Overview of the Mandible
A comparative study of the lower jaw or mandible in vertebrates explores the evolutionary adaptations, structural variations, and functional modifications of this critical skeletal element across different vertebrate groups. Here’s an overview of key aspects to consider:
- The mandible, or lower jaw, is a key element of the skull in vertebrates, serving critical roles in feeding, respiration, and vocalization.
- It consists of bones or cartilaginous elements, depending on the vertebrate group, and varies in complexity and structure across taxa.
Evolutionary Development
- Jawless Vertebrates (Agnatha):
- Gnathostomes (Jawed Vertebrates):
- The mandible evolved from the first branchial (gill) arch, termed the mandibular arch, during the transition from jawless to jawed vertebrates.
- The lower jaw initially consisted of multiple bones in primitive vertebrates.
Comparative Anatomy in Vertebrate Groups
Cartilaginous Fishes (Chondrichthyes):
- Examples: Sharks, rays, and skates.
- The mandible comprises cartilaginous elements called Meckel’s cartilage.
- Adapted for capturing and processing prey, often with a robust jaw musculature.
Bony Fishes (Osteichthyes):
- Examples: Teleosts, lungfish, and coelacanths.
- The mandible typically consists of multiple bones, including the dentary, angular, and articular.
- Dentary bones bear teeth, adapted for a wide range of feeding strategies, from filter-feeding to predation.
Lower Jaw or Mandible of Frog (Amphibians)
Comments
The lower jaw of frog is devoid of teeth and the mandible consists of Mento-meckelian, dentary, angulosplenial coronary process and articular facet for quadrate.
- Mento-meckelian
- The anterior extremity of Meckel’s cartilage becomes ossified forming a small cartilage bone called Mento-meckelian.
- It is found at anterior symphysis of mandibles.
- Angulo-splenial
- It is a long, curved lower jaw bone, forming major part of posterior region of each mandible.
- It has a small projection called the coronary process.
- Its anterior end is pointed, while posterior end contains an articular surface for quadrate cartilage.
- Dentary
- It is closely applied anteriorly to angulo-splenial and is a dagger-shaped bone.
- It extends up to Mento-meckelian bone. It is dagger shaped both in outer and inner.
- Examples: Frogs, salamanders, and caecilians.
- The mandible is simplified compared to fishes and supports fewer bones, with the dentary being dominant.
- Amphibians show adaptations for both aquatic and terrestrial feeding.
Lower Jaw or Mandible of Varanus (Reptiles)
Comments
[I] Mandible in inner view comprises of following parts
- Articular
- It is the posteriomost bone of the lower jaw ramus, articulating with quadrate dorsally.
- It extends behind angular process which terminates into articular cartilage.
- Angular
- Small splint-like bone, found between dentary and articular.
- Perforated by angular foramen. Supra-angular. It is an elongated, nearly rectangular bone, found in the middle of each ramus and contains a pair of mandibular foramina. Coronoid. It is a small process constituting dorsal side of middle ramus and gives rise to backwardly directed coronoid process. Splenial. It is a membranous bone in a groove on the inner side of dentary.
[II] Mandible Outer view
- Mandible outer view consists of, foramina for branches of mandibular nerve, a tests, coronoid, foramina for branches of mandibular nerve, articular cartilages articular suprangular, mental foramen angular and dentary.
- Dentary :- It forms major part of lower jaw ramus and contains 8 to 10 small, conical, pleurodont teeth, along dorsal edge in inner view. While dentary in outer view consists of 8 teeth, mental form and foramina for branches of mandibular divisions of V nerves.
- Examples: Snakes, lizards, turtles, and crocodilians.
- The mandible is primarily composed of several bones, including the dentary, angular, surangular, and articular.
- In snakes, the lower jaw is highly mobile and connected by ligaments for consuming large prey.
- Crocodilians have a powerful mandible with a robust dentary adapted for crushing and tearing.
Lower Jaw or Mandible of Fowl (Birds) (Aves)
Comments
Lower Jaw or Mandible of Fowl The lower jaw of fowl is devoid of teeth. Each ramus is made of the following bones :
- Articular
- It expands from posterior end of each ramus and is continued with Meckel’s cartilage.
- It contains mandibular condyle dorsally.
- Angular : It lies below articular and forms lower border of jaw.
- Supra-angular : It forms upper margin of posterior mandible and contains a small coronoid process.
- Splenial : It is a thin bone found along the inner surface of middle of mandible.
- Dentary :
- It forms anterior half of mandible and joins with fellow dentary at an anterior symphysis.
- It is devoid of teeth and contains a mandibular foramen.
- The mandible is lightweight and adapted for flight efficiency.
- It consists of the dentary and other fused bones, forming the beak.
- Teeth are absent, and feeding adaptations are diverse, reflecting diet (e.g., seed-eating, carnivory).
Lower Jaw or Mandible of Rabbit (Mammals)
Comments
In rabbit, the lower jaw of each side IS made of a single dentary bone. The two dentaries are joined together at a mandibular symphysis.
Mandible (Outer view)
- Postero-Iateral surface contains large mesenteric fossa for attachment of masseter muscle.
- Condyle and angle are separated by a deep notch posteriorly.
- Teeth are incisor, premolars and molars.
- Several processes such as condyle, angular process and coronoid process present.
Mandible (Inner view) Consists of incisor tooth, diastema, premolar teeth, molar teeth, coronoid process, condyle, angular process and mandibular foramen.
- The mandible is highly specialized, consisting of a single bone, the dentary.
- It articulates with the squamosal bone of the skull via the temporomandibular joint.
- Dentition is heterodont (differentiated teeth) and reflects dietary habits (herbivory, carnivory, omnivory).
Functional Adaptations
- Feeding Mechanisms:
- Filter feeding (e.g., some fishes).
- Prey capture (e.g., amphibians, reptiles).
- Mastication (e.g., mammals).
- Respiration:
- The mandible in early vertebrates supported structures aiding respiration (e.g., gill arches).
- In tetrapods, it is less involved in respiration but essential in feeding and vocalization.
Evolutionary Trends
- Progressive reduction in the number of bones in the mandible, culminating in a single bone in mammals.
- Increased specialization of the mandible for specific ecological roles.
- Transition from primarily cartilaginous to bony structures.
Comparative Insights
- The mandible reflects ecological and functional demands, such as dietary habits and environmental pressures.
- Fossil records trace the evolutionary trajectory of the mandible, offering insights into vertebrate diversification.
Conclusion
The lower jaw or mandible demonstrates remarkable evolutionary plasticity in vertebrates, showcasing a continuum of adaptations from primitive structures in jawless fishes to highly specialized forms in mammals. Studying its comparative anatomy deepens our understanding of vertebrate evolution, ecology, and functional morphology.
