Lecture 11: Vertebrate Skeletal System 3

Question 1

The pectoral girdle is composed of dermal and endochondral segments. Describe the dermal components.

Answer 1

• Dermal components support endochondral components and articulate them with axial skeleton.
• Dermal components provide surfaces for muscle attachment.
• Dermal components include:
• Cleithrum (-a): Main pectoral elements in bony fishes (incl. crossopterygians, lung fishes, and chondrosteans)
• Clavicle: Lost in holosteans and teleosts.
• Interclavicle

Question 2

The pectoral girdle is composed of dermal and endochondral segments. Describe the endochondral components.

Answer 2

• Endochondral components carry the limb articulation:
• Glenoid cavity
• Endochondral components serve as major base of attachment for limb muscles.
• Endochondral components include:
• Procoracoid
• Coracoid
• Scapula

Question 3

Describe the shark pectoral girdle

Answer 3

• In the shark and other chondrichthians, the pectoral girdle only consists of endochondral components.
• The main component is the coracoid bar:
• Carries the fin attachment
• Is typically “U”-shaped
• Scapular processes sit on top of lateral ends of coracoid

Question 4

Describe the pectoral girdle in bony fish

Answer 4

• In many osteichthyes, all crossopterygians, lungfishes, and chondrosteans, the cleithrum and clavicle of the pectoral girdle are united with the dermal components of the skull. Other dermal components in bony fishes may include the supracleithrum and posttemporals.
• The clavicle is lost in holosteans and teleosts.
• Be able to identify the Opercular, Subopercular, and Interopercular in the drawing on slide 7

Question 5

Describe Dermal Components in Tetrapods

Answer 5

• In tetrapods, the dermal components are separate from the dermal skull components.
• The cleithrum is not found in any modern forms.
• The interclavicle appears in fossil amphibians (embolomerous amphibians).
• Clavicles and interclavicles are present in:
• • Lizards
• • Sphenodon
• In birds:
• • Clavicles + interclavicle = Furculum
• • Coracoids brace wings against sternum.
• Clavicles and interclavicles present in monotremes.
• Clavicles lost in many mammals.

Question 6

Describe the pectoral girdle in tetrapods

Answer 6

• Connection between pectoral girdle and skull is lost in fossil and modern amphibians, allowing for more freedom of movement of head.
• Cleithrum and clavicle are long, slender rods located along anterior margin of scapula in primitive tetrapods.
• Fate of original coracoid bar in tetrapods:
• • Becomes paired
• • Forms a scapulocoracoid cartilage on each side with a glenoid fossa in the middle:
• • Scapula is located above the fossa.
• • Coracoprecoracoid region is located below the fossa.
• Anurans:
• • Interclavicle is a new, diamond-shaped, unpaired element that appears first in fossil amphibians but is lost in modern amphibians.
• • Cleithrum is rudimentary in modern anurans.
• Salamanders:
• • Dermal components are lost in modern salamanders.
• • Much of the endochondral girdle remains unossified.

Question 7

Describe the pectoral girdle in the anurans

Answer 7

• Interclavicle is a new, diamond shaped, unpaired element.
• Appears first in fossil amphibians but is lost in modern amphibians.
• Cleithrum is rudimentary in modern anurans.
• Appears first in fossil amphibians but is lost in modern amphibians.

Note: The frog pectoral girdle consists of both dermal components and endochondral components. Most prominent in this photo are the large calcified suprascapular cartilages. From the venter are the coracoids and the clavicles. These serve as braces for the forelimbs when the frog lands after jumping.

