Skull Design

Skulls and jaws are not solid bone but are made up of many plates.  They can be loosely or tightly connected.  Biologists and paleontologists study the arrangement of the plates to determine relationships between similar animals.

During the late Carboniferous and Early Permian some important divisions in animal design occurred.  The most important is the amniotic egg, which frees terrestrial animal’s dependence on water to lay their eggs.  The second major division is in skull design.  The number of fenestrae (holes) behind the orbit (eye socket).  These holes are to attach large jaw muscles.  The transition from amphibian to reptilian was as slow as fish to amphibian.  But when the division between reptilian and proto-mammal-like happened in a relatively short amount of time.

[A] Anapsid skull - The most primitive form does not have any holes behind the orbit.  All the first terrestrial animals had this kind of skull.  Fish, amphibians and turtles still do.  Turtles are the only truly terrestrial representative of this group to persist beyond the Triassic.

[B] Synapsid scull - One fenestra (hole) behind the orbit.  Pelycosaurs and all their decedents, Therapsids and mammals, have this arrangement.  (Feel the bone ridge next to your eye.  At the side of your face is a soft area where your jaw muscles attach.  That is your fenestra.)

Several other characteristics differentiate the Synapsids.  One is specialized regions of the tooth row: incisor, canine, and molar.  A second is the relationship between the jaw and ear.  The dividing attribute of mammals is that three rear jaw bones reduce in size and become part of the ear.  Third, their fore limbs tend to be strong.

[C] Diapsid skull - Two fenestra (holes) behind the orbit developed soon after.  All reptiles and birds have the .  Archosaurs, lizards, snakes, crocodiles, dinosaurs, birds.  Dinosaurs and birds have an additional fenestra (hole) in front of the orbit.

[D] Euryapsid skull - Marine reptiles have a skull with only one hole behind the orbit.  But the skull plate and teeth arrangements are not similar to synapsids (mammal-like).  The plate and teeth are instead similar to a diapsid.  It has been determined that the connecting arch of bone between the double fenestrae (holes) disappeared.

Common Synapsid Characteristics

(mammals and mammal-like)

1. differentiation of the tooth row into specialized regions, incisor-like, canine-like, and molar-like

2. fore limbs are relatively large and powerful compared to the hind limbs

3. rear bones of the lower jaw have become intimately involved with the ear region

Common Sauropsid Characteristics

 (reptiles, dinosaurs, birds)

1. little tendency for the tooth row to develop specialized regions, a retained primitive feature

2. fore limbs tend to be relatively small and the hind limbs are often much larger

3. rear bones in the lower jaw tend not to become associated with  the ear, a retained primitive feature

Animal Feet

Plantigrade stance = entire foot (toes to ankle) lands on the ground (like most reptiles and humans)

Semi-digitigrade stance = only the ankle is held off the ground (like T. rex and dogs)

Digitigrade stance = metatarsals are held off the ground (walks on point) (like Sauropods and elephants)

Two Main Branches of Dinosaur

Dinosaurs groups are not classified by what they eat, if they walk on two or four legs, or how big they get.  Paleontologists arrange them by the structures of their bones.  For example, in the exclusively herbaceous Ornithischian line there are both bipeds and quadrupeds, but the name means “birdlike pelvis” disclosing one of the group’s major characteristics.  Another similarity among Ornithischians is that most have no front teeth.  Instead they have beaks.  On the other hand, all Saurischian dinosaurs started off as bipedal and carnivorous.  But an early split in the lineage produced the Sauropods and Theropods.  Sauropods are all herbaceous and mostly quadrupedal.  Theropods are all carnivorous bipeds.

The earliest Ornithischian dinosaurs were small bipedal herbivores with long thin arms and long clawed hands.  As Ornithischian evolved, their front limbs lengthened and their hands became more robust allowing them to stand on four legs.  The larger the animal, the more hoof-like the hands became.  Some Ornithischian, like the Ceratopsians, became quadrupeds.  Others, like Hadrosaurs, walked on four legs but ran on two.  All Ornithischians walked on their toes.  Their footprints resemble Theropods but without claws.  They have hoof-like unguals that resemble those of horses.

Hadrosaurs are the only Ornithischians known to have hollow limb bones.  Ornithischian vertebrae are not hollowed out with air passageways as in the Saurischian dinosaurs.

Interestingly birds do not come from the Ornithischian line, which have beaks and birdlike pelvises, but are more closely related to Theropods like T-rex.  The first bird-like reptiles are found mid to late Jurassic.  The best examples are those of the famous Archaeopteryx, a feathered bird with teeth and long tail.  By the Cretaceous, birds developed into modern forms.  They had lost their tail bones, their breast bones enlarged allowing strong flight, and their hand bones fused together.  Today, only a few species of birds have remnants of teeth.

Ornithischian (means bird hipped)

Hadrosaurs, Pachycephalosaurs, Ceratopsians, Ankylosaurs, and Stegasaurians

Saurischian (means lizard hipped)

Theropods (including birds), Segnosaurs, and Sauropods

Dino Tidbits

The first nearly complete dinosaur found in the United States is a Hadrosaur, the duck-billed dinosaur.  Found near Haddonfield, New Jersey in 1858 and named Hadrosaurus foulkii.

Very few dinosaurs grew less than 6 feet; the smallest is only 16 inches long.  Mammals, lizards, amphibians, birds, and insects rule the micro world.

The largest dinosaur is a Sauropod, Argentinosaurus huinculensis, at 130 feet long and 110 tons.

Less than 10 new species of dinosaur are named each year.  These often are later reclassified as a variant of another species.

80% of all dinosaurs are known from fewer than 5 specimens.  Of those 50% are known from only one.

The only known descendants of the dinosaurs are the birds.

Paleobotany and the Radiation of the Dinosaurs: a new kind of plant

The climate of the Mesozoic was hot and humid.  The equator was dryer, sometimes arid.  The poles were not ice bound and dinosaurs lived in temperate jungles on Antarctica.  Since the Pennsylvanian Period, (320 Ma) ferns, horsetails, mosses, cycads, conifers, cypress, and ginkgoes filled the forests.  Herbivorous dinosaurs raked leaves off plants as they walked through the open forests.  They did not chew but had gizzard stones to pulverize the vegetation.  Wood from these plants are laden with indigestible chemicals and resins.  Animals had to eat great quantities of vegetation while continuously moving to obtain sufficient nutrition and to keep from destroying their habitat.  Herds had to remain small.

Near the beginning of the Cretaceous a new land plant appeared, the angiosperm.  These flowering trees and shrubs diversified quickly.  The largest known Cretaceous angiosperm trees had a 4” diameter trunk and only 16 feet tall.  They had leaves similar to oaks, poplars, maples, and sycamores.  No grass existed until after the K/T boundary.

As the angiosperm plants diversified, so did the herbaceous dinosaurs.  The end of the Cretaceous saw an explosion of dinosaur species and great herds roamed the continents.  Of all the known species of dinosaurs, almost 50% lived at the end of the Cretaceous.  Scientists credit this spread to the angiosperm, a much better food source.