Chordata: Skeletal System & Classification

• The skeletal system consists of bone and cartilage, supporting the vertebrate body, providing skeletal muscle attachment, and storing minerals like calcium and phosphorus.
• The axial skeleton runs along the body's midline, including the skull, vertebrae, and ribs.
• The appendicular skeleton includes fins or limbs and their supporting bones (pectoral and pelvic girdles).

Phylum Chordata

• Includes mammals, birds, reptiles, amphibians, and fish.
• Named for the notochord, a flexible rod supporting the body and providing muscle attachment sites, often replaced by vertebrae.
• Synapomorphies of chordates include a notochord, pharyngeal gill slits, a muscular post-anal tail for locomotion, and a dorsal hollow nerve cord (precursor to the CNS).

Subphylum Cephalochordata (Lancelets)

• The first group to diverge from other chordates.
• Retain all four chordate synapomorphies.
• Marine burrowing suspension feeders that use fleshy cirri around the mouth to grab food particles from the water.
• They lack an appendicular skeleton and paired fins, making them poor swimmers.
• Tiny food particles are trapped in the mucus inside pharyngeal gill slits.
• Notochord, dorsal hollow nerve cord, and muscular post-anal tail are present.
• Segmental muscles produce side-to-side movement during swimming.

Subphylum Urochordata (Tunicates)

• Named for their tails.
• Larvae have all four chordate synapomorphies, whereas adults only have pharyngeal gill slits.
• Larvae are free-swimming; adults are sessile filter feeders attached to the ocean bottom, coated in a thick tunic.
• Larval tunicate anatomy includes an incurrent and excurrent siphon for water flow, pharyngeal gill slits, a notochord, a dorsal hollow nerve cord, and a muscular post-anal tail for swimming
• Adult tunicates possess an incurrent and excurrent siphon, an enlarged pharynx with pharyngeal gill slits, and a tunic.
• Adults do not have a notochord, dorsal hollow nerve cord, or tail

Subphylum Vertebrata (Vertebrates)

• Chordates with a head and brains protected by a skull.
• Vertebral elements enclose and protect the dorsal hollow nerve cord.
• Possess a heart with two or more chambers, erythrocytes with hemoglobin, and kidneys.

Class Myxini (Hagfishes)

• Have a cartilage skull, no jaws (but have palps), gills, and retain a notochord throughout life.
• Limited to small, soft, decomposing food due to fleshy mouths.
• Defend themselves with slime glands that produce clogging slime when attacked and mixed with sea water.
• Hagfishes have a small circular mouth lacking jaws, fleshy palps, multiple gill openings, a notochord, a dorsal hollow nerve cord, slime glands, and a muscular post-anal tail with a caudal fin fold.

Class Petromyzontida (Lampreys)

• Ammocoete larvae are superficially similar to cephalochordates
• Adults have a suction-cup-like mouth with rasping teeth; they attach to hosts to feed on blood.
• Adults have seven pairs of gill openings as well as pharyngeal gill slits in larvae.
• Lampreys possess a notochord, a dorsal hollow nerve cord, a muscular post-anal tail, and dorsal and caudal fin folds for locomotion.

Clade Gnathostomata (Vertebrates with Jaws)

• Vertebrates with jaws and a mineralized skeleton.
• Jaws broaden the variety of food sources.
• Some members have secondarily lost the mineralized skeleton and returned to cartilage.
• Median fins (dorsal, anal, caudal) provide stability, and paired fins (pectoral, pelvic) aid in steering.

Class Chondrichthyes (Cartilaginous Fishes)

• Sharks, rays, chimeras, and their relatives have skeletons mostly made of cartilage (derived trait).
• Possess a chondrocranium, cartilaginous gill arches, and jaws derived from anterior gill arches (palatoquadrate upper jaw and Meckel's cartilage lower jaw).
• Vertebrae replace the notochord and are composed mostly of cartilage with remnants of the notochord persisting as intervertebral discs.
• Paired pectoral and pelvic fins are supported by pectoral and pelvic girdles.
• Sharks use a dorsal fin for stability and caudal fin at the posterior end of body to propel themselves.

Class Actinopterygii (Ray-Finned Fishes)

• Represented by over 27,000 species with diverse feeding and reproductive strategies and sizes.
• Jaw movement is enabled by maxilla, premaxilla (upper jaw), and dentary (lower jaw) bones.
• An operculum covers and protects the gills, which are supported by bony gill arches.
• Vertebrae consist of a centrum, neural arch, neural spine, haemal arch, and haemal spine.
• Pleural ribs articulate with vertebrae.
• Dorsal and anal fins are along the midline, and the caudal fin is at the posterior end.
• Paired pectoral and pelvic fins are supported by pectoral and pelvic girdles.
• Muscular area between anal fin and caudal fin forms caudal peduncle.

