Bullfrog Skeletal Reconstruction



Bullfrog Skeleton Reconstruction



The skeleton of the frog consists chiefly of bony and cartilaginous elements.  The functions of a skeleton include providing support for the body,  protection of delicate internal organs and attachment surfaces for muscles.  In vertebrates, the axial skeleton consists of the skull, vertebral column, sternum (breast bone) and ribs (which are not present in amphibians).  The vertebral column of frogs is made up of 10 vertebrae, the first of which (called the atlas) articulates with the base of the skull.  The atlas is the only cervical vertebra in the frog.  The next seven vertebrae are abdominal vertebrae, which is the large sacrum with two strong transverse processes that join with the ileum.  The last vertebra is the long and highly modified urostyle.  Note:  Most vertebrates have a tail supported by caudal vertebrate, but frogs and toads are atypical in that they lack any tail and are therefore called anurans (“tail less amphibians”).



1. Skull
2. Axis Cervical vertebrae)
3. Abdominal vertebrae
4. Cervical Vertebrae
5. Urostyle
6. Scapula
7. Ilium
8. Ischium
9. Humerus
10. Radio-ulna
11. Carpals
12. Metacarpals
13. Phalanges
14. Femur



The appendicular skeleton includes the limbs and the pectoral and pelvic girdles that support them.  In most vertebrates the forelimbs consist of three major bones — the humerus, radius and ulna, along with the smaller bones of the hand (carpals, metacarpals and phalanges).  Note that in the frog the radius and ulna have become fused into a single bone, the radio-ulna.  Likewise, the hindlimbs consist of three major bones — the femur, tibia and fibula, along with the smaller bones that make up the feet (tarsals, metatarsals and phalanges). Once again, in frogs and toads the tibia and fibula have become fused into a single bone, the tibio-fibula.  The pectoral girdle consists of four pairs of bones (the suprascapula, scapula, coracoid, and clavicle).  The last three pairs are connected to the sternum.  In frogs, the pelvic girdle, which supports the hindlimbs, is formed by the fusion of the ilium, ischium and non-ossified pubis.  Each femur fits into a socket on the pelvic girdle called an acetabulum.  Note that the pelvic girdle and limb structure are well adapted for giving a powerful, synchronous thrust of both hind limbs in swimming and jumping


By reassembling an amphibian skeleton, students will learn the bones and modifications of a vertebrate.


Dermestid beetles, small aquarium with air-vented lid,  dermestid bedding, food, & water supply, freeze-dried bullfrogs, tweezers, small container with lid for bones, 20% H2O2, thin piece of wood, paints, and glue.


  1. Place the freeze dried frog into a small aquarium of dermestid beetles.
  2. Leave the frog skeleton in the aquarium for several days to several weeks until all flesh has been stripped from the skeleton.
  3. Carefully remove all bones from the aquarium and use forceps to carefully pull away any flesh that remains..
  4. Rinse the bones in running water thoroughly.
  5. Bleach the bone in 20% hydrogen peroxide solution until the bones appear white.
  6. Dry the bones and articulate them. (use the above diagram to help in the arrangement of the bones.)
  7. Bones should be mounted on a thin piece of wood.
  8. You may be creative and place your frog in a scene of your choosing — playing football or basketball, swimming in a pond, practicing ballet, etc.







Amphibians   All Materials © Cmassengale  


Amphibian Evolution:

Arose from lobe-fined ancestor called Crossopterygians
Land plants & insects provided new food source
Had primitive lungs & short, limb like fins for short periods on land
Appeared during late Devonian
Icthyostega early amphibian with 4 limbs, lungs, & a tail for swimming


Four limbs with claws on digits (toes)
Lungs instead of gills
Both internal & external nares (nostrils)
Three chambered heart (two atria & one ventricle)
Double loop blood circulation to lungs & rest of body cells

Skin with keratin (protein) to prevent water loss
Necks to more easily see & feed
Most with smooth, moist skin to take in dissolved oxygen
Some with oral glands to moisten food they eat
Webbed toes without claws
Ectothermic – body temperature changes with environment
Show dormancy or torpor (state of inactivity during unfavorable environmental conditions)
Hibernate in winter and aestivate in summer
Aquatic larva called tadpole goes through metamorphosis to adult
Metamorphosis controlled by hormone called thyroxine

American Toad Tadpole photograph

External fertilization with amplexus (male clasps back of female as sperm & eggs deposited into water)
Eggs coated with sticky, jelly like material so they attach to objects in water & do not float away
Eggs hatch into tadpoles in about 12 days


  • Males with vocal sacs to croak
  • Digested system adapted to swallow prey whole
  • Well developed muscular system


