Comparative Vertebrate Anatomy

BIO 0304214

TERMS AND CONCEPTSHomology refers to features of two or more organisms sharing common ancestry it also refers to the correspondence in type of structure between parts or

organs of different animals clearly linked through time by continuity of fossil record shown to develop similarity in the embryo from identical primordial. different appearance, common general make-up

example: the skeletons of the forelimbs of cats, the wings of birds, the arms of humans, and the wings of a bat.

Analogy refers to features of two or more organisms sharing common

function correspondence in function of a structure in similar or different

organs or organ parts.example: the fin of a fish and the flipper of a whale

the scales of fishes and reptiles

Homoplasy refers to features of two or more organisms which may be related

by similarity of appearance but cannot be explained by either homology and analogy.

example: mimicry and camouflage

HOMOPLASY/HOMOPLASTIC ORGANS Shared anatomy Generally, not homologous E.g. Insect wings vs. leaf

Clade: A group of organisms consisting of a single common ancestor and all descendants of that ancestor

Cladistics: A method of classification of living organisms based on the construction and analysis of cladograms

Ontogeny the developmental history of an organism begins with embryogenesis, the development after fertilization includes post embryonic changes: aging or senescence and death. genes are the primary operants a single lifetime

Phylogeny the evolutionary history of group of species it requires hundreds or thousands to hundred of millions of years.

Cephalization it refers to the centralization or localization of nervous structures with

accompanying dominance of the head. the pronounced tendency for the anterior end of the body to become more

and more distinctly separated and differentiated from the rest of the body=head.

localization within the head the main part of the nervous system - i.e. the brain – and the most important sense organs

it is more and more prominent as one ascends the animal kingdom

Homologies are anatomical features, of different organisms, that have a similar appearance or function because they were inherited from a common ancestor that also had them. For instance, the forelimb of a bear, the wing of a bird, and your arm have the same functional types of bones as did our shared reptilian ancestor. Therefore, these bones are homologous structures. The more homologies two organisms possess, the more likely it is that they have a close genetic relationship.

Parallelism or parallel evolution, is a similar evolutionary development in different species lines after divergence from a common ancestor that did not have the characteristic but did have an initial anatomical feature that led to it. For instance, some South American and African monkeys evolved relatively large body sizes independently of each other. Their common ancestor was a much smaller monkey but was otherwise reminiscent of the later descendant species. Apparently, nature selected for larger monkey bodies on both continents during the last 30 million years.

Convergence , or convergent evolution, is the development of a similar anatomical feature in distinct species lines after divergence from a common ancestor that did not have the initial trait that led to it. The common ancestor is usually more distant in time than is the case with parallelism. The similar appearance and predatory behavior of North American wolves and Tasmanian wolves (thylacines) is an example .

Analogies   are anatomical features that have the same form or function in different species that have no known common ancestor.  For instance, the wings of a bird and a butterfly are analogous structures because they are superficially similar in shape and function.  Both of these very distinct species lines solved the problem of getting off of the ground in essentially the same way.  However, their wings are quite different on the inside.  Bird wings have an internal framework consisting of bones, while butterfly wings do not have any bones at all and are kept rigid mostly through fluid pressure.  Analogies may be due to homologies or homoplasies, but the common ancestor, if any, is unknown. 

Vertebrate beginnings

Among the oldest & best known = ostracoderms •fishes that occurred in the late Cambrian period (see The Cambrian Explosion) through the Devonian (about 400 - 525 million years before present) •had bony plates and scales (&, therefore, were easily fossilized) •jawless vertebrates called 'armored fishes'

THE EARLIEST VERTEBRATES: JAWLESS OSTRACODERMS

1. Until recently, ostracoderms are the earliest articulated vertebrate skeletal fossils.

2. They are found in the late Cambrian deposits in the United States, Bolivia and Australia.

3. They were small, heavily armored, jawless, and lacked paired fins. 

4. In 1999, researchers described two fishlike 530-million-year-old vertebrates, Myllokunmingia and Haikouichthys from the Chengjiang deposits.

5. These fossils push back the origin of vertebrates to at least the early Cambrian.

6. Although they possess many typically vertebrate characters, such as the W-shaped myomeres, a heart, cranium, and fin rays, they lack evidence of mineralized tissues (which may explain the extreme rarity of vertebrate fossils prior to the late Cambrian).

7. The earliest Ostracoderms were armored with bone in their dermis and lacked paired fins that later fishes used for stability.

8. The ostratocderms are not considered to be a natural evolutionary assemblage, but a convenience for describing several groups of heavily armored extinct jawless fishes, such as the heterostracans. 

