LECTURE OUTLINE PART 1
WHY SHOULD WE STUDY ANATOMY OF THE VERTEBRATES IN THIS MANNER?
1. Illustration of basic biological principles
Evolution
Adaptation
2. Practical applications
Animal Biology
Medical Sciences
..enhances explanation (five fingers, appendix etc.)
3. Scholarship value………to learn
THE HISTORY OF ANATOMY AS A DISCIPLINE
The first interest in anatomy centered around humans….
Galen (ca. 250 a.d.)
Greek physician and writer
Wrote a treatise on Human Anatomy based on
Barbary Apes……this was accepted dogma for
over 1200 years!
Andreas Vesalius (ca. 1550)
Wrote and illustrated De Humani Corporis Fabrica
Carl Linné (ca. 1750)
Wrote Systema Naturae
Developed the concept of binomial nomenclature
Georges Cuvier (ca. 1800)
Wrote Histoire Naturelle
Established Vertebrate Paleontology and Comparative
Anatomy as biological disciplines
The “father” of Comparative Anatomy
Jean Baptiste de Lamarck (ca. 1800)
Early champion of evolutionary change but his
proposed mechanism (inheritance of acquired
characters) was faulty.
Cleared up a number of invertebrate taxonomic
problems
Louis Agassiz (ca. 1850)
Swiss paleontologist who worked with fossil fishes
and started the MCZ at Harvard.
Did not believe in evolutionary change.
Charles Darwin (ca. 1850)
Wrote The Origin of Species which outlined the
mechanism for evolutionary change….natural
selection
Darwin stressed the importance of variation.
Genotypic Variation can be expressed three ways in the phenotype…..
Physiology, Morphology and Behavior
Phenotypic variation can be easily measured and reflects the relative success of variants.
The frequency of variation actually represents the relative reproductive success (or offspring produced) of animals……this is known as Darwinian Fitness.
Organisms produce more offspring than necessary to replace the parents, and because there is genotypic and phenotypic variation in these numerous offspring, they show differential survival.
Genotypic Abundant Differential Natural
Variation + Offspring + Survival = Selection
= Differential Reproductive Success of Genotypes
Over time, and in the face of environmental change, animals may exhibit adaptation……
From a comparative anatomical standpoint:
Adaptation: Any structure contributing to the fitness of an organism and brought into existence for its current function through the process of natural selection.
Corollaries:
Aptation: Any structure contributing to the individual fitness of an organism.
Exaptation: Any structure contributing to individual fitness but not brought into existence by natural selection for its current function.
e.g. the avian feather, the rattlesnake rattle
Additionally, we need to make a distinction between the terms “Anatomy” and “Morphology”……
· Anatomy connotes mere observation and description
· Morphology interprets structures from an evolutionary perspective
In order to properly understand morphological and phylogenetic relationships, we must understand the following concepts:
Homology: Relationship between structures that share common ancestry.
Human, horse and seal forelimb
Homoplasy: Relationship between structures that share common appearance
Bird, bat and pterosaur wings
Analogy: Relationship between structures that share common function.
Bird and insect wing
Structures in different organisms may exhibit more than one of these relationships…
Derived Characters: Evolutionarily new (additional)
characters relative to typical closely related species
Specialized vs. Generalized
Specialized: Showing many derived characters
relative to closely related species
Generalized: Showing characteristics common to
many organisms across broad taxonomic lines.
Primitive vs. Advanced:
Primitive: Closer to the origin of a particular lineage
Advanced: Closer to the terminus of a particular
lineage
Simple vs. Complex:
Simple: Structures lacking complexity relative to
other similar structures
Complex: Structures exhibiting more components and
features relative to other similar structures
Rudimentary vs. Vestigial:
Rudimentary: Structure not showing level of
development typical of same structure in other
taxa….but with evidence that it could proceed to
that level.
Vestigial: Structure that appears to be moving in a
direction of a lower level of development from what
it is in other taxa.
Parallelism vs. Convergence:
Parallelism: Independent development of similar
character(s) from a common ancestor potential
Convergence: Independent development of similar
character(s) without common ancestor potential
Coevolution: Character changes resulting from predator-
prey “tug-of-war”, and other reciprocal competitive
relationships.
