FOSSIL 6: VERTEBRATE
Back again with me, Kemal, the author of the blog "Catatan Belajarku" (My Learning Notes). So, how’s the spirit, bro? Still excited to talk about fossils? Hehehe. Alright, let’s keep going!
VERTEBRATE
A. Definition
Animals that have a backbone. Vertebrates first appeared during the Ordovician period and still exist today. They initially lived in the ocean as simple, worm-like organisms.
B. Characteristics
- Possess segmented vertebrae that function as the axial support.
- The brain is protected by a cranium (skull).
- The skeleton is internal (endoskeleton), composed of hard bone, spongy bone, cartilage, and ligaments.
- The body has bilateral symmetry.
C. Taxonomic Level
- Subphylum: Vertebrate
D. Classifications
a. Chondrichthyes
Their body structure is made of cartilage. Key features:
- Teeth are not fused with the jaw.
- They lack a swim bladder.
- They have spiral valves in their intestines.
b. Osteichthyes
Their body structure is made of true/hard bone. Key features:
- Fusiform (spindle-shaped) body.
- Swim bladder functions as lungs.
- A single gill opening on each side of the body, covered by a bony flap called the operculum.
c. Myxini (Hagfish)
- Their mouth has four pairs of tentacles at the tip.
- The living pouch has a channel to the pharynx.
- They have 5–15 pairs of gill pouches.
d. Cephalospidomorphi (Lampreys)
- Possess a sucking mouth with horn-like teeth.
- The living pouch is not connected to the mouth.
- They have 7 pairs of gill pouches.
e. Amphibia
- Can live in both water and on land.
- Cold-blooded (body temperature follows the environment).
- Breathe using lungs.
f. Reptilia
- Crawling animals.
- Warm-blooded.
- Their bodies are covered with scales.
g. Aves (Birds)
- Animals that can fly.
- Their bodies are covered with feathers.
- Limbs include legs and wings.
h. Mammalia (Mammals)
- Females have mammary glands.
- Warm-blooded.
- Reproduce sexually.
E. Morphology
In general, vertebrate fossil identification focuses on two morphological aspects: bone morphology and tooth morphology.
a. Bone Morphology
Vertebrate bones are classified into two types: cranial bones and postcranial bones.
-
Cranial bones refer to the bones in the skull, primarily functioning to protect the brain. These bones are usually flat and plate-like in shape.
-
Postcranial bones include the supporting bones of the body and limbs. Limb bones are typically round and elongated, while supporting bones show more variation in shape.
 |
| Cranial bone ©️ thepoundlanespaniel.com |
 | ||
Pisces
|
The bones that make up each vertebrate class have distinct characteristics. For example, fish have bones shaped like spines, reptiles tend to have flat bones, and birds have lightweight, hollow bones.
b. Tooth Morphology
Teeth are very important in fossil identification, especially for classifying mammals down to the species level.
In non-mammalian animals, tooth structure tends to be simpler, with little variation in shape. The main difference is usually in tooth size, not form.
In contrast, mammalian teeth are more complex and can be clearly divided into different types:
- Incisors – front teeth used for cutting
- Canines – pointed teeth for tearing
- Premolars – transitional teeth used for grinding and tearing
- Molars – back teeth used for grinding food
 |
| Teeth of Vertebrates (Non-Mammals) |
 |
| Teeth of Vertebrates (Mammals) |
- Enamel: A hard layer or tissue that covers the crown of the tooth and contains calcium.
- Dentin: A yellowish layer or tissue made of calcified material, located just beneath the enamel.
- Cementum: A hard layer with a strong structure that covers the root of the tooth.
 |
| Parts of a tooth |
2. Tooth Formula
Because mammalian teeth have distinctive shapes that differ from one another, mammal teeth can be represented using a tooth formula. Below is the tooth formula for mammals.
Let’s take the example of a rabbit, which has a tooth formula of 2033/1023. In this formula, the numerator (top number) is 2033, and the denominator (bottom number) is 1023.
The numerator represents the arrangement of teeth in the upper jaw, while the denominator shows the arrangement in the lower jaw. Both numbers represent half of the jaw (either the left or right side) because mammals have bilateral symmetry — meaning the left and right sides are mirror images. So, the upper left and upper right jaws both have 2-0-3-3 teeth, and the lower left and lower right jaws both have 1-0-2-3 teeth.