Subphylum Urochordata = tunicates  Chordate 'ancestor' of vertebrates: 

sessile (like adult tunicates)  tail evolved as adaptation in larvae to increase mobility  'higher forms' - came about by retention of tail (neoteny) 

Tunicate larva - also called 'sea squirt'  notochord is confined to the tail  notochord is lost during metamorphosis into sessile adult  possess pharyngeal slits 

Subphylum Cephalochordata= Amphioxus (or Branchiostoma) 

Vertebrate features: notochord dorsal, hollow nervous system pharyngeal gill slits  'circulatory' system - vertebrate pattern with 'pumping vessels' (but no heart) 

HETEROSTRACANS

1. The heterostracans represent an awkward design that probably filtered particles from the bottom. 

2. Unlike ciliary filter-feeding protochordates, ostracoderms sucked in water by muscular pumping.

3. Therefore, a few authorities believe they may have been able to feed on soft-bodied animals. 

4. The Devonian saw a major radiation of heterostracans that never evolved jaws or paired fins.

OSTEOSTRACANS

1. Coexisting with heterostracans, this group developed paired pectoral fins that stabilized their movement.

2. Their jawless mouth was toothless.

3. They had a sensory lateral line, paired eyes, and inner ears with semicircular canals. 

4. Although the head was well armored, they lacked any axial skeleton or vertebrae.

EARLY JAWED VERTEBRATES

1. All living and extinct jawed vertebrates are called gnathostomes in contrast to agnathans.

2. Living agnathans, the lampreys and hagfishes, are often called cyclostomes.

3. Gnathostomes constitute a monophyletic group; all derived organisms share these features.

4. Agnathans, defined by the absence of jaws, may be paraphyletic.

6. Placoderms appeared in the early Devonian and were heavily armored; some were large.

7. Acanthodians contemporary with placoderms may have given rise to bony fishes.

Coccosteus (top, Middle Devonian), Campbellodus (left, Late Devonian),

and Bothriolepis (bottom right. Late Devonian)

Chordate Origins & Phylogeny

Comparative vertebrate anatomy : the study of structure, of the function of structure, & of the range of variation in structure & function among vertebrates: Kingdom: Animal Phylum: Chordata Subphylum: Vertebrata

Chordates are well represented in marine, freshwater and terrestrial habitats from the Equator to the high northern and southern latitudes. The oldest fossil chordates are of Cambrian age. The earliest is Yunnanozoon lividum from the Early Cambrian, 525 Ma (= million years ago), of China

-The Phylum Chordata includes the well-known vertebrates (fishes, amphibians, reptiles, birds, mammals). -The vertebrates and hagfishes together comprise the taxon Craniata.

-The remaining chordates are the tunicates (Urochordata), lancelets (Cephalochordata), and, possibly, some odd extinct groups. With few exceptions, chordates are active animals with bilaterally symmetric bodies that are longitudinally differentiated into head, trunk and tail. The most distinctive morphological features of chordates are the notochord, nerve cord, and visceral clefts and arches

During their embryonic development, all chordates pass through a stage called the pharyngula (Fig 1) with these features:

•a dorsal, tubular nerve cord ("1") running from anterior to posterior. At its anterior end, it becomes enlarged to form the brain. •a flexible, rodlike notochord ("2") that runs dorsal to the digestive tract and provides internal support. In vertebrate chordates, it is replaced by a vertebral column or backbone long before maturity. •pairs of gill pouches. These lateral outpocketings of the pharynx are matched on the exterior by paired grooves. In aquatic chordates, one or more pairs of gill pouches break through to the exterior grooves, forming gill slits ("3"). These provide an exit for water taken in through the mouth and passed over the gills. •a tail that extends behind the anus

There are three subdivisions of the chordates: 1.Cephalochordata 2.Tunicata The cephalochordates and tunicates never develop a vertebral column. They are thus "invertebrates" and are discussed with the other invertebrates. 1.Craniata The vast majority of chordates have a skull enclosing their brain, eyes, inner ear, etc.). All but one group of these (the hagfishes) also convert their notochord into a vertebral column or backbone thus qualifying as vertebrates.

FIVE CHORDATE HALLMARKS

1. Notochord

2. Dorsal Tubular Nerve Cord

3. Pharyngeal Pouches and Slits

4. Endostyle or Thyroid Gland

5. Postanal Tail

An endostyle is a longitudinal ciliated groove on the ventral wall of the pharynx which produces mucus to gather food particles

Vertebrate characteristics: 1 - notochord (at least in the embryo) 2 - pharynx with pouches or slits in wall (at least in the embryo) 3 - dorsal, hollow nervous system

4 - vertebral column

1. This feature as well as the other three is always found at some embryonic stage of all chordates.

2. The notochord is the first part of the endoskeleton to appear in the embryo.

3. It serves as an axis for muscle attachment; it can bend without shortening and permits undulation.

4. In protochordates and jawless vertebrates, the notochord persists throughout life.

5. In vertebrates, a series of cartilaginous or bony vertebrae form from mesenchymal cells derived from blocks of mesodermal cells lateral to the notochord.