Lateral, Medial, Proximal, Distal
Sagittal, Transverse and Frontal planes
Phylum Chordata
Subphylum Urochordata
Subphylum Cephalochordata
Subphylum Vertebrata
These groups share the following characters:
1. Dorsal hollow nerve cord
2. Pharyngeal gill slits at some point in their life
3. Notochord at some point in their life
4. Postanal tail
How do the vertebrates relate to other members of the Animal Kingdom?
Generally, this depiction of animal phyla implies a rather linear relationship….i.e. mollusks gave rise to the arthropods and the arthropods led the echinoderms, etc.
In reality, these extant taxa were derived from ancestral groups common to both or all three groups.
Today we use PHYLOGENETIC SYSTEMATICS or CLADISTICS to attempt to work out these relationships.
The resulting cladograms give us a good estimate of taxonomic relationships.
Using these techniques, we know that the higher invertebrates diverge into two groups…..
The Echinoderms on one hand, and the Mollusks, Annelids and Arthropods on the other……
…..and the Echinoderms have a close evolutionary relationship to the Chordates.
It’s through the Echinoderm ancestry that we can trace the origin of the vertebrates.
But what evidence is there for this?
In order to understand the evidence, we must first examine some of the details of early animal development….
Zygote through Gastrulation.
The zygote undergoes cleavage until there is a hollow ball stage….this process is blastulation and the result is the blastula.
The blastula then shows a unilateral invagination, the result of which is an interior cavity known as the archenteron or primitive gut. This developmental stage is now known as the gastrula and this process is called gastrulation. The opening to the Archenteron is called the blastopore.
Mollusks, Arthropods, Annelids
Deuterostomous: Blastopore forms anus
Echinoderms, Chordates
Ectoderm
Endoderm
Mesoderm
Mesoderm leads to the development of numerous organs and organ systems, and is essential to the formation the coelom or body cavity.
Two ways in which the body cavity forms:
Schizocoely: Mesoderm splits to form coelom
Enterocoely: Mesoderm balloons from gut forming
Coelom
The Echinoderm-Chordate connection:
Primitive sessile arm feeders gave rise to echinoderms and to the hemichordates….the latter developing a pharyngeal filter feeding mechanism…..and Hemichordates are related to Echinoderms due to a strong larval similarity.
Hemichordates possess 2 of the 4 major chordate characteristics….pharyngeal gill slits and dorsal hollow nerve cord
The Echinoderms, Hemichordates and Chordates share a number of characters not shared with in most invertebrates.
Deuterostomy, Entercoelous Radial Cleavage, Mesodermal
Skeleton,
The Chordates contain the vertebrates, the lancelets and the tunicates….the latter of which is most primitive, and which probably gave rise to early vertebrates.
The life cycle of Urochordates exhibits larvae that are free swimming, pharyngeal gill-feeders, that greatly resemble the lancelets and primitive vertebrate larvae (lamprey ammocoete larvae)
This has led to three theories as to the origin of the vertebrates.
Garstang Theory (1920s): Larval stage of urochordates and others becomes reproductive; adult stage is lost, through paedomorphic progenesis
Gutmann Theory (1980s): Chordate-like ancestor developed within the invertebrates and echinoderms, hemichordates and urochordates were offshoots.
Dipleuruloid Theory (of Jollie; 1980s): Small, ciliated and bilateral form was ancestral to echinoderms and protochordates and progressively became more active and vertebrate-like.
Paedomorphosis: Larval Reproduction
Progenesis: Somatic development ceases; reproductive system matures; in adult all (or most) other characters remain larval.
Neoteny: Somatic development slows for some features; reproductive and other systems mature; retention of one or a few larval features in the adult.