Since mammals have four types of teeth, the numbers correspond to the order of tooth types: I C P M — Incisors, Canines, Premolars, and Molars.
So, 2033 means the rabbit’s upper jaw (left or right) has:
-
2 incisors
-
0 canines
-
3 premolars
-
3 molars
And 1023 means the rabbit’s lower jaw (left or right) has:
-
1 incisor
-
0 canines
-
2 premolars
-
3 molars
This means rabbits don’t have canines, which classifies them as herbivores.
How’s that? Pretty clear, right? Hehe. Below is an example of a tooth arrangement. Try to guess which species it is! (Scroll slowly so the answer doesn’t pop out right away).
Based on observation, this species has a tooth formula of 2123/2123, and the species is a monkey. However, humans also have the same tooth formula.
3. Types of Molars
-
Tribosphenic: The surface of the molar consists of 3 cusps (points). Found in species like the platypus and marsupials.
Quadrate: The surface of the molar consists of 4 cusps (points). Found in species like porcupines, raccoons, and some primates.
Bunodont: The cusps are rounded with blunt peaks. Found in hominids, bears (Ursidae), and sea lions (Otariidae).
Hypsodont: Teeth with high crowns and enamel that extends well past the gum line. Found in animals like cattle, deer, and horses.
Brachyodont: Teeth with short crowns, just slightly above the gum line, and having at least one root. Found in humans.
Lophodont: Characterized by long, numerous ridges (called lophids) on the tooth surface. Found in elephants, tapirs, and rodents.
- Selenodont: The main cusps are elongated into crescent-shaped ridges. Found in deer (Cervidae), cattle (Bovidae), and horses (Equidae).
- Secodont: Sharp, pointed cusps. Found in carnivores.
Differences between molars of carnivores, herbivores, and omnivores:
-
Carnivores: Upper and lower molars are large, sharp, and triangular, known as the carnassial type.
 |
| ©️ sciencesource.com |
Herbivores: The cusps of the molars are joined together forming grooves, known as the selenodont type.
 |
| ©️ museum2.utep.edu |
Omnivores: The cusps of the molar crowns are short and tend to be rounded, known as the bunodont type.
 |
| ©️ museum2.utep.edu |
F. The Evolutionary History of Vertebrates
a. Cambrian Period
The first vertebrate fossils, known as ostracoderms, appeared during this time.
b. Early Silurian Period
The first jawed fish evolved.
c. Early Devonian Period
Four major groups of jawed fish emerged:
-
Acanthodians
-
Placoderms
-
Cartilaginous fish (e.g., sharks)
-
Bony fish (e.g., tuna)
d. Late Devonian Period
These jawed fish began to replace all primitive vertebrates.
e. Late Paleozoic Era
Acanthodians and placoderms went extinct. However, cartilaginous and bony fish continued to evolve and exist today.
f. Carboniferous Period
Amphibians began to evolve, marked by the development of lungs and limbs. Vertebrates diverged into two major groups:
-
Animals living in cold and humid environments, which developed fur and internal pouches. These later evolved into mammal-like reptiles.
-
Animals living in hot and dry environments, which lacked fur and pouches. These evolved into reptiles, birds, mammals, and amphibians.
g. Late Paleozoic Era
Mammal-like reptiles became the dominant terrestrial vertebrates.
h. Mesozoic Era
Reptiles reached the peak of their evolutionary development.
i. Triassic Period
Marked the peak of vertebrate diversification and dominance.
j. Late Triassic Period
The first true mammals appeared.
k. Jurassic and Cretaceous Periods
Terrestrial animals dominated, and the first birds began to appear.
l. End of the Mesozoic Era
A mass extinction event occurred, wiping out many dominant reptilian species, including the dinosaurs.
G. Uses of Vertebrate Fossils
a. Biostratigraphic Purposes
In Indonesia, vertebrate fossils play a significant role in determining the age and depositional environments of terrestrial sediments. This is primarily because much of Indonesia’s geological record—particularly in regions such as Sumatra, Java, and Sulawesi—is dominated by Tertiary and Quaternary strata. During these periods, vertebrates were the dominant fauna, as dinosaurs had already gone extinct by the end of the Cretaceous.
 |
| Biostratigraphy of Java based on index fossils (Puspaningrum, 2016) |
 |
| Biostratigraphy of Sulawesi based on index fossils (Puspaningrum, 2016) |
 |
| Biostratigraphy of Flores based on index fossils (Puspaningrum, 2016) |