Question 8

Describe the pectoral girdle in the amniotes: Reptiles and Turtles

Answer 8

• In Most reptiles:
• Cleithrum disappears early in reptiles.
• Endochondral girdle similar to that of fossil amphibians
• In crocodilians and dinosaurs, the glenoid fossa is formed by articulation of both the scapula and procoracoid, which meet at an angle at the fossa.
• In turtles:
• Dermal elements have disappeared into the plastron.
• Pectoral girdle is triradiate:
• Anterior ventral prong = downward extension of scapula
• Connected to clavicle plate embedded in plastron
• Posterior ventral prong = procoracoid

Question 9

Describe the pectoral girdle of the amniotes
Pelycosaurs
Therapsids and Monotremes
And Birds

Answer 9

• Pelycosaurs: Added a true coracoid
• Therapsids and monotremes:
• Development of acromion process
• Coracoid is fully developed in monotremes but is reduced to a coracoid process in other mammals.
• Birds:
• Endochondral pectoral girdle similar to dinosaurs.
• Coracoids serve as wing braces against sternum.
• Clavicle and interclavicle fuse to form furcula (“wishbone”)
• Note: The coracoid of reptiles and birds is really the procoracoid.

Question 10

Describe the pectoral girdle of the amniotes: primitive mammals, and other mammals.

Answer 10

• Primitive mammals (including monotremes):
• Both paired clavicles and interclavicle are present
• Other mammals:
• Interclavicles are lost
• Clavicles are often reduced or lost in running and bounding forms such as ungulates and carnivores.
• Bats have elongated clavicles and coracoid processes (along with elongated forelimbs).
• Entire coracoid plate, along with procoracoid, are lost in marsupials and placentals.
• Original anterior margin of scapula develops a shelf-like process (scapular spine) and an acromion process which serve as attachment sites for muscles that were originally attached to coracoid plate.
• The scapular spine separates the supraspinous fossa from the infraspinous fossa.
• Musculature that was originally attached to the coracoid plate has shifted to the scapula.

Question 11

Describe the sternum

Answer 11

• Found only in some tetrapods, including:
• Amphibians: Urodeles: cartilaginous plate Anurans: composed of several elements
• Birds: Well-developed keel (carina) attached to sternum of flying birds: provides a large surface area for attachment of flight muscles.
• • Note that a carina was also present in some pterosaurs = example of convergent evolution.
• • Birds with a well-developed carina = carinates; Birds without a well-developed carina = ratites.
• Mammals
• Lost in:
• Turtles
• Snakes
• Snakelike lizards

Question 12

Describe the pelvic girdle

Answer 12

• The pelvic girdle lacks dermal components in all vertebrates.
• The pelvic girdle consists of three paired endochondral bones:
• Pubis: Located ventral and anterior to the other two components.
• Ischium: Located ventral and posterior to the other two components.
• Ilium: Located dorsal to the other two components.
• All three bones carry limb articulation in most vertebrates:
• Acetabulular fossa
• Ilium articulates with sacral vertebra(e) either directly or via sacral ribs.
• Pubis and ischium are primarily for muscle attachment.
• In bony fishes, the pelvic girdle consists of a pair of ventral triangular plates fused along their midline. They are never articulated with the axial skeleton.

Question 13

Describe the pelvic girdle of sharks

Answer 13

• The shark pelvic girdle is entirely cartilaginous.
• The main portion is the puboischiadic bar with two small spurs that are called the iliac processes.
• These cartilages are probably not homologous with elements of the same names in the tetrapod pelvic girdle.
• SAT Slide 25

Question 14

Describe the pelvic girdle in tetrapods

Answer 14

• In general, the ventral part of the tetrapod pelvic girdle ossifies from two centers:
• Collectively this ossification forms the puboischiadic plate:
• The puboischiadic plate provides an area of origin of many of the hindlimb muscles.
• The obturator foramen is an opening between the pubis and ischium which provides a passageway for a nerve (obturator nerve) supplying the limb muscles.
• The third element, the ilium, lies dorsal to the other two.
• The acetabulum develops above the puboisciadic plate and forms from all three elements except in crocodilians.
• The ilia are attached to the sacral vertebrae, and the pubis and ilium from each side fuse to each other along the midline.
• This creates a bony ring through which must pass the digestive, urinary, and genital systems.
• The size of the bony opening is an important factor in vertebrates that lay large eggs or bear live young.
• See Slide 28

Question 15

Describe the Necturus Pelvic Girdle

Answer 15

• In the Necturus all three pairs of elements are represented in the pelvic girdle.
• The ilia are in the form of long slender bars that articulate with the single sacral vertebra.
• The ischia are paired, flat, posterior plates.
• The pubis is cartilaginous in the form of an anterior triangular-shaped plate of cartilage.
• See Slide 29

Question 16

Describe the pelvic girdle in frogs

Answer 16

• In the frog pelvic girdle, the elements are ossified.
• The single slender element in the middle is the caudal vertebra (urostyle).
• The two long slender elements on either side are the ilia.
• The ischia are fused into a single plate posteriorly, below which are the fused pubic bones.

