Comparative anatomy review

Provide a reason why the skeleton is important to comparative anatomy

1. Bone is readily fossilized
   - minimally susceptible to taphonomic variance

2. Skeleton is generally conservative (evolutionarily)

3. Skeleton is evolutionarily "plastic" enough to respond to major environmental and habitat changes

What is the function of a secondary palate? Provide one group which possesses a secondary palate

To separate the nasal cavity from the oral cavity so you can eat and breath

Crocodilians, Mammals

Which types of vertebrae are found in amphibians?

Cervical, Trunk, Sacral and Caudle

Provide three modifications to the skeleton found in birds and the region of the skeleton these are found in.

1. The sternum is changed to the keel by becoming longer outward wise to allow for more flight muscle attachment. Axial skeleton

2. Caudle vertebrae are merged to form the Pygostyle . Axial skeleton.

3. small fibula and elongated tibia called a tibiotarsus. Appendicular skeleton

What is cranial kinesis and what does it help with? Provide a group which does not have cranial kinesis.

Extra point of articulation in the skull to help with feeding. Crocodilian and mammals do not have it.

Provide 2 functions of the skeleton in vertebrates

Support the body

Mineral storage

Which group contains vertebrae in which the intercentra are dominant and the pleurocentra are ost?

Amphibians

What is another term for muscular limbs with well- defined joints and digits? In which group did this first appear?

Chiridium

early tetrapods

Provide 3 modifications to the skeleton of snakes and the region of the skeleton these are found in

Loss of limbs. Appendicular skeleton

Extra zygoatocity. Axial skeleton

Loss of jugal. Cranium

What are the two main types of ossification

Intramembranous Ossification and Endochondral Ossification

The skull of vertebrates can be divided into 3 major regions/components. What are those regions/components?

Chondrocranium, Splanchnocranium, Dermatocranium

Define aspidospondyly

Noting is connected in vertebrae

Provide a characteristic of the appendicular skeleton in Chondrichthyes.

Chordina bar or claspers

Briefly define endochondral ossification

Precursor of cartilage is replaced by invading bone

What important structures do the neural and hemal arches surround?

Neural arches: dorsal hollow nerve cord

Hemal arches: dorsal aorta

Provide a trend we observe in the evolution of the cranial skeleton of vertebrates

Chondrocranium and Splanchnocranium decrease while Dermatocranium stays the same

Provide 3 modifications to the skeleton in mammals and the region of the skeleton these are found in.

Limbs shift under the body. Appendicular

Secondary Palate

No cranial kinesis

Provide 2 major changes to the skull that occur in the evolution of synapsid.

loss of Post orbital

Chondrocranium and Splanchnocranium decrease while Dermatocranium stays the same

Loss of cranial kinesis

Briefly define intramembranous ossification?

No precursor of cartilage. Begins in the dermis

Which types of vertebrae are found in Mammals?

Cervical lumbar, thoracic, sacral, caudle

Which regions of the skull are found in all vertebrates?

Chondrocranium and Splanchnocranium

What are the two hypotheses for the origin of paired
fins?

Gill Arch theory

Ventral lateral Fin-Fold Theory

Provide the support and the anti support for the Gill arch theory

Support

Comparative anatomy and embryology:
• 1. Mandibular, hyoid and gill arches all develop from visceral arches
• 2. Spiracle, associated with mandibular arch, could represent “vestigial” gill-slit
between first and second arch (mandibular and hyoid arch)
• 3. Pseudobranch (vestigial gill filaments) could represent gill filaments of the
mandibular arch
• Points 2 and 3 suggest that the original function of the mandibular arch (V1) was
respiration (vs. current role as Jaws)

anti-support

Fossil record: Lack of “intermediate” fossil forms
• Developmental genetics: V1 in living agnathans does not contribute
to the gill arches but instead to the cartilages that surround the mouth and support the tongue
• Comparative anatomy:
• Cranial nerve V (trigeminal) innervates jaws in gnathostomes but innervates an area anterior to the pharynx in modern agnathans and not associated with gill function

Provide the support and the anti support for the Ventral lateral Fin-Fold Theory

This theory proposes that paired fins (pectoral and pelvic) evolved from continuous lateral folds of skin and muscle running along the sides of early vertebrates. Over time, parts of these folds became specialized and were retained as paired fins.

