Laboratory Exercise VIII, continued

Vertebrate skeletal system diversity demonstrations

1) Note the skull of Amia calva and accompanying diagrams:

                The skull of Amia demonstrates the variety of dermal (dermatocranium) bones present in fossil and primitive

extant fish that have been secondarily reduced or fused in tetrapods.  Note also the often clear distinctions among

splanchnic, chondrocranial and dermatocranium elements that are not quite so obvious in tetrapods.

Compare the specimen and labeled diagram with the generalized dermatocranium elements as summarized in

lecture.  Though not precisely the same, the two are quite similar.

2) Note the skulls of the bear and turtle:

                Note that, as an anapsid Reptile, the turtle possesses no skull fenestrae; while the bear, as a derivative of synapsid

Reptilian stock (via Pelycosaurs and Therapsids) possess a synapsid fenestra.  List some vertebrates that would

demonstrate the third form, the Diapsid skull.

3) Note the skulls of the Necturus and squirrel:

                The Necturus demonstrates the primitive vertebrate condition in which all the teeth appear the same (the

“homodont”; “same” “teeth”).  All amphibians and many reptiles (e.g. the Iguana) demonstrate this condition.

In contrast, most reptiles and all mammals demonstrate the “heterodont” (“different” “teeth”) condition.

4) Note the skulls of the squirrel, cat, and bear:

                Note in the squirrel the presence of a diastema, large incisors and prominent tympanic bulla

                In contrast, note in the cat the lack of diastema, prominent canines, carnassials and prominent zygomatic arch.

What insights can be gained about these two species’ diet based on their dentition?

As an omnivore, the bear possesses characteristics largely in between those of the squirrel and cat.  What

characteristics of dentition are intermediate and how are they intermediate?

5) Note the skulls of the shark and turtle:

                Note that the upper jaw of the shark is composed of the palatoquadrate cartilage, made of calcified hyaline

cartilage.  The lower jaw is Meckel’s cartilage.

The turtle illustrates well how, with the elaboration of the dermatocranium, elements of the splanchnocranium 

become reduced and “reassigned.”  The figure diagrams how Meckel’s cartilage forms a “scaffold” around which

dermal elements form.  The dermal elements indicated are the Dentary, Surangular, and Angular.  In the sea

turtle (the example given), the posterior tip of Meckel’s cartilage becomes ossified as the Articular bone that

articulates with the skull.  (What is the site on the skull of articulation with the jaw?)

On the skull note i) the dermal bones present, ii) the now-ossified splanchnocranium derived Articular bone and

iii) the cavity within the inside of the jaw that, in life, contained the rest of the cartilaginous Meckel’s cartilage.

Note also that turtles do not have teeth.  Rather, turtles posses a “beak” composed of heavily keratinized

epithelium that forms a sheath around the dentary bone.

6) Note the skulls of Necturus, the turtle and cat:

These three skulls demonstrate a sequence of development of the secondary palate- a “table” of bones that serves

to separate the flow of air from the nares to the trachea from the pharynx proper.  Necturus has no secondary

palate and the primary palate (composed of dermatocranium bones) is composed of the Vomer and Palatine

bones.  Note also the Premaxilla bone.  The Maxilla is missing, but would be found in a position lateral and

caudal to the premaxilla.

The turtle has a partial-bony secondary palate composed of processes (or “shelves”) that emerge from the

margins of the Maxilla.

The cat has a complete secondary palate composed of processes that emerge from the margins of the premaxilla,

maxilla and palatine bones.  This bony shelf, along with an extension of soft tissue (the “soft palate”) creates a

complete partition between the pharynx and the air channel between the nares and trachea.

