lecture 6 - origin of jaws and paired appendages

pharyngeal arches, hox patterning,nthe gnathostome radiation

palatoquadrate

upper jaw

meckelian

lower jaw

opposing jaws - anatomical character

jointed upper and lower skeletal bars surrounding the mouth

• homologous to the first pair of pharyngeal gill arches (mandibular arch)

jaws originally evolved to bend forward to

expand the oral cavity, creating such to enhance respiratory ventilation (buccal pumping) over the gills

opposing jaws - functional explanation → over times, repetitive mechanical force selected for

biting and grasping, allowing vertebrates to transitions from passive suspension-feeding to active predation

true teeth - anatomical character

hard, mineralized tool growing directly on the jaw bones

primitive teeth (seen in early fossil placoderms) grew on

bone surface and lacked roots, preventing replacement

true teeth - functional explanation

appeared alongside or shortly after jaws to significantly improve the ability to capture, tear, and process prey

paired pelvic appendices and pelvic girdle - anatomical character

presence pelvic fins/limbs posteriorly, supported by a pelvic girdle, alongside the anterior pectoral appendages

paired pelvic appendices and pelvic girdle - functional explanation

dramatically enhanced swimming stability, steering, and maneuverability

myelinated neurons - anatomical character

nerve axon wrapped insulating sheaths of myelin

myelinated neurons - functional explanation

increases speed of electrical nerve impulse conduction across the nervous system

• allowed for complex sensory integration and split-second motor reflexes necessary for an active, predatory lifestyle

horizontal semicircular canal fluid maintains

orientation

horizontal semicircular canal - anatomical character

a third canal added to the inner ear

horizontal semicircular canal - functional explanation

provides 3-d spatial orientation, allowing to coordinate balance and track movement precisely in all physical planes

true stomach - anatomical character

highly specialized, acidic digestive chamber situated along the GI tract

true stomach - functional explanation

allowed for ingestion and storage of larger food particles, chemically break down animals tissues before they passed into the intestine

elastin - anatomical character

presence seen within connective tissues

elastin - functional explanation

provides structural flexibility and resilience, supporting high-pressure cardiovascular systems and flexible, moving body walls

second round of WGD - genetic character

gnathostomes underwent a second, separate duplication events after the linages split

second round of WGD - functional explanation

provided jawed vertebrates with a massive reserve of duplicated genes

serial theory

anatomical/ how

the serial theory posits that

jaws are homologous to the anterior gill arches (or brachial arches) of ancestral jawless vertebrates

gill =

mandibular

serial theory stage 1 - ancestral jaw state

series of pharyngeal gill slits open behind the mouth cavity, supported by jointed, cartilaginous basket0like structure known as gill arches

first gill arch =

mandibular arch

second gilll arch

hyoid arch

serial theory stage 2 - transitional migration

jointed first pharyngeal arch bent forward a d migrated anteriorly toward the oral cavity

• allowed the arch to act as a physical valve to prevent back flow and assist in pumping water over the respiratory gills

serial theory stage 3 - jawed gnathostome state

mandibular arch has fully enclosed the oral margin to become functional jaw hinge

• upper to palatoquadrate

• lower to heckles cartilage yoih

hyoid arch has modified its upper element into the

hyomandibula, which acts as a suspensory bridge, mechanically anchoring the newly formed jaw joint to the brain case to provide structural stability during forceful biting

serial theory evidence - neural crest origin

unlike the rest of the postcranial skeleton (which develops from mesoderm), both the jaw cartilages and posterior gill arches are uniquely derived from migratory embryonic neural crest cells.