6. In most vertebrates, the notochord is entirely displaced by vertebrae although it remains persistant as the intervertebral discs.

NOTOCHORD

The notochord is made up of layers, where the external three are of collagen and elastin. The central part contains germinative chordoblasts that surround a core of large, vacuolated cells.

Notochord = rod of living cells ventral to central nervous system & dorsal to alimentary canal

Fate of notochord during development: •Head region - incorporated into floor of skull •Trunk & tail - surrounded by cartilaginous or bony vertebrate (except in Agnathans) Adults: •Fishes & amphibians - notochord persists the length of the trunk & tail but is constricted within the centrum of each vertebra •Reptiles, birds, & mammals - notochord almost disappears during development (e.g., remains as a pulpy nucleus in the vertebrae of mammals) •Protochordates - notochord remains as the chief axial skeleton •Agnathans - lateral neural cartilages are located on notochord lateral to the spinal cord

DORSAL TUBULAR NERVE CORD

1. In most invertebrate phyla, the nerve cord is ventral to the alimentary canal and solid.

2. In chordates, the single cord is dorsal to the alimentary canal and is tubular.

3. The anterior end enlarges to form the brain.

4. The cord is produced by the infolding of ectodermal cells on the dorsal side of the body.

PHARYNGEAL POUCHES AND SLITS

1. Pharyngeal slits lead from the pharyngeal cavity to the outside.

2. They form by the inpocketing of the outside ectoderm and the evagination of the pharynx endoderm.

3. In aquatic chordates, the two pockets break through to form the pharyngeal slit.

4. In amniotes these pockets may not break through and only grooves are formed.

Pharynx - region of alimentary canal exhibiting pharyngeal pouches in embryo; pouches may open to the exterior as slits:

•permanent slits - adults that live in water & breathe via gills •temporary slits - adults live on land

Dorsal, hollow central nervous system - consists of brain & spinal cord & contains a central cavity (called the neurocoel)

5. In tetrapods, the pharyngeal pouches give rise to a variety of structures, including the Eustachian tube, middle ear cavity, tonsils and parathyroid glands.

6. The perforated pharynx functions as a filter-feeding apparatus in protochordates.

7. Pharyngeal pouches or slits are not unique to chordates; hemichordates also have pharyngeal slits.

8. Fishes added a capillary network with gas-permeable walls; this network evolved into gills.

ENDOSTYLE OR THYROID GLAND

1. Recently, the endostyle was recognized as a shared chordate character.

2. The endostyle or its derivative, the thyroid gland, is found in all chordates.

3. Some cells in the endostyle secrete iodinated proteins homologous with the iodinated-hormone-secreting thyroid gland of adult lampreys and the remainder of vertebrates.

POSTANAL TAIL

1. The postanal tail, plus musculature, provided motility for larval tunicates and Amphioxus to swim.

2. This was increased in fishes but became smaller or vestigial in later lineages.

SUBPHYLUM UROCHORDATA: TUNICATA 1. There are about 3000 species of tunicates identified.2. They occur in all seas and at all depths.3. Most are sessile as adults although a few are free-living.4. The tunic is the tough, nonliving test that surrounds

them and contains cellulose. 5. In most species, only the larval form bears all the

chordate hallmarks; adults lose many of these characters.

6. During adult metamorphosis, the notochord and tail disappear; the dorsal nerve cord is reduced.

7. Urochordata is divided into Ascidiacea, Larvacea and Thaliacea.

SUBPHYLUM VERTEBRATA

1. Chordates with backbones or spinal columns.

2. About 57,739 species of vertebrates have been described.

3. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals.

4. Fish (including lampreys, but traditionally not hagfish, though this is now disputed), amphibians, reptiles, birds, and mammals (including humans) are vertebrates.

CLASSIFICATION OF THE PHYLUM CHORDATA

Group Protochordata (Acrania) Subphylum Urochordata Subphylum Cephalochordata Group Craniata Subphylum Vertebrata Superclass Agnatha Class Myxini

Class Cephalaspidomorphi Superclass Gnathostomata Class Actinopterygii Class Sarcopterygii Class Amphibia Class Reptilia Class Aves Class Mammalia