Vertebrate Adaptations Evolution of the Skeletal System

General Trends in the Skeleton n Simplification through bone loss, bone fusion, and ossification. n Reduced bone mass, and therefore, less energy invested in skeleton (important because mammals are endotherms). n Increased skeletal strength.

General Trends in the Skeleton n Improved articulations. n Loss of indeterminant growth and consequent improved articulations and strength. –Consider the consequences of allometric growth for an animal with indeterminant growth.

Evolutionary Trends Involving the Skull n Neurocranium (cartilage bone) –In early chordates, the neurocranium served as a support for the brain. –With the formation of sensory capsules (olfactory, optic, and otic) it assumed a protective function.

Neurocranium, Dermocranium, and Splanchnocranium

Sensory Capsules

Evolutionary Trends Involving the Skull –Bones contributed by the neurocranium: supraoccipital (nim = not in mammals). Exoccipitals Basioccipital (nim - except fused). Occipital condyles (amphibians and reptiles have 1, mammals have 2) Basisphenid (contains sella turcica) Presphenoid Mesethmoid (nasal septum)

Evolutionary Trends Involving the Skull –Bones contributed by the neurocranium: Petrous (houses inner ear) Mastoid Turbinate bones

Evolutionary Trends Involving the Skull n Dermocranium (dermal bone) –Protection for neurocranium –Aid in capturing food –Bones contributed by the dermocranium: Dorsal series –premaxilla –nasal –septomaxilla –maxilla

Evolutionary Trends Involving the Skull –Bones contributed by the dermocranium: Dorsal series continued –frontal –parietal –postparietal –jugal –squamosal –quadratojugal –tabulare

Evolutionary Trends Involving the Skull –Bones contributed by the dermocranium: Ventral series –premaxilla –prevomer –macilla –palatine –pterygoid –ectopterygoid –jugal –quadratojugal –parasphenoid

Evolutionary Trends Involving the Skull n Splanchnocranium (cartilage bone) –Composed of palatquadrate cartilage and Meckel’s cartilage. –The palatoquadrate becomes the quadrate in non-mammals, and the incus and alisphenoid in mammals. –Meckel’s cartialge becomes the articular in non-mammals, and the malleus in mammals.

Evolutionary Trends Involving the Skull n Overview: –There is a progressive assimilation of cranial components. –Multiplication of chondral elements. –Williston’s law (reduction in dermal bone) –Reduction of visceal jaws. –Evolution of sound conduction routes. –Evolution of mandibular suspensorium.

Evolutionary Trends Involving the Skull n Overview continued: –Dissociation of skull and pectoral girdle. –Reduction of interorbital space. –Progressive compounding of bones. –Division of occipital condyles. –Formation of temporal fossae. –Formation of secondary palate.

Formation of Temporal Fenestrae n Dermocranium is laid down over the neurocranium. n All cranial musculature is thus between the dermocranium and the neurocranium.

Evolution of Temporal Musculature

Formation of Temporal Fenestrae n With the advent of the amphibians, the dermocrnium began to interfere with the operation of the jaw musculature. –To allow for the belly of the adductor mandibulae during contraction, the amphibians evolved a temporal notch (this anapsid solution also occurs in the chelonians)

Formation of Temporal Fenestrae –Muscle attachment shifts from the neurocranium to the edges of the temporal fenestrae. Crocodilians: progressed little. Lizards and snakes: edge of fenestrae and top of dermocranium. Mammals: top of dermocranium.

Evolution of the Secondary Palate n Primary palate –Forms complete roof of mouth. –Broken only by intenal nares. –Retained in fishes and amphibians. –Problems: terrestrialization results in breathing problems when the mouth is open. Impossible to breathe when food is in mouth.

Evolution of the Secondary Palate –Bones of the primary palate prevomers parasphenoid palatines ectopterygoids

Evolution of the Secondary Palate n Solution to the problem –Snakes: the trachea extends far forward ventrally. –Turtles and lizards: a shelf is formed over the primary palate anteriorly - includes the maxillary and premaxillary bones, also the palatine to some degree.

Evolution of the Secondary Palate n Solution to the problem continued: –Crocodilians: the secondary palate extends completely over the primary palate, even more completely than mammals. i.e., in mammals the last 1/2 to 1/3 is soft. In crocodiles this facilitates manipulation of food under water. Probably not for breathing since they can go several hours without breathing.

Evolution of the Secondary Palate n Solution to the problem continued: –Mammals: extensive as in crocodilians - permits brathing while eating, this is necessary as mammals are endotherms and consequently have high metabolic rates. –Birds: reduced bony content, but still extensive soft tissue.

Evolution of Teeth. n All gnathostomes either have teeth, or evolved from ancestors with teeth. n Those without teeth have tooth-like structures. n True teeth: –Outer layer of enamel. –Deep layer of dentine.

Evolution of Teeth. n True Teeth continued. –Innermost pulp layer with connective tissue, blood vessels, and nerves. –Enamel equals 96% inorganic materials, very hard non-living substance. –Dentine is very bone-like, has living matter.

Evolution of Teeth. n Location of teeth. –Thought to be modified denticles originally found on all integumentary scales or plates over all the body in early fishes. –Denticles could thus occur wherever ectoderm was, i.e. as far back as the branchial bars of some fishes. –Trend towards limitation of the area of dispersal.

Distribution of Teeth

More on Teeth n Number of teeth: –trend toward reduction in number, but increase in size and anchorage. n Cycles of Replacement –Early vertebrates: continuous and unlimited –Primitive verts: polymodal replacement (many ways of replacement).

