CVA: The Cranial

Skeletal System

The skeletal system provides support, protection, a lever system for movement, and serves as a reservoir for minerals like calcium and potassium.

Cranium

The cranium is the skull composed of three main components: Splanchnocranium, Chondrocranium, and Dermatocranium.

Splanchnocranium

The most ancient part of the skull, supporting the pharyngeal arches and jaws, formed via cartilage or bone.

Chondrocranium

A series of cartilage or bony elements that underlies the brain and forms the braincase in Chondrichthyes.

Dermatocranium

The outer casting of the skull formed of dermal bones; 1st arose in ostracoderms (agnathans) and providing the outer covering of the brain in most vertebrates.

Mandibular Arch

The first pharyngeal arch that gives rise to the jaw structures in vertebrates.

Metaustylic

A jaw attachment type where the quadrate connects to the articular bone found in most amphibians, reptiles, and birds.

Euautostylic

A jaw attachment where the mandibular arch connects directly to the skull without the hyomandibula, seen in placoderms.

Hyostylic

A jaw attachment type where the jaw is attached via the hyomandibula, found in most modern bony fishes.

Craniostylic (Sq—D)

A jaw attachment type where the dentary articulates with the squamosal (temporal) bone, characteristic of mammals.

Amphyistylic

A jaw attachment type attached to 2 articulations—hyomandibula (skull), palatoquadrate (skull)—found in early sharks, some Osteichthyes, and most modern sharks.

Temporal Fenestra

An opening in the skull behind the eye socket, significant in the evolution of synapsids and mammals.

Heterodonty

The condition of having different types of teeth for various functions, commonly found in mammals.

Diphyodonty

A dental condition characterized by having two sets of teeth during an individual's lifetime: primary (milk) teeth and permanent teeth.

Braincase

The portion of the skull that encases the brain, formed by fused cranial elements.

Operculum

A bony plate that covers the gill slits in many fish species.

Suction Feeding

A feeding mechanism enabled by mobile jaws and cranial kinesis in fish, allowing them to draw in food.

Secondary Palate

separates the nasal cavity from the oral cavity, allowing for simultaneous breathing, chewing, and suckling; some turtles and crocs have this but found in all mammals

Exoskeleton (some vertebrates have this)

Formed from integument (bone or keratin)

Endoskeleton

Froms from the medoderm

Pharyngeal Arches/ slits consists in 5 segements

Top/Bottom: Phayngobranchial, Epi…, Cerato…, Hypo…, Basi…

“Please excuse Charlie, he burped”

Paleostylic

No jaw/ jawless

Early Vertebrates

no formed elements of skull evident

Ostraderms

had bony head shields (dermal)

Cyslostomes

lack bone entirely; branchial arches present; 1x median nostril

Ancient gnathostomes

Cont.d ancient agnathan pattern, but w/ the addition of jaws

Placoderms

heavy dermal bony shield on head; upper jaw fixed to skull

Acanthodii

similar to modern Chondrichthyes; possessed mandibular bone to reinforce jaw

Chondrichthyes

The dermatocranium is absent; the chondrocranium forms the entire braincase; ethmoid, orbital, and otoccipital regions fuse; the splanchnocranium forms the jaw and gill arches

Chondrichthyes’ jaw

suspended by ceratohyal, Meckel’s cartilage…NOT palatoquadrate; ant. most gilss reduced to spiracle or absent; its protrusion during feeding helps maintain streamlining during swimming

Actinopterygii

exhibit tremendous diversity due to adaptive radiation; possess numerous extra facial bones; an operculum covering gill slits; and bones of the pectoral girdle attached to the posterior rim of the skull; allowing for suction feeding enabled by mobile jaws

Sarcopterygii

the upper jaw is often fused to the braincase; has clade Rhipodstians that incl. 4pods; labyrinthodont teeth (highly infolded), hinged joint b/w ethmoid and oticooptical units, notochord extended into skull.

Actinopterygii nasal

sac doesn’t open internally to mouth

4pods nasal

sac open to mouth via internal nares or choanae

Nasolacrimal duct

drains lacrimal gland to nasal sac

Early 4pods

Facial bones reduced or fused, with hymomandibula moving to inner ear as stapes, quadrate-articular jaw joint, loss of operculum, decoupling of pectoral girdle from head, and simplification of dermatocranium and reduction of splanchnocranium.

Primitive Amnoites

The palatoquadrate is separated into the epiterygoid and quadrate, introducing temporal fenestrae openings.

Rhynchocephalians (Lepidosouria)

Characterized by complete upper and lower temporal bars that define fenestrae, with 1 lower and 2 upper tooth rows.

Squamates (Lepidosauria)

Lower temporal bar lost in lizards; both bars lost in snakes; features a quadrate-articular jaw joint, and snake skulls are not fully fused allowing for more movable links.

Archalosaouria: Tesudines

Anapsid skulls with inward emarginations from the edge of dermatocranial bones; lacks teeth and has keratinized tooth plates instead.

Archosauromorphs

clade incl. crocs, birds, non-avian dinos contianing mandibular and antorbital fenestra (@ least ancestrally)

Birds

Modified diapsid; Inflated braincase; Built largely obliterated; jaw bones form beak; Absent temporal bar lost and lower bar forms jugal bar

Dinos

fully diapsid and usually possess both antorbital and mandibular fenestra

Synapsids

1x temporal fenestra; in mammal lineage, enlarges and eventually merges w/ orbital

Zygomatic arch

squamosal- jugal bar bounding fenestra

Therapsids

larger temporal fenestrae; heterodont dentition

Cynodonts

2o palate, larger braincase

Mammalia

Characterized by many composite bones formed by the fusion of ancestral bones, possessing 2 occipital condyles, a fully ossified nasal capsule, and turbinate bones that host nasal epithelium.

Middle ear

3 bones derived from ancestral jaw elements

Malleus

from ancestral articular

Incus

from ancestral quadrate

Stapes

from ancestral hyomandibula