7) Note the skull and associated structures of the Necturus and the plastimount of the cat hyoid:

First, some important terminology with respect to the visceral/splanchnic skeletal elements.  There are SEVEN (7) or less visceral arches, depending on the vertebrate considered.  In those vertebrates with jaws, the first two visceral arches have special status- the mandibular arch (visceral arch I) and the hyoid arch (visceral arch II).  In these vertebrates the remaining visceral arches are now termed the "branchial arches", starting with number 1 (the re-named 3^rd visceral arch).  Thus:

                                GENERALIZED VERTEBRATE:                                        JAWED VERTEBRATE:

                                Visceral arch I,                                                      è           Mandibular arch (JAW)

                                Visceral arch II,                                                     è           Hyoid arch (HYOID APPARATUS)

                                Visceral arch III,                                                   è           Brancial arch #1

                                Visceral arch IV,                                                   è           Branchial arch #2

                                Visceral arch V,                                                     è           Branchial arch #3

Etc…. (depending on the vertebrate)               è           Etc…

The hyoid apparatus of the Necturus is composed of elements derived from the 2^nd visceral arch (the “hyoid

arch”) through the 5^th visceral arch (the 3^rd branchial arch) as indicated in the accompanying diagrams. Identify the following

elements: body and cornua.

The hyoid of the cat is also composed of elements from the 2^nd - 5^th visceral arch.  Note the distribution of these

visceral arch elements across the cat hyoid apparatus.

The same holds true for the human, as shown in the accompanying diagram.

Limbs and the pectoral girdle

8) Note the forelimb demonstration in the glass-fronted box:

                This demonstrates the homologies among major vertebrates of the elements of the forelimb.  Using your

knowledge of the elements of the cat forelimb (on the right) as your guide, appreciate the conservation of

elements found in the bird, frog and turtle (in clockwise sequence from the cat).

9) Note the pig foot, horse lower leg, and diagrams of the generalized tetrapod è “Ungulate” series

                You should be familiar with the basic elements of the tetrapod limb.  Among the ungulate mammals- herbivores

specialized for swift- footedness.

They share the common  feature of walking on the “toes” (the phalanges indicated in the generalized tetrapod). 

However, there is a broad distinction made between those species the walk on an “odd” number and “even”

number of toes.

Artiodactyls (deer, camels, pigs, hippopotami, goats, antelope) have reduced the number of toes walked on to an

“even” number- the 3^rd and 4^th digits.  This is exemplified by the pig's foot.

Perissodactyls (tapirs, rhinoceros, zebra and horse) use an “odd” number of toes- the 2^nd, 3^rd and 4^th digits.

The extreme expression of this trend toward “odd” number of toes is shown by the horse and zebra, in which the

toes are reduced to a single digit (the 3^rd) and the remaining toes are reduced (m4 and m2 in the diagram) or lost. 

Note the extended single toe of the horse.  This is termed the “cannon bone”.  There is a remarkably complete

fossil record for the horse, clearly demonstrating the reduction of elements from 5 to 4 to 3 to 1 over the course of

several million years.

10) Note the plastimount of the shark pectoral girdle, and the skeletons of the Necturus, bird and cat.

                The trend among vertebrates is the establishment of an endochondral girdle composed of the Procoracoid and

Scapula that serve as the site of attachment for the forelimb.  Among the fish these elements are greatly

elaborated as a series of dermal elements that connect the endochondral elements to the skull. 

Among tetrapods, the trend in dermal element elaboration is reversed in gradual steps.

Note the elements of the primitive shark pectoral girdle- the Coracoid bar (a procoracoid), scapula and

Suprascapula.  All of these are endochondral.

Note the pectoral girdle of Necturus- a generalized primitive amphibian.

                Endochondral- coracoid (a procoracoid), scapula and suprascapula. 

The dermal elements have been secondarily lost in this animal.

Note the pectoral girdle of the bird.

                Endochondral- coracoid (a procoracoid) and scapula.

                Dermal- Clavicle and Interclavicle.  Together they form the Furcula or “wishbone”.

Note the pectoral girdle of the cat.

                Dermal elements have been reduced to a small sliver of a clavicle.

                The endochondral elements have been enlarged for the attachment of major limb muscle masses-

scapula (major element), Coracoid (a “true” coracoid, not the procoracoid of the previous species) that

is fused to the scapula as a “process”

Tail and axial skeleton

11) Note the cat and bird skeleton and string human vertebrae

                All three animals have a clearly defined sacral vertebral region- that point where the pelvic girdle meets the

vertebral column.

The tail of the cat forms a sequence of vertebral elements.

What are the equivalent structures in the bird and human, animals without tails?  In other words, what is the fate

of the vertebral elements of the tail in vertebrates without tails?