More on Teeth n Cycles of Replacement cont. –mammals and some reptiles: unimodal replacement. –Neither of these replacements occur all at once.

More on Teeth n Tooth form –modified with respect to diet. Crushing = rounded and flattened. Grinding = flattened (only mammals). Slashing = canines. Poison conducting (snakes, lizards, and Blarina). Shearing = carnassials (only mammals).

More on Teeth n Tooth form cont. –Occurrence of 2 types: heterodont. –Occurrence of 1 type: homodont. –Types of teeth reflect diet.

Functional Evolution of the Mandibular Suspensorium. n Initial detection of sound was via waves received through solids (ie gross structures of the body). n Derived condition involves detection of air-borne sound waves.

Functional Evolution of the Mandibular Suspensorium. n The sound conducting system in all vertebrates involved the mandibular arch and its attachments to the skull. It thus became necessary to consider the evolution of the mandibular suspensorium.

Functional Evolution of the Mandibular Suspensorium. –Paleostyly: the agnathan condition in which some of the visceal arches are directly associated with the skull. –Autostyly: exhibited by the placoderms. Here, the mandibular arch is suspended from the cranium by itself. In this condition there is intervention by the hyomandibula.

Morphology at bottom represents Autostyly. Paleostyly is not shown.

Functional Evolution of the Mandibular Suspensorium. –Euamphistyly: In the primitive post- placoderm fishes the epibranchial portion of the second visceral arch suspends the rear portion of the mandibular arch. This is a true double suspension. The hyomandibula is proximal to the otic capsule and also to the spirical. In this condition and the following, the hyomandibula is ideally suited for the transmission of sound waves directly to the otic capsule.

Euamphistyly is the second from from the bottom.

Functional Evolution of the Mandibular Suspensorium. –At this point, we have a division in our evolutionary scheme. Elasmobranchs and teleosts have a hyostylic suspension which is solely via the hyomandibula. This is OK in terms of sound conduction since they are aquatic. In this condition, the hyomandibula is the only link to the otic region of the skull.

Functional Evolution of the Mandibular Suspensorium. –The second evolutionary line contains those with the Metaautostyly condition, which is derived directly from euamphistyly, and is characteristic of non- mammalian tetrapods. In this condition: hyomandibula no longer serves in jaw suspension. Hyomandibula is modified as a columella, the inner ear ossicle of non-mammalian tetrapods.

Metautostyly is at the top left.

Serves only for the conduction of sound. What are the selective pressures? The importance of delicate air-borne sound waves and their relation to terrestrialiation. Recall that all this takes place in the vicinity of the spirical and its cavity. One end of the hyomandibula butted against the otic capsules, the other end against the spiracular cavity. This cavity acted as a resonating chamber for air- borne sounds (not possible in water?) Functional Evolution of the Mandibular Suspensorium.

When this cavity becomes covered by a membrane (tympanum - ear drum) it becomes known as the middle ear cavity and provides no dimunition of sound. Also, in this stage, the columella is supported via cartilaginous struts by the quadrate. It is thus able to detect both ground borne and air-borne sound waves. Ground waves are via the articular-quadrate articulation, struts, columella, and otic capsule.

Functional Evolution of the Mandibular Suspensorium. –From the metautostyly condition of non- mammalian tetrapods we see the evolution of Cranioamphistyly in mammals (birds still have the typical reptilian condition). Mammals were no longer ground crawling as were reptiles and thus any sound conduction via solids is almost completely gone. At the same time (circa Therapsids) the jaw was becoming shortened to facilitate leverage and differend feeding modes.

Functional Evolution of the Mandibular Suspensorium. The jaw articulation moved anteriorly. The major articulation was between the dentary - articular and squamosal-quadrate. Eventually the articulation became completely dentary - squamosal. Ultimately, the reduced quadrate and articular are functionless except for sound conduction. They lie close to the tympanum. The articular ultimately lies against the tympanum and becomes the malleus. The quadrate and columela (hyomandibula) become the icus and stapes respectively.

Cranioamphi- styly is at upper right.

Evolution of the Postcranial Skeleton. n Functional units of the post-cranial skeleton. –Visceal skeleton –Vertebral column –Ribs –Sternum –Girdles –Paired appendages –Unpaired appendages

Postcranial Skeleton n We need to know a little more about bone. n What sorts of forces operate on bony tissue? –Compression –Tension –Shear –Torsion

Forces operating on bone n Examples –Compression…….Graviportal limbs of elephants. –Shear……………..Greater trochanter of the femur. –Torsion…………...Vertebrae & Femur –Tension…………..Sternum

Forces operating on bone n Bone is living tissue, and accommodates whatever forces are applied to it. –As an example, someone who loses a lot of weight quickly will still possess a robust skeleton designed to carry a lot of weight. However, with time the skeleton will reabsorb a considerable amount of tissue and become more gracile.

Forces acting on bone. n We can look at cross-sections of bone and determine exactly what kinds of forces were applied to the bone. –Note - a bone is not solid in cross section. –“force lines” within the bone become ossified for increased strength.

Changes resulting from terrestrialization. n What are some of the problems associated with a terrestrial life style? –Support –Stability –Locomotion –Respiration –Dessication. n Note: some of these same issues are faced by aquatic forms as well.

In an aquatic environment, the water acts as a skeleton. Terrestrial organism often have their mass arranged over only a few points of support. Compare and contrast the articulations of the 2 joints shown here.