✅ Support for the Fin-Fold Theory:

1. Embryological Evidence:

   • In many vertebrate embryos, pectoral and pelvic fins develop from continuous lateral thickenings — similar to what the theory suggests.

   • These ridges appear simultaneously on both sides and at both fin positions (pectoral and pelvic), hinting at a common origin.

2. Genetic Evidence:

   • Shared expression of Hox genes and T-box genes (like Tbx4 and Tbx5) in both pectoral and pelvic regions suggests a shared genetic regulatory pathway, supporting a common evolutionary origin from a continuous fin fold.

3. Fossil Support:

   • Fossils like Osteostracans and early jawless fishes show long fin-like lateral flaps, possibly remnants of fin folds.

   • Some early jawed fishes (e.g., placoderms) have both pectoral and pelvic fins, showing an early and coordinated appearance of paired fins.

❌ Anti-Support / Criticisms of the Theory:

1. Lack of Direct Fossil Evidence for a Continuous Fold:

   • While lateral flaps exist in some fossils, there's no definitive fossil showing a true continuous lateral fin fold from which paired fins evolved.

   • Some argue the folds seen are too inconsistent or irregular to support the theory.

2. Segmented Development:

   • Pectoral and pelvic fins do not always develop in continuity with each other in embryos; instead, they arise as distinct buds at specific positions, suggesting separate origins rather than from a continuous structure.

3. Alternative Theories (e.g., Gill-Arch Theory):

   • Competing hypotheses propose different origins, like the gill-arch theory, which argues paired fins evolved from modified gill supports, casting doubt on the fin-fold idea.

🧠 Summary:

Support 🟢	Anti-Support 🔴
Embryonic lateral ridges	No clear fossil evidence of a full fin-fold
Shared genetic control (e.g., Tbx genes)	Independent development of fins in some embryos
Fossils show fin-like flaps	Competing theories (e.g., gill-arch theory)

In which group did endochondral bone first appear?

Osterithyians

Provide 1 major trend in the evolution of the axial skeleton of vertebrates

Specialization of the vertebrae

What 3 groups of vertebrates are capable of flight?
Briefly explain the differences in the appendicular
skeleton of each.

Aves: reduce digits, Arm is specialized for flight, carpels and metacarpals are fused

Pterosaurs: Thumb is specialized for flight leaving the other digits free for use

Bats: Digits are specialized for flight by elongating to form the wing

Provide 3 modifications of the skeleton that occurred during the transition from aquatic to terrestrial environments.

Limbs with digits

zygapophyses to prevent twisting

pelvic girdle attached to vertebrae

In which group does the humerus and femur first appear?

Rhipthian fishes

Which types of vertebrae are found in teleosts

Thoracic and Caudle

Provide the 3 types of cartilage found in vertebrates.

Hyaline cartilage, Fibrocartilage, Elastic cartilage

Provide 3 modifications to the skeleton of Testudines and the region of the skeleton which is modified.

Ribs expanded in shell Axial

No teeth Cranial

Vertebrae in shell Axial

Where can dermal bone be found in the postcranial
skeleton?

Pectoral Girdle

Which group contains vertebrae in which the pleurocentra are dominant and the intercentra are lost?

amniotes

What are the first two visceral arches referred to as?
What is their function?

Mandibular Arch-Jaws

Hyoid Arch- support

In which group did the exoskeleton first appear?

Ostractoderms, conodonts

In which group did pelvic fins first appear?

Placoderms

Provide the 3 major types of cells found in bone and
what they do.

