mother by the placenta.

Primates & The Evolution of Homo sapiens

Primate evolution provides a context for understanding human origins Some General Primate Characteristics Most primates have hands and feet adapted for grasping, and have large brains and short jaws, giving them a short face. Have forward-looking eyes and flat nails. The opposable thumb is only found in monkeys, apes, and humans.

Modern Primates Examples of prosimians (“premonkeys”) are lemurs of Madagascar and the lorises, pottos, and tarsiers that live in tropical Africa and southern Asia. The antropoids include monkeys, apes, and humans.

Humanity is one very young twig on the vertebrate tree Paleonanthropology is the study of human origins and evolution. Hominoid refers to great apes and humans; fossils are more closely related to chimpanzees, gorillas, or orangutans. Hominid refers to the twigs of evolutionary tree that are more related to us than others living species.

Some Major Features of Human Evolution Brain size- Hominoids of about 6 million years ago had brains with volumes of about 400-450 cm3; modern human brains average 1300 cm3. Jaw Shape- Hominoids had prognathic (longer) jaws. During human evolution, shortening of jaws resulted in flatter faces with more pronounced chins.

Bipedal Posture- Hominoid ancestors walked on all four limbs when on the ground like modern apes. Reduced Size Difference Between the Sexes- In hominoids the size difference between males and females is a major feature of dimorphism. Modern ape males weigh about twice as much as females. Monkey males are about 1.35 times heavier than females and human males average 1.2 times the weigh of females.

Some Key Changes in Family Structure- To identify evolutionary changes in social behavior, researcher rely more on comparisons between humans and other extant hominoids. In contrast to the social organization of most ape species (as well as most monkeys), monogamy prevails in most human cultures. Newborn human infants are dependent on their mothers and the duration of parental care for offspring is much longer in humans than in other hominoids. The extended parental car, couples with the large brain size, enhanced learning and is associated with the behavioral complexity of humans.