BIO153H5 Chapter Notes - Chapter 34: Marsupial, Archaeopteryx, Muscular System
Chapter 34: The Origin and Evolution of Vertebrates
- vertebrates are animals with a vertebrae: the series of bones that make up the vertebral column (or backbone)
Concept 34.1: Chordates have a Notochord and a Dorsal, Hollow Nerve Cord
- Chordates are bilaterian (bilaterally symmetrical) animals, and within Bilateria, they belong to the clade of animals
known as Deuterostomes
- The cephalochordates and the urochordates are two groups of invertebrate deuterostomes that are more closely
related to vertebrates than to other invertebrates
DERIVED CHARACTERS OF CHORDATES
- All chordates share a set of derived characters, though many possess them
only during embryonic development
- Four key characters of chordates are: a notochord; a dorsal, hollow nerve
cord; pharyngeal slits or clefts; and a muscular, post-anal tail
NOTOCHORD
- The notochord is a longitudinal, flexible rod located between the digestive tube and the nerve cord, and it’s
composed of large, fluid-filled cells encased in fairly stiff, fibrous tissue
- It provides skeletal support throughout most of the length of a chordate, and in larva or adults that retain it, it
also provides a firm but flexible structure against which muscles can work during swimming
- In most vertebrates, a more complex, jointed skeleton develops around the ancestral notochord, and the adult
retains only remnants of the embryonic notochord (in humans, it’s reduced)
DORSAL, HOLLOW NERVE CORD
- The nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a tube located dorsal to
the notochord, and is unique to chordates
- Other animal phyla have solid nerve cords, and in most cases they’re ventrally located
- The nerve cord of a chordate embryo develops into the central nervous system: the brain and spinal cord
PHARYNGEAL SLITS OR CLEFTS
- The digestive tube of chordates extends from the mouth to the anus, and the pharynx is posterior to the mouth
- In all chordate embryos, a series of pouches separated by grooves forms along the sides of the pharynx, and these
grooves (pharyngeal clefts) develop into slits that open to the outside of the body
- pharyngeal slits allow water entering the mouth to exit the body without passing through the digestive tract
- in vertebrates (with the exception of vertebrates with limbs, the tetrapods), these slits and the structures that
support them have been modified for gas exchange, and are known as gill slits
- in tetrapods, the pharyngeal clefts don’t develop into slits; instead, they play a role in development of parts of the
ear and other structures in the head and neck
MUSCULAR, POST-ANAL TAIL
- chordates have a tail that extends posterior to the anus, although usually reduced during embryonic development
- most non-chordates have a digestive tract that extends nearly the whole length of the body
- contains skeletal elements and muscles, and it helps propel many aquatic species in water
EARLY CHORDATE EVOLUTION
- LANCELETS
• the most basal group of living chordates are lancelets
• since lancelets branch off near the base of the chordate phylogenetic tree, its
suggested that the ancestral chordate may have looked something like a lancelet
• rather than a brain, lancelets have a slightly swollen tip on the anterior end of the
dorsal nerve cord, but the Hox genes that organize parts of the brain are still present,
suggesting that the vertebrate brain is an elaboration of an ancestral structure similar
to the lancelet’s simple nerve cord tip
- TUNICATES
• tunicates are suggested to be more closely related to other chordates than they are to lancelets
• chordate characteristics of tunicates are most apparent during their larval stage, which can be very brief
• once a tunicate settles on a substrate, it
undergoes radical metamorphosis, in which many
chordate characteristics disappear
• loss of characters seems to have occurred after
the lineage branched off from other chordates
- ancestral chordates had genes associated with
vertebrate organs (genes which are found in tunicates
and vertebrates, but absent in non-chordate
invertebrates)
Concept 34.2: Craniates are Chordates that have a Head
- chordates with a head are known as craniates
- the origin of the head (consisting of the brain at the anterior end of the dorsal nerve cord, eyes and other sensory
organs, as well as a skull) enabled chordates to coordinate more complex movement and feeding behaviours
DERIVED CHARACTERS OF CRANIATES
- due to gene duplication, craniates possess two or more sets of Hox genes (lancelets and tunicates have one)