PHYLUM CHORDATA
SUBPHYLUM UROCHORDATA
SUBPHYLUM CEPHALOCHORDATA
SUBPHYLUM VERTEBRATA
SUPERCLASS AGNATHA
CLASS PTERASPIDOMORPHI
CLASS MYXINI
CLASS CEPHALASPIDOMORPHA
SUPERCLASS GNATHOSTOMATA
CLASS PLACODERMI
CLASS ACANTHODII
CLASS CHONDRICHTHYES
SUBCLASS ELASMOBRANCHII
SUBCLASS HOLOCEPHALI
CLASS OSTEICHTHYES
SUBCLASS SARCOPTERYGII
SUPERORDER DIPNOI
SUPERORDER CROSSOPTERYGII
SUBCLASS ACTINOPTERYGII
SUPERORDER PALAEONISCIFORMES
SUPERORDER NEOPTERYGII
CLASS LABYRINTHODONTIA
CLASS AMPHIBIA
SUBCLASS LEPOSPONDYLI
SUBCLASS LISSAMPHIBIA
ORDER CAUDATA
ORDER ANURA
ORDER APODA
CLASS REPTILIA
SUBCLASS PARAREPTILIA
ORDER TESTUDINATA
SUBCLASS EUREPTILIA
ORDER SQUAMATA
ORDER CROCODYLIA
ORDER SPHENODONTIDA
SUBCLASS SYNAPSIDA
CLASS AVES
SUBCLASS SAURIURAE
SUBCLASS ORNITHURAE
SUPERORDER PALAEOGNATHAE
SUPERORDER NEOGNATHAE
CLASS MAMMALIA
SUBCLASS HOLOTHERIA
INFRACLASS MONOTREMATA
INFRACLASS MARSUPIALIA
SUBCLASS THERIA
INFRACLASS EUTHERIA
VERTEBRATE ONTOGENY
Early vertebrate development follows the pattern:
Cleavage ---------> Blastulation ----------> Gastrulation
…..the details of this process depend on the amount and distribution of yolk in the egg and zygote
Eggs may be classified by the amount of yolk present:
1. Microlecithal (little yolk)
2. Mesolecithal (moderate yolk)
3. Macrolecithal (much yolk)
Eggs may be classified by the distribution of yolk:
1. Isolecithal (even distribution)
2. Telolecithal (polarized distribution)
Based on the amount and distribution of yolk there are three cleavage patterns:
1. Holoblastic (cleavage furrows through entire Zygote)
2. Meroblastic (cleavage furrows through only portion of the zygote)
3. Discoidal (cleavage furrows restricted to
the cap of the zygote)
Taxonomic Patterns of Blastulation
Lancelets, Placental Mammals
Microlecithal, Telolecithal, Holoblastic
Primitive Fish, Amphibian
Mesolecithal, Telolecithal, Holoblastic
Elasmobranchs, Teleosts
Macrolecithal, Telolecithal, Meroblastic
Retiles, Birds, Monotremes
Macrolecithal, Telolecithal, Discoidal
Vertebrate Gastrulation
Two patterns:
1. Invagination
Lancelets, Anamniotes
2. Delamination
Amniotes
Other Developmental events:
Amniotes are characterized by the possession of the Amnion during development, along with three other membranes…….These are known as the Extraembryonic Membranes.
Each arises uniquely relative to the germ layers involved.
Derivation of the extraembryonic membranes:
Amnion…………….Ectoderm + Mesoderm
Chorion…………….Ectoderm + Mesoderm
Allantois……………Mesoderm + Endoderm
Yolk Sac……………Mesoderm + Endoderm
Neural Tube Formation
The vertebrate dorsal hollow nerve cord forms in the ectoderm, beginning with the neural groove which proceeds to form a neural tube.
As the neural tube closes dorsally, bilateral germ cells are formed and “pinch off”. This mass of cells is known as the Neural Crest.
Neural Crest cells later migrate to other parts of the body to form a variety of structures:
Dorsal Root ganglion Neurons
Sympathetic and Parasympathetic Ganglia
Chromaffin and Calcitonin Cells – Produce hormones
Schwann Cells
Parts of the Meninges
Branchial Cartilage Cells
Pigment Cells (except retina and CNS)
Odontoblasts
Facial Dermis
Vasoreceptors
Sensory Capsules and parts of the Neurocranium
Cephalic Armor and Derivatives
Somites, Coelom Formation and Mesodermal Differentiation
Segmental masses of Mesoderm arise dorsally and laterally following gastrulation. These masses are known as Somites.
As development proceeds, the somatic tissue expands and migrates. This leads to coelom formation and differentiation into three distinctive regions:
Epimere
Sclerotome…………… Vertebral Column
Myotome……………….Trunk musculature
Dermatome…………… Contributes to Skin
Mesomere…………………. Kidneys, Gonads
Hypomere
Somatopleure………..Parietal Linings
Splanchnopleure…….Visceral Linings
Vertebrate Birthing Summary
A. Ovipary (Egg-laying)
B. Vivipary (Live-bearing)
1. Lecithotrophic
2. Placentotrophic
FUNCTIONAL RELATIONSHIPS
Size and Shape
As animals increase in size, their morphological characters do not do so proportionally
Geometric similarity of animal’s support system does not (and cannot) pertain to the entire body size range.