Class Lissamphibia (Amphibians)

• Includes salamanders, frogs, and caecilians (6,150 species).
• Have four digits on forelimbs (if present), skin glands (mucous or poison), and fat bodies (energy storage).
• The foramen magnum is a large opening for spinal cord attachment, and occipital condyles articulate with the first vertebra.
• Consists of atlas (cervical vertebra), trunk vertebrae, and sacral vertebra.
• Frogs have a urostyle, while salamanders have caudal vertebrae.
• Pectoral girdle includes suprascapula, scapula, clavicle, and coracoid; pelvic girdle includes ilium, ischium, and pubis.
• Consists of humerus, radioulna, carpals, metacarpals, and phalanges.
• Hindlimbs includes femur, tibiofibula, astragalus, calcaneum, tarsals, metatarsals, and phalanges.

Clade Testudines (Turtles)

• Upper jaw (maxilla and premaxilla) and the lower jaw (mandible) lack teeth and are fused.
• Jaws are covered with a sharp, knife-like ridge of keratin.
• They possess a foramen magnum and a single occipital condyle.
• They have multiple cervical vertebrae for greater range of motion.
• Dorsal vertebrae are fused to the shell, and the sacral vertebra articulates with the pelvic girdle.
• The shell consists of a carapace (dorsal) and plastron (ventral).
• Their pectoral girdle includes a scapula, acromial process, and coracoid. and is modified to accommodate the shell
• Consists of humerus, radius, ulna, carpals, metacarpals, and phalanges.
• Their pelvic girdle includes ilium, ischium, and pubis.
• Hindlimbs includes femur, tibia, fibula, tarsals, metatarsals, and phalanges.

Clade Lepidosauria (Snakes, Lizards, Tuataras)

• Upper jaw (maxilla and premaxilla) and the mandible (lower jaw) are fused to different degrees.
• The quadrate bone in snakes attaches the dentary to the skull, allowing for a large jaw opening.
• Single occipital condyle for skull articulation with the first vertebra.
• Multiple cervical vertebrae enable a greater range of motion.
• Trunk vertebrae and sacral vertebra are part of dorsal body axis.
• Pectoral girdle includes a scapula and coracoid.
• Consists of humerus, radius, ulna, carpals, metacarpals, and phalanges.
• Pelvic girdle includes ilium, ischium, and pubis.
• Hindlimbs includes femur, tibia, fibula, tarsals, metatarsals, and phalanges.

Clade Crocodylidae (Crocodiles, Alligators, Caimans, Gharials)

• Have a thick, less mobile skull with tightly fused upper and lower jaws.
• Single occipital condyle for skull articulation with the first vertebra.
• Long series of thoracic vertebrae with ribs and lumbar vertebrae without ribs.
• Rib cage composed of sternal and pleural ribs.
• Pectoral girdle includes a scapula and coracoid.
• Consists of humerus, radius, ulna, carpals, metacarpals, and phalanges.
• Pelvic girdle includes ilium, ischium, and pubis.
• Hindlimbs includes femur, tibia, fibula, tarsals, metatarsals, and phalanges.

Clade Aves (Birds)

• Tightly fused upper and lower jaws covered by a hard keratin bill or beak.
• Single occipital condyle for skull articulation with the first vertebra.
• Multiple cervical vertebrae and thoracic vertebrae with ribs are present.
• Uncinate processes overlap and brace the ribs for flight stresses.
• Synsacrum (fused lumbar and sacral vertebrae) and sternum with a keel for pectoral muscle attachment.
• Pygostyle is a flattened plate from fused caudal vertebrae for feather attachment.
• Structure of avian wing: humerus, radius, ulna, carpometacarpus, and phalanges.
• Furcula, a fused clavicle often called the "wishbone".
• Long, thin scapula dorsal to the rib cage.
• Sturdy procoracoid articulating with the sternum and humerus.
• Pelvic girdle includes ilium, ischium, and elongated pubis.
• Hindlimbs includes femur, tibia, fibula, tibiotarsus, tarsometatarsus, and phalanges.

Class Mammalia

• Fused upper and lower jaws with visible sutures.
• Two occipital condyles for skull articulation with the first vertebra.
• Multiple cervical vertebrae and thoracic vertebrae with ribs are present.
• Lumbar vertebrae (lacking ribs), and sacral vertebrae are present.
• The sternum has sternal rib elements.
• Number of caudal vertebrae varies among species; the coccyx is present in humans.
• Mammalian pectoral girdle (cat) consists of a broad scapula and a small clavicle.
• Presence of humerus, radius, ulna, carpals, metacarpals, and phalanges.
• Mammalian pelvic girdle includes ilium, ischium, and pubis.
• Composed of femur, tibia, fibula, patella, tarsals, metatarsals, and phalanges.

Modifications for Locomotion in Mammals

• Cursorial (running): Limbs are directly under the body, clavicle is reduced or absent, and can be digitigrade (walking on toes) or unguligrade (walking on the tips of toes).
• Plantigrade (walking): Walking using posterior limbs, feet are modified to have a large area of stability e.g., humans, bears, and raccoons.
• Arboreal (climbing): Large clavicle pushes out the forelimb, long limbs, long digits, and a prehensile tail in some species.
• Saltatorial (jumping): Hindlimbs are longer than forelimbs, with a large tail for balance.
• Fossorial (burrowing): Strong, short forelimbs with wide bones and long claws.
• Volant (flying): Elongated metacarpals and phalanges support the skin forming the wing.