- divergence of sequences in the duplicate genes led to additional genetic complexity, this may have made it
possible for craniates to develop more complex morphologies than those of lancelets and tunicates
- one feature unique to craniates is the neural crest, a collection of cells that
appears near the dorsal margins of the closing neural tube in an embryo
- neural crest cells disperse throughout the body, where they give rise to a
variety of structures
- in aquatic craniates, the pharyngeal clefts evolved into gill slits (associated
with muscles and nerves that allow water to be pumped through the slits)
- craniates also have higher metabolic rates and a much more expensive
muscular system, muscles lining their digestive tract that aid digestion, and
a heart with at least two chambers
- HAGFISHES
• most basal group of craniates is Myxini, the hagfishes
• have a skull made of cartilage, but lack jaws and vertebrae
• swim using segmental muscles to exert force against their notochord
(a strong, flexible rod of cartilage)
Concept 34.3: Vertebrates are Craniates that have a Backbone
- during the Cambrian period, a lineage of craniates gave rise to vertebrates
- with a more complex nervous system and a more elaborate skeleton, vertebrates became more efficient at two
tasks: capturing food and avoiding being eaten
DERIVED CHARACTERS OF VERTEBRATES
- after they branched off from other craniates, vertebrates underwent another gene duplication which involved a
group of transcription factor genes called the Dix family (associated with nervous system and skeleton innovation)
- in some animals, the vertebrae are small prongs of cartilage arrayed dorsally along the notochord
- in the majority of animals, the vertebrae enclose the spinal cord and take over mechanical roles of the notochord
- aquatic vertebrates also acquired dorsal, ventral, and anal fins stiffened by bony structures called fin rays
- LAMPREYS
• are the most basal lineage of living vertebrates
• their skeletons are made of cartilage, but contains no collagen (instead, it’s a stiff matrix of other proteins)
• the notochord is the main axial skeleton in adults
• also have a flexible sheath around their rod-like notochord, and along this length are pairs of cartilaginous
projections extending dorsally that partially enclose the nerve cord
FOSSILS OF EARLY VERTEBRATES
- conodonts were slender, soft-bodied vertebrates with prominent eyes
- hunted with the help of their large eyes, impaling prey on a set of barbed hooks at the
anterior end of the mouth which were made of dental tissues that were mineralized
(composed of minerals such of calcium)
- vertebrates with additional innovations emerged during the Ordovician, Silurian, and
Devonian periods
ORIGINS OF BONE AND TEETH
- the human skeleton is heavily mineralized bone, and cartilage plays a minor role
- the vertebrate skeleton evolved initially as a structure made of un-mineralized cartilage (not a bony skeleton)
- one hypothesis for the process of mineralization is that it’s associated with a transition in feeding mechanisms
- early chordates were suspension feeders, but over time became larger and were able to ingest larger particles
- mineralization of the vertebrate body may have begun in the mouth, later incorporated into protective armour
- only in mor4e diverse vertebrat4es did the endoskeleton begin to mineralize, starting with the skull
Document Summary
Chapter 34: the origin and evolution of vertebrates vertebrates are animals with a vertebrae: the series of bones that make up the vertebral column (or backbone) Concept 34. 1: chordates have a notochord and a dorsal, hollow nerve cord. Chordates are bilaterian (bilaterally symmetrical) animals, and within bilateria, they belong to the clade of animals known as deuterostomes. The cephalochordates and the urochordates are two groups of invertebrate deuterostomes that are more closely related to vertebrates than to other invertebrates. All chordates share a set of derived characters, though many possess them only during embryonic development. Four key characters of chordates are: a notochord; a dorsal, hollow nerve cord; pharyngeal slits or clefts; and a muscular, post-anal tail. The notochord is a lo(cid:374)gitudi(cid:374)al, fle(cid:454)i(cid:271)le (cid:396)od lo(cid:272)ated (cid:271)et(cid:449)ee(cid:374) the digesti(cid:448)e tu(cid:271)e a(cid:374)d the (cid:374)e(cid:396)(cid:448)e (cid:272)o(cid:396)d, a(cid:374)d it"s composed of large, fluid-filled cells encased in fairly stiff, fibrous tissue.