Osteoblast-synthesize bone

Osteocytes-maintain bone

Osteoclast-breakdown bone

Provide an extant group of vertebrates which possess a notochord throughout their lives

Cyclostones and Petromyzontiformes (hagfish)

Explain the difference between the jaws of Chondrichthyes and Osteichthyes?

Chondrichthyes: meckel's cartilage

Osteichthyes: Novel Jaw including Premaxilla, Maxilla and Dentary

Provide 3 modifications to the skeleton observed in either Amphibians, Actinopterygians or Chondrichthyes (mix and match) along with the group and the region of the skeleton the modification occurs in

🐸 1. Amphibians – Axial Skeleton

• Modification: Reduction in the number of trunk vertebrae and fusion of caudal vertebrae into a urostyle

• Details: In frogs, many tail vertebrae are reduced and fused into a rod-like structure called the urostyle, aiding in jumping and shock absorption.

• Region: Axial skeleton (vertebral column)

• Group: Amphibians

🦈 2. Chondrichthyes (Cartilaginous Fish) – Cranial Skeleton

• Modification: Cartilaginous skull with no dermal bones and highly developed rostrum

• Details: Sharks and rays lack bony skulls; instead, their chondrocranium is made entirely of cartilage. Many have an elongated rostrum (snout) used for sensory detection.

• Region: Cranial skeleton

• Group: Chondrichthyes

🐟 3. Actinopterygians (Ray-finned Fish) – Appendicular Skeleton

• Modification: Highly mobile fin rays (lepidotrichia) supported by modified pectoral girdles

• Details: The pectoral fins of many ray-finned fish (like teleosts) are adapted for fine maneuverability, thanks to bony fin rays and specialized girdle articulation.

• Region: Appendicular skeleton (fins and girdles)

• Group: Actinopterygians

Provide 3 of the 5 types of caudal fins found in vertebrates and a group which possess each.

🦈 1. Heterocercal Tail

• Description: The vertebral column extends into the upper lobe of the tail, making it asymmetrical.

• Function: Provides lift during swimming, useful for animals without a swim bladder.

• Example Group: Chondrichthyes (e.g., sharks)

🐟 2. Homocercal Tail

• Description: The external appearance is symmetrical, although the vertebral column may angle slightly into the upper lobe.

• Function: Efficient for fast, sustained swimming in open water.

• Example Group: Actinopterygians (e.g., most modern bony fishes like tuna and bass)

🐠 3. Diphycercal Tail

• Description: The vertebral column runs straight to the end of the tail, and the fin is symmetrical above and below.

• Function: Common in species with slow, precise movements or those that dwell near the bottom.

• Example Group: Lobe-finned fishes (e.g., coelacanths and lungfish)

What trend is observed in the evolution of the appendicular skeleton of both archosaurs and mammals. Explain the importance of this.

🔄 Trend: Shift from sprawling to upright limb posture

📘 Summary:

Group	Limb Posture Evolution	Importance
Archosaurs	Sprawling ➡ Upright (dinosaurs, birds)	Efficient walking/running, flight adaptations
Mammals	Sprawling ➡ Upright	Better support, endurance, diverse locomotion

What are heterotopic bones? Provide one example

bones that develop in unusual or abnormal locations — that is, outside the typical skeleton. They form in soft tissues like muscles, tendons, or skin, often due to genetic programming, injury, or mechanical stress.

Patella (kneecap)

Skulls of amniotes are cool! What are the two different temporal fenestration types in amniotes, and which bones contribute towards the temporal bars in each condition?

Synapsids

Temporal bar (bordering the fenestra):

• Formed mainly by the postorbital and squamosal

diapsids

• Upper (supratemporal) fenestra

• Lower (infratemporal) fenestra

• 🦴 Temporal bars (bones forming the fenestra borders):

  • Upper bar: Postorbital + Squamosal

  • Lower bar: Jugal + Quadratojugal

The evolutionary origin of the bones of the middle ear is cool! What is the evolutionary origin of the bones of the middle ear? (i.e., what bones in fishes are homologous to the bones of the inner ear). When in the evolution of vertebrates did each element become incorporated into the middle ear?