  • Anura – frogs & toads
  • Urodela – salamanders & newts
  • Apoda – caecilians
  • Trachystoma – sirens or mud eels

Anuran Characteristics:

  • Both terrestrial & freshwater species
  • Tadpole with tail, gills, & two-chambered heart
  • Adults without a tail, four limbs, & lungs
  • Frog skin smooth & moist for cutaneous respiration, while toads is rough & warty (poison glands)





  • Long hind limbs for jumping
  • Long, forked tongue hinged at front of mouth

Urodela Characteristics:

  • Includes salamanders & newts
  • Have elongated bodies with a tail & four limbs 
  • Smooth, moist skin for cutaneous respiration
  • Less able to stay on dry land than anurans

Spotted salamander photograph
Spotted Salamander

  • Size from a few centimeters long to 1.5 meters
  • Nocturnal when live in drier areas
  • Newts are aquatic species

red-spotted newt photograph
Red Spotted Newt

  • Lay eggs in water or damp soil
  • Some bear live young
  • May or may not go through tadpole stage (some hatch & look like small adult)

Apodan Characteristics:

  • Includes caecilians
  • Tropical, burrowing, worm like amphibians
  • Legless
  • Small eyes & often blind
  • Eat worms & other invertebrates
  • Average length 30 centimeters, but can grow up to 1.3 meters
  • internal fertilization
  • Female bear live young


Trachystoma Characteristics:

  • Includes mud eels or sirens
  • Known as “rough mouth” amphibians
  • Found in eastern U.S. & southern Europe
  • Have minute forelimbs & no hindlimbs

Mud Eel or Siren

External Frog Anatomy:

  • Live double life on land & water
  • Powerful hind legs for jumping & swimming fold under body when at rest
  • Bulging eyes to stay submerged but still see predators
  • Blinking eyelids protect eyes from dust & dehydration
  • Nictitating membranes clear to moisten eye & see underwater
  • Internal nostrils or nares allow frog to breathe underwater
  • Tympanic membranes or eardrums behind each eye transmit sound through bone called columella to inner ear
  • Eustachian tubes connect mouth & middle ear to equalize pressure

  • Males croak or make sound to attract females & ward off other males
  • Have protective coloration from cells called chromatophores
  • Granular glands secrete foul tasting or poisonous substance
  • Mucus glands lubricate skin for oxygen to be dissolved & absorbed

Internal Frog Anatomy:
Skeletal System

  • Nine spinal vertebrae (1 cervical in neck, 7 trunk, & 1 sacral supporting hind legs)
  • Urostyle long, slim bone connecting sacral vertebrae & trunk
  • No rib cage, but pectoral girdle forms shoulders & connects front legs
  • Pelvic girdle connects to hind legs

Digestive System

  • Tongue sticky, forked, & hinged at front of mouth so can be extended out to catch insects
  • Can pull eyes inward to help swallow food
  • Two, sharp, backward-pointing  vomerine teeth in roof of mouth help prevent prey from escaping
  • Maxillary teeth line the edge of the upper jaw
  • Alimentary canal (mouth, esophagus, stomach, small & large intestines, and cloaca) is where food is digested, absorbed & wastes eliminated
  • Stomach makes gastric juices to break down food
  • Pyloric sphincter muscle controls movement of food from stomach into first part of small intestine called duodenum
  • Liver makes bile to digest fats; stored in gall bladder
  • Pancreas makes pancreatic juice to digest food in small intestine
  • Ileum is coiled mid portion of small intestine
  • Mesentery is a fanlike membrane holding the intestine in place
  • Wastes collect in large intestine & then move into cloaca along with eggs, sperm, & urine until they leave body through the anus



Circulatory System

  • Need more oxygen to burn increased amount of food needed to live on land
  • 3 chambered heart (right atrium receives deoxygenated blood from body, left atrium receives oxygenated blood from lungs, & ventricle pumps blood to lungs & rest of the body)
  • Double loop blood circulation (pulmonary from heart to lungs & systemic from heart to rest of body)
  • Conus arteriosus carries blood from ventricle to body cells

Respiratory System

  • Tadpoles use gills to breathe
  • Adult frogs breathe through lungs & moist skin (cutaneous respiration)
  • Glottis is the opening into throat & lungs

Excretory System

  • Carbon dioxide excreted through skin & lungs
  • Kidneys filter blood & store urine in urinary bladder until leaves cloaca

Nervous System

  • Olfactory lobes at base of brain detect smells
  • Cerebrum behind olfactory lobes controls muscles
  • Optic lobes detect sight
  • Cerebellum controls balance & coordination
  • Medulla oblongata controls heart rate & breathing
  • Cranial nerves connect brain & spinal cord, while spinal nerves branch off the spinal cord to muscles & sensory receptors