serial theory evidence - hinged geometry

both structure form a homologous spatial arrangement consisting go jointed upper and lower bars that hinge in the middle

serial theory evidence - neuromuscular homology

cranial nerves and muscles that operate the jaw hinge are serial homologous to the muscular and nervous pathways associated with the posterior respiratory arches

in gnathostomes, first pharyngeal arch much be

free of Hox gene expression to form Meckle’s cartilage and the palatoquadrate

• ectopic expression of hox genes actively inhibited jaw formation and transforms it into the second-arch skeletal elements

serial theory - Cohn’s evolutionary discovery

seminal developmental studies in lampreys revealed that a Hox gene is expressed in their embryonic mandibular arch; demonstrated that jaw evolution in the gnathostome stem lineage was biochemically unlocked by the selective silencing of Hox expression erialsin the first pharyngeal arch.

serial theory explain what anatomical structures were modified, but not

why these modification were favored by natural selection

mallet’s hypothesis is the

ventilatory model

initial evolutionary driver for jaw development was not

feeding or biting, but rather respiratory ventilation

vending the first pair of gill arches forward did not initially serve

a predatory function; it served to expand the oral cavity

• allowed the animal to suck larger volumes of water into the mouth

by flexing these skeletal bars, the fish could drive water

backward over the respiratory gills via buccal pumping to enhance gas exchange

mallat’s hypothesis stage 1 ancestral agantham system - somatic context

passive ciliary suspension feeding supported by unjointed or weakly jointed gill arches

mallat’s hypothesis stage 1 ancestral agnathan system selective pressure

increasing metabolic demands (larger body and active swimming) required more efficient oxygen uptakes

mallat’s hypothesis stage 2 - active buccal pumping - anatomical adaptation

anterior-most gill arch (mandibular arch) enlarged, jointed, and gained powerful branchial muscle attachments

mallat’s hypothesis stage 2 - active buccal pumping - selective pressure

need to actively pump water into the oral cavity to pass over the respiratory gill for gas exchange (buccal pumping)

mallat’s hypothesis stage 3 - valvular occlusion - anatomical adaptation

mandibular arch bent forward during pumping compression, sealing the oral margin

mallat’s hypothesis stage 3 - valvular occlusion - selective pressure

prevented back flow of water during the compression phase; established a highly mobile, functional hinge positioned at the oral boundary

mallat’s hypothesis stage 4 - prey grasping - anatomical adaptation

joined mandibular hinge was structurally reinforced to slam shut with physical force

mallat’s hypothesis stage 4 - prey grasping - selective pressure

natural selection co-copted (exapted) this moving valve to grab, pinch, and hold highly mobile prey items

mallat’s hypothesis stage 5 - predatory true jaws - anatomical adaptation

madibualr arch fully specialized into the upper (palatoquadrate) and lower (meckels cartilage) jaws, supplemented by mineralizing surface teeth and supported by the second (hyoid) arch

mallat’s hypothesis stage 5 - predatory true jaws - results

coordinated macrophage and active predation drove the massive radiation of jawed vertebrates

the split of the ancestral single, midline nasal opening (monorhiny) in agnathans into

paired, lateral nostrils (diplorhiny) in gnathostomes was a crucial prerequisite that physically “unlocked” the evolution of the jaw

by clearing the midline of the head, this spatial reorganization allowed

embryonic neural crest cells to migrate directly forward to floor the cranium and facilitate the development of the upper jaw

mallet’s hypothesis is that most widely accepted

functional/adaptive model for the origin of jaws

the serial theory remains accepted as the correct

structural/anatomical homology framework

Rather than viewing them as mutually exclusive, modern paleontology and embryology have unified them:

Jaws are anatomically derived from the first gill arch (Serial Theory), but the selective pressure that drove this physical transformation was respiratory, not trophic (Mallatt's Hypothesis)

in addition, transition from monorhiny to diplorhiny is integrated into the revised serial theory of jaw evolution because it solves

the spatial and embryonic "how” – a shows how the head was structurally reorganized to allow the first pharyngeal arches to bend forward into functional, opposing biting structures

reptivitve mechanical use of these jointed mandibular elements for ventilation led to

muscular and skeletal reinforcement

• ability to close the mouth forcefully allowed for grasping and biting objects

• ancestral jawless fished to become mobile and active predators (macrophage)

