I. General Chordate Characteristics Chordates are distinguished as a group by the presence of four embryonic features that may persist into adulthood in some species, but disappear as development progresses in most others: Figure 1: Chordate Characteristics a) Notochord: describes a rigid, fluid-filled rod running along the dorsal (back) surface of the developing embryo. This structure serves in structural support. In some primitive chordates, the notochord persists into adulthood, but in most chordates it disappears (gives rise to the disks between the vertebrae of the backbone). b) Dorsal, Hollow Nerve Chord: long, hollow tube above the notochord that forms from the infolding of the ectoderm along the embryo s dorsal surface (see figure 2). Develops into the central nervous system (brain & spinal cord). Figure 2: Neurulation: Formation of Dorsal, Hollow Nerve Cord
c) Pharyngeal Gill Slits: can give rise to the following structures as development progresses: Filter-feeding devices in some aquatic chordates Gills in aquatic vertebrate-chordates (chordates with a backbone) Inner-ear bones (hammer, anvil, & stirrup) of terrestrial vertebrate-chordates. d) Muscular, Postanal Tail: extends beyond the anus; may be retained into adulthood to function in the propulsion of the animal in aquatic environments or helping to maintain balance while walking or running in land animals. II. Craniate & Vertebrate Chordates Craniates are all chordates w/a skull or Cranium. Evolution of a skull led to the formation of related structures (teeth, jaws, etc) that allowed members of this group to pursue a predatory lifestyle. A unique feature 1 st seen in craniate chordates are Neural Crest Cells that form during neurulation & give rise to the following structures: a) Facial bones of the cranium, & the teeth. b) Facial dermis & the cranial nerves. Figure 3: Neurulation: Neural Crest Cells Neurulation Dorsal Hollow Nerve Cord Neural Crest Cell Fates Vertebrates are all chordates w/a cranium that also possess a backbone. Vertebrates exhibit all characteristics of chordates & craniates as well as the following features: a) Exhibit a high degree of cephalization (brain + sensory structures associated with ears, nose, eyes, etc). Extensive cephalization allows for a greater processing of sensory information in order to more rapidly respond to changes in the environment, increasing the likelihood of survival.
b) Possess an Axial Skeleton composed of the cranium & vertebrae serves to protect the brain & spinal cord & provides the main support structure for central trunk of the body. The axial skeleton may be associated with an Appendicular Skeleton supporting 2 pairs of appendages (legs, fins, etc). *Axial skeleton: skull & vertebral column **Appendicular Skeleton: 2 pairs of fins (pectoral & caudal) c) The skeleton is composed of bone &/or cartilage; living cells within the nonliving matrix of bone allows the vertebrate skeleton to grow with the animal. d) Possess a Closed Circulatory System that exhibits the following characteristics: Chambered heart (2, 3, or 4) that pumps blood through a series of vessels. The blood is oxygenated as it passes through gills or lungs. Since it is confined to a vessel network, the blood is under pressure as it flows through the vascular system, allowing for the rapid exchange of nutrients & wastes with body tissue. This promotes a more rapid metabolism which, in turn, can support a more active (predatory) lifestyle. III. Vertebrate Diversity Figure 4: Agnathans: Jawless Fish Lampreys Hagfish
Agnathans describe fish that lack true hinged jaws. Modern representative of this group are rare & include: a) Hagfish: cartilaginous skeleton lacking a vertebral column. Thus the notochord persists as the main axial skeleton. Lacks paired appendages & are thus not agile swimmers. b) Lampreys: external parasites w/cartilaginous skeletons. Notochord is surrounded by a cartilaginous pipe from which extend pairs of cartilaginous projections that partially enclose the nerve chord (may be a vestige of an early stage in the evolution of the vertebral column). Like hagfish, lampreys lack paired appendages. Jawed-Fish (Gnathostomes) Gnathostomes are