Consider the following relationships:
Surface Area (S) ∞ l2
Surface area of an object increases in proportion to the square of its linear dimension
Volume (V) ∞ l3
Volume of an object increases with the cube of its linear dimension.
For example:
In a sphere… Diameter increases 10 times
Surface area increases 100 times
Volume increases 1000 times
This has consequences…
Consider an animal’s legs the shape of which is
likened to a column.
A column’s strength is proportional to its x.s. area.
As an animal grows larger, legs must get larger faster than trunk to keep up with volume…hence heavier limbs.
Example: The Elephant, which is Graviportal
Here, large size is adaptive for a number of reasons:
1. Free from predation
2. Ability to traverse great distances
3. Less food required per unit body weight
4. Low surface to volume ratio…heat up, cool down more slowly
We often see a disproportionate change in size between body parts…this is known as Allometry
Surface to volume ratio has an effect on metabolic rate
Heat loss ∞ Surface Area
Heat Capacity ∞ Volume
So, animal size profoundly affects its metabolism.
As animals increase in size, other morphological aspects of surface area must attempt to keep up with the volume:
Gut Kidneys
Teeth Cerebral Cortex
BIOMECHANICS
The Mechanics of Movement
Force (F) = Ma (Mass X Acceleration)
Force is a vector quantity…possesses magnitude and direction
Bone-Muscle systems act as machines….A machine is a mechanism that transmits force from one place to another. These forces can be illustrated in a simple bone-muscle system:
Fi = in force (applied to system)
Fo = out force (against resistance)
li = in lever arm (power arm) or
moment arm of pull
lo = out lever arm (load arm) or
moment arm of resistance
These quantities can be expressed as a ratio…e.g. between lo and li
lo
_____ = Velocity Ratio
li
The Velocity Ratio is a measure of how quickly a limb moves for any given unit of force.
Further, in a ratio expression between Fo and Fi,
Fo
______ = Mechanical Advantage
Fi
Mechanical Advantage is a measure of the efficiency of limb movement…doing more work for less energy output
Finally, the turning force can be calculated
Fi X li = in system Torque
Fo X lo = out system Torque
This is a measure of how strong the movement is…the power of the limb movement
The Mechanics of Body Support
As adaptations to counteract the effects of gravity, tetrapods have evolved a number of mechanisms. Many show similarities to man-made devices of similar function.
The “Ungulate Support Mechanism”
Acts as a sling
Allows balance of movement
Relieves stress on musculature
Structural elements are subject to three types of stress:
1. Compression
2. Tension
3. Shear
The response to stress is evident in bone, where areas of heavy stress are often reinforced with bone spicules = trabeculae
The gross morphology of bones is an expression of their response to stress
FLUID DYNAMICS
Vertebrates living in water are aquatic…..
· All fishes are Primary Swimmers …their
ancestors swam
· Other swimming vertebrates are Secondary
Swimmers…their ancestors passed through
a terrestrial stage
Advantages of swimmers over non-swimmers:
· Gain access to a wide variety of aquatic foods
· Escape terrestrial predators
· Oceans and inland waterways are favorable
avenues of dispersal
Organisms that swim in water have certain requirements:
1. They must reduce resistance
2. They must propel themselves through a dense medium
3. They must maintain orientation….steer
4. They must avoid harm from crushing pressures
The spindle-shaped body of most aquatic organisms is an adaptation to reduce Drag
Two types of Drag….
Frictional Drag: Resistance produced by layers of water sliding past each other
Pressure Drag: Force required to displace water---negative and positive pressures
Other adaptations to reduce drag and turbulence:
· Loss of a functional neck
· Reduction of projections from body
· Reduction in limb segments
Fishes, being primary aquatic organisms, show a large number of swimming adaptations…
The Sailfish is the fastest fish, showing sustained speeds of up to 68 m.p.h. It has a preponderance of red muscle and has special folds within which its dorsal fin fits to minimize turbulence and drag
MORPHOLOGICAL EFFICIENCY
Countercurrent Exchange
The passing of heat or diffusible materials between currents of gas or liquid passing one another in opposite directions.
This is the basis for a number of adaptations in vertebrates.
Energy Recycling
As a fish bends along its longitudinal axis during swimming the convex side stretches, and its elasticity contributes to the power stroke as it bends in the opposite direction.
END PART 1