• Jawed fishes had multiple bones in their lower jaw. The articular and quadrate formed the jaw joint, and the hyomandibula braced the jaw against the skull.

• In early tetrapods, the hyomandibula lost its jaw-support role and became the columella, acting as a sound-conducting bone from the outer skull to the inner ear.

• In synapsids (mammal ancestors), the dentary bone expanded, taking over the jaw joint role. The articular and quadrate became smaller and gradually shifted function.

• In mammals, the articular and quadrate completed their migration into the middle ear, evolving into the malleus and incus respectively.

• 📜 Homologous Structures & When They Changed

  Middle Ear Bone	Fish Homolog	Original Function in Fish	When It Became Part of Middle Ear
  Stapes / Columella	Hyomandibula	Supported jaw/skull articulation and helped with gill breathing	🐟 Early tetrapods (~360 MYA) – First used to transmit sound
  Incus	Quadrate	Upper part of jaw joint (connected to articular)	🦎 In early mammal-like reptiles (therapsids) – Began shifting toward the middle ear (~250–200 MYA)
  Malleus	Articular	Lower jaw joint bone	🐀 Fully incorporated in mammals (~200 MYA), as the dentary took over the jaw joint

Flying is cool! What are the three hypothesis of the origin of flight and briefly discuss them?

• Arboreal Hypothesis

  • Early birds hopped from tree to tree to escape predators

  • Feathers aided in longer jumps and eventually
    gliding

  • Over time, gliding gave way to active flight

• Insect-net or Ground Up Hypothesis

  • Early birds were cursorial dinosaurs that ran and jumped to capture quick moving prey (insects)

  • Feathers helped with higher jumping and batting down prey

• Wing Assisted Incline Running (WAIR) Hypothesis

  • Feathered forelimb aided bird ancestors in climbing steep inclines or vertically up trees

  • Based on evidence from newly hatched modern birds which use their developing wings for this purpose

Provide all of the possible bones that can be found in the pectoral
and pelvic girdle of vertebrates.

🟦 Pectoral Girdle (Shoulder Girdle)

Connects the forelimbs to the axial skeleton (vertebral column or skull)

🦴 Dermal Bones (from the dermis)

• Clavicle – collarbone; often braces the scapula

• Cleithrum – prominent in fish; reduced or lost in most tetrapods

• Supracleithrum – found in bony fish

• Postcleithrum – found in some fish

• Interclavicle – midline bone, especially in reptiles

• Posttemporal – connects girdle to the skull in some fish

🦴 Endochondral Bones (from cartilage)

• Scapula – the shoulder blade

• Coracoid – supports the glenoid fossa; reduced or fused in mammals

• Procoracoid – a separate coracoid element in some reptiles and amphibians

• Suprascapula – dorsal extension of scapula in amphibians/reptiles

• Glenoid Fossa (joint socket, not a separate bone but an important feature)

🟩 Pelvic Girdle (Hip Girdle)

Connects the hindlimbs to the axial skeleton (via the sacrum)

🦴 Endochondral Bones (entire girdle is endochondral)

• Ilium – articulates with the vertebral column

• Ischium – forms the lower rear part of the pelvis

• Pubis – forms the lower front part of the pelvis

• Acetabulum – socket for the femur (again, not a separate bone but a critical joint feature)

📝 Summary Table:

Girdle	Bone Type	Possible Bones
Pectoral	Dermal	Clavicle, Cleithrum, Supracleithrum, Postcleithrum, Interclavicle, Posttemporal
Pectoral	Endochondral	Scapula, Coracoid, Procoracoid, Suprascapula
Pelvic	Endochondral	Ilium, Ischium, Pubis