Question 17

Describe the pelvic girdle of turtles

Answer 17

Seen are the fused pubic bones and the fused ischium. The ilium is directed dorsally. May not be NTK.

Question 18

Describe the pelvic girdle in dinosaurs

Answer 18

• True dinosaurs are divided into two groups based on the structure of their pelvic girdle:
• Saurischians (lizard-like hip)
• • Triradiate pelvic girdle
• Ornithischians (bird-like hip)
• -Tetraradiate pelvic girdle

Question 19

Describe the saurischian pelvic girdle

Answer 19

• The modern alligator pelvic girdle (left) and the extinct saurischian dinosaur pelvic girdle (right) are examples of the saurischian type of pelvic girdle. Note that acetabulum is open in both examples.
• The pubic bones (PU) are fused anteriorly and form one prong. The two ischia form two additional prongs, forming a triradiate pelvic girdle.
• SAT: Slide 33

Question 20

Describe the ornithischian pelvic girdle

Answer 20

• The bird pelvic girdle (left) and the extinct ornithischian dinosaur pelvic girdle (right) are examples of the ornithischian type of pelvic girdle.
• The pubic bones (PU) are separate and in the dinosaur, the pubic bones have cranial extensions (tetraradiate).
• SAT 34

Question 21

Describe the mammalian pelvic girdle

Answer 21

• In mammal-like reptiles and mammals, the ilium extends anteriorly rather than dorsally or posteriorly.
• In marsupials (i.e., opossum) marsupial bones may extend anteriorly from the pubic bones to support the marsupium (pouch).
• The obturator foramen is large and may represent the pubioischiadic fenestra found in the pelvic girdles of other tetrapods such as turtles. The passageway for the obturator nerve and artery is found along a groove in the superior pubic ramus
• In life, the obturator foramen is mostly covered by a fibrous membrane, the obturator membrane, which increases the surface area for muscle attachment.
• The acetabulum is closed, compared to many reptiles, and opens either laterally or lateroventrally. As in most tetrapods, it is formed from all three pelvic bones.
• The ilium forms a strong joint (the sacroiliac joint) with multiple sacral vertebrae.

Question 22

Describe the major features of the four limb elements

Answer 22

• Humerus:
• Single, proximal bone of the forelimb
• Head articulates with glenoid fossa of scapula
• Distal end articulates with ulna (trochlea) and radius (capitulum)
• Ulna:
• In anatomical position, ulna and radius are parallel and in the same plane with ulna lying medial to radius.
• Semilunar notch of ulna forms hinge joint with trochlea of humerus
• Olecranon process of ulna serves as lever arm for triceps complex.
• Radius:
• Concave depression on head (proximal end) of radius rotates on captiulum of humerus.
• Narrow neck of radius distal to head spins within annular ligament attached to ulna.
• Note that during pronation and supination, ulna is fixed, while head of radius spins and distal end of radius rotates over head of ulna.
• Wrist articulation is between distal radius and proximal bones of carpus.

Question 23

Describe the phalangeal formulas

Answer 23

• The tetrapod hand typically consists of five digits with varying numbers of phalanges. Phalangeal number may be indicated by listing numbers of phalanges per digit beginning with the radial side:
• Primitive and mammal-like reptiles: 2:3:4:5:3
• Advanced reptiles and modern mammals: 2:3:3:3:3
• Humans: 2:3:3:3:3

In cetaceans (whales) and extinct marine reptiles (i.e., icthyosaurs) the phalangeal formula may reach as high as 13 or 14 in a single digit. = hyperphalangy