fish & all other vertebrates that possess hinged jaws. The first jaws evolved from the modification of the skeletal rods or gill arches that supporting the gill slits. The remaining gill slits, no longer required for suspension feeding, remained as the major sites of respiratory gas exchange. Figure 5: Jaw Evolution Gnathostomes exhibit 2 major evolutionary breakthroughs that enabled them to out-compete & replace agnathans. a) Jaws, in conjunction with teeth, enabled these animals to grip food firmly & slice it up. Thus, gnathostomes were able to exploit food supplies that were unavailable to agnathans. b) Two sets of paired fins, along with the tail, enable gnathostome fish to maneuver accurately while swimming. This, coupled with the evolution of jaws, enabled these animals to become active predators. Gnathostomes: Cartilagenous Fish Gnathostomes include various classes of fishes. Two extant groups of fishes include the Chondrichthyes (cartilaginous fish) & the Osteichthyes (bony fishes). Chondrichthyes include sharks & rays that exhibit the following characteristics: Table 1: Chondrichthyes vs Ostiechthyes Fish Skeleton Respiration Senses Buoyancy Nutrition Reproduction Chondrichtyes (sharks & rays) Cartilage Swim to pass water over gills Ostiechthyes (ray,lobed,lungfish) Bone Operculum & muscles pump water over gills Lateral line system, ampullae of Lorenzini Lateral line system Oils in liver decrease density & increase boyuancy Swim bladder adjusts density Mostly carnivorous Carnivorous or herbivorous Internal fertilization (oviparity, oviviparity, viviparous) External fertilization (oviparity)
Of all bony fish, Ray Finned Fish are the most numerous & possess fins supported by long flexible rays. Figure 6: Ray-Finned Fish Lobed Fined Fish & lungfish have a central appendage in their fins containing many bones and muscles. The fins are very flexible & potentially useful for supporting the body on land. Figure 6.1: Lobe-Finned Fish Tetrapods, or land vertebrates, are most likely descended from lungfish. Morphological evidence of this relationship includes the presence of tooth enamel & 4 similarly sized limbs that have a similar structure & position. Mitochondrial DNA analysis also supports a close evolutionary relationship. The two sets of paired appendages of ancestral lungfish were modified as legs that could support the animal on land. Include amphibians & amniotes. a) Amphibians: include frogs, salamanders & caecilians. Eggs lack a shell & dehydrate quickly in dry air. Fertilization is external in most species & eggs may develop directly in water or within either the male s or female s mouth, back, or stomach. Some species encase their egg masses into foam that resists desiccation. b) Amniotes: include birds, mammals, & reptiles. Evolved from a common amphibian ancestor & produce an egg with a shell that retains water & thus can be laid on dry land. In some amniotes the shell is calcareous whereas others produce leathery shells. Most mammals have dispensed with the shell, with the embryo implanting in the uterin wall.
Amniotic Egg Amniotes (reptiles, birds, & mammals) have specialized Extraembryonic Membranes within the amniotic egg. These structures function in gas exchange, waste storage, & the transfer of stored nutrients to the developing embryo. a) Chorion: outermost extraembryonic membrane. Together with the membrane of the allantois, the chorion aids in gas exchanges gas between the embryo & surrounding air, which diffuses freely through the shell. b) Amnion: encloses the embryo in a fluid-filled amniotic cavity that protects against desiccation & cushions against mechanical shock. c) Yolk Sac: contains stored nutrients that are delivered to the embryo by blood vessels that service this sac. d) Allantois: stores metabolic wastes produced by the embryo. Figure 7: Amniotic Egg Figure 8: Vertebrate Survivorship Curves Type I (K-strategy): most newborns survive, with the oldest individuals dying at the highest rate (e.g elephants). Type II (K-strategy): individuals of all ages die at the same rate (e.g. songbirds). Type III (R-strategy): most newborns die, with the oldest surviving at the highest rate (e.g. fish, amphibians).