evolution of teeth

1. ancestral external scales

2. snout and mouth migration

3. biting teeth on jaws

evolution of teeth stage 1 - ancestral external scales -

ancestral dermal odontodes (scales) embedded within skin 9ectoderm) of Paleozoic jawless vertebrates

structural homology to teeth

bone attachment base, a core of dentine, an inner pulp cavity, and a an outer cap of hyper-mineralized enameled

early scales functioned primarily as

sensory organs to detect changes in water temperature and pressure rather than as heavy armor

evolution of teeth stage 2 - south and mouth migration

• Physical migration of the odontogenic (tooth-forming) tissue boundary.

• As jaws began to assemble in early stem-gnathostomes, this competent epidermal tissue rolled over the lip margin, carrying the sensory odontodes from the external snout surface directly into the newly forming oral cavity.

evolution of teeth stage 3- predatory true teeth

• Final transition where these migrated scales became anchored directly onto the upper and lower jawbones.

• Once inside the mouth, the tissue was structurally reinforced for biting, grasping, and macrophagy, while retaining the ancestral sensory nerve network, which explains why our teeth remain highly sensitive to temperature and pressure today.

types of jaw attachment - paleostylic

none of the arches attach themselves directly to the skull (no suspension)

types of jaw attachment - euautostylic

found in placoderms; mandibular arch is suspended from the skull by itself, without help from the hyoid arch

• only really have lower jaw, upper is fused

types of jaw attachment - amphistylic

early sharks, some bony fishes; jaws are attached to the brain case through two primary articulation, anteriorly by a ligment connecting the palatoquadrate to the skull and posteriorly by the hyomandibula

types of jaw attachment - hyostylic

most bony fishes; mandibular arch is attached to the brain case primarily through the hyomandibula

types of jaw attachment - metautosylic

most amphibians, reptiles and birds; jaws are attached too the brain case directly through the quadrate, a bone formed in the posterior part of the palatoquadrate; hyomandibula has no part in supporting the jaw

types of jaw attachment - craniostylic

mammals; entire upper jaw is incorporated into the brain case, but the lower jaw is suspended from the dermal squamosal bone of the brain case

evolution of pectoral girdle

3D CT scans of the Devonian placoderm Kolymaspis sibirica demonstrated that the pectoral girdle was structurally integrated with the sixth branchial (gill) arch.

• This proves that the shoulder has a pharyngeal origin, arising at the head-trunk interface.

evolution of pelvic girdle

developmentally distinct, originating purely from the trunk lateral plate mesoderm later in evolution

evolution of pectoral and pelvic fins

• Primitive folds of skin acted as passive hydrofoils, generating hydrodynamic lift and stabilizing the fish during swimming.

• This passive lift served as a crucial pre-adaptation (exaptation) before muscular, segmented pectoral and pelvic fins arose in more advanced gnathostomes to allow active steering and braking

fins control and

stabilize movement

fins have the ability to exert pressure or to

create resistance provided a more efficient way of maneuvering

dorsal and anal fins control the tendency to

roll or yaw

• prevents unwanted rolling to keep fish upright

• keeping fish pointing int the right direction

paired fins (pelvic and pectoral) control

pitch, facilitated accerlation, coasting speed and deceleration

• causing head to point upward or downward

placoderms

• skin plate

• no teeth

• core heavy armor on the head and neck; endoconwdral bone

placoderms - craniovertebral joint -

a ball-and0sicket joint on either side of he cranium that articulates with the vertebral column

• flexion of the cranium backward increases the gape of the mouth

placoderm body

• mostly makes, or covered in small scare

• had bony plates instead of teeth

Acanthodii

spiny-sharks

acanthodii characteristics

• Typically two or more pairs of pelvic fins (up to 6), none with girdle

• Bony opercle (gill chamber cover)

• Branchiostegal rays (bony structures associated with the gills)

• Oldest fossil record to late Ordovician

• Extinct during Permian