Question 24

Describe the major features of the hind limb elements

Answer 24

• Femur:
• The femur possesses a greater trochanter.
• In many mammals there is also a fourth trochanter.
• The internal trochanter of the reptilian femur is replaced by the lesser femur in mammals.
• The adductor crest is for the attachment of the adductor muscles.
• It may be replaced by the linea aspera in some mammals (i.e., humans).
• Tibia:
• The tibia possesses an anterior crest, the cnemial crest which forms the anterior border of the tibia (shin).
• Fibula:
• Many mammals (i.e., humans) possess a separate fibula that plays a role along with the tibia in forming the ankle (tibio-tarsal) joint.
• In other mammals, the fibula may be partially or completely fused with the tibia.

Question 25

Describe the evolution of the tetrapod limb

Answer 25

• Tetrapods may have evolved from a crossopterygian ancestor.
• i.e.: the rhipidistian Eusthenopteron
• Presumably, then, the tetrapod limb can be traced back to the crossopterygian lobe fin.
• All tetrapod limbs are built on the same basic anatomical plan.
• Forelimbs and hind limbs are almost identical.
• In the crossopterygian limb skeleton, fin bones may be arranged in several series that correspond to the series of bones in the tetrapod limb.
• In the rhipidistian Eusthenopteron, the series of fin bones are even more like those of a tetrapod limb
• See Slide 57

Question 26

Describe the components of a tetrapod limb

Answer 26

• Stylopod:
• Proximal single element: Humerus or femur
• Articulates with girdle
• Zeugopod:
• Two parallel elements: Radius (fibula) and ulna (tibia)
• Autopod(proximal to distal):
• Carpals (tarsals) - Basipodium
• Metacarpals (metatarsals) - Metapodium
• Phalanges - Acropodium

Question 27

Describe the elements of the carpus and tarsus

Answer 27

• The carpus and tarsus primitively consisted of twelve elements:
• Three proximal elements: radiale, intermedium, ulnare in carpus and fibulare, intermedium, and tibiale in the tarsus.
• Four centralia
• Five distal carpalia or tarsalia
• See Slide 59

Question 28

Describe the centralia in the formula in tetrapod forelimbs

Answer 28

• In the forelimb, the centralia are almost always reduced:
• There are never more than two in primitive reptiles
• A single centrale is a common reptilian and mammalian pattern.
• The fifth distal carpal element is typically lost.
• The phalangeal formula for both primitive and mammal-like reptiles = 2-3-4-53.
• In advanced reptiles the phalangeal formula = 2-3-3-3-3.

Question 29

Describe the phalangeal formula in extinct reptiles and cetaceans.

Answer 29

• In cetaceans and extinct marine reptiles the phalangeal formula may reach as high as 13 or 14 in a single digit.
• This is referred to as hyperphalangy.
• It is an example of convergent evolution.

Question 30

Describe the intratarsal joint in reptiles

Answer 30

• In the hind limb, the general pattern is for the calcaneus (heel) to develop from the fibulare, while the astragalus (talus or ankle bone) develops from the tibiale + intermedium + centrale.
• Reptiles also have an intratarsal joint:
• In crocodilians foot movement occurs between the astragalus and the calcaneus.
• In dinosaurs the astragalus and calcaneus are closely attached to the fibula and the tibia, and distal tarsals are joined to the metatarsals.
• In birds, the situation is similar to that in dinosaurs.

Question 31

Describe the phalangeal structure of ungulates.

Answer 31

• Ungulates are the hoofed mammals.
• There tends to be a reduction of the digits in both forelimb and hind limb.
• Perissodactyls:
• The axis of the limb extends through the middle (third) toe.
• These mammals tend to reduce or lose digits lateral to the third toe, and in modern forms have one or three toes (odd-toed).
• Examples include the horse with one toe and the rhinoceros with three toes.
• Tapirs have have four toes, but the axis still runs through the third toe, so they are also perissodactyls.
• Artiodactyls:
• The axis of the limb extends between the third and fourth toes.
• These mammals have an even number of toes—often two (cloven-hoofed).
• Examples include camels, pigs, sheep, and cattle.