Chapter 14 –Overview of the Clade Thyreophora

Introduction to Thyreophora (shield bearers)

Characteristics

Dermal Armor! in rows parallel to the midline of the body

Bony plates and spines embedded in the skin and interlocked with adjacent plates forming a continuous shield across the neck, throat, back and tail, and also covering the top of the head and often the cheeks in Ankylosauria

Developed into parascapular spines and parasagittal plates along the neck, back and tail in Stegosauria

Quadrupedal limb posture

Short limbs with hindlimb lengths exceeding forelimb lengths by 50%

Toes ended in broad hooves

Deeply inset cheek teeth and Beaks (rhampothecae)

Almost no species had premaxillary teeth

Teeth are small and triangular, with numerous vertically oriented ridges

Ankylosaurs were rarely longer than five meters, although some reached eleven meters in length, and weighed up to 4000 kilograms

Stegosaurs were three to nine meters in length; 300 to 6500 kilograms

Ankylosaur Geological range & diversity

Middle Jurassic to Late Cretaceous (~175 - 65 million years)

Maximum diversity – about equal number of genera (9 and 10) during the Early and Late Cretaceous

At least 23 genera (28 species) throughout their time on earth

But 6 or 7 genera are known from only fragmentary skeletal remains

Found in North America, Europe, Mongolia, China and Australia

Stegosaur Geological range & diversity

Early Jurassic to Early Late Cretaceous (~170 - 95 million years)

Maximum diversity - 6 species during the Late Jurassic

A dozen species (10 genera) throughout their time on earth

Found in North America, Europe, China, India and Africa

Clades and Species of Thyreophora

Basal forms: Scutellosaurus, Emausaurus, and Scelidosaurus

Ankylosauria (fused lizard)

Divided into two clades: Ankylosauridae and Nodosauridae

Differentiated based on presence (Ankylosauridae) or absence (Nodosauridae) of a tail club

Ankylosaurid skulls had horns, while Nodosaurids had large spikes projecting from their sides

Notable species

Ankylosaurus, Euplocephalus, Hylaeosaurus, Nodosaurus, Pinacosaurus, and Sauropelta

Stegosauria (roofed lizard)

Long, thin, relatively small heads

Stegosaurs had small brains based on brain casts - brains were only an estimated 0.001% of body weight

Relatively large olfactory bulbs suggest Stegosaurs had a good sense of smell

Marsh and others suggest an enlargement of the spinal cord canal in the hip vertebrae housed a second brain (to control the back legs and tail)

It has recently been shown that the enlargement was probably used for a glycogen body, as occurs in some living birds

Notable species

Stegosaurus and Kentrosaurus

thyreophorans as living animals

Reproduction and Growth and Social Life

Ankylosaur social behaviour mostly unknown

Pinacosaurus may have lived in herds

twelve specimens in an accumulation in china

Generally only individual skeletons or isolated remains are found

Stegosaur social behaviour mostly unknown

No nests, eggs, or hatchlings

Only a few juvenile and adolescent specimens

Some sexual dimorphisms among adults

Kentrosaurus appears to have lived in herds

Mass accumulation of disarticulated but associated fossils at Tendaguru

Ankylosaur Skeletal Conditions

Ankylosaur skeletons from Asia are nearly complete & articulated, sometimes preserved in upright poses or on their sides

Those from North America are only partial skeletons & often found upside down, sometimes in transitional or marine sedimentary beds

Suggests bloated carcasses floating upside down from weight of armor

Ankylosaur Defensive moves

Calculations based on limb size and body mass suggests that Ankylosaurs were slow

Running - <10 km/hr; walking - ~3 km/hr

They were probably quite agile when attacked by predators, however

Hunkering down and/or Tail-clubbing

Ankylosaurs were covered with bony armor

With legs folded under body, an ankylosaur would have been difficult to flip over

Nodosaurids had shoulder spines and massive shoulder muscles

Planted rear legs and rotated their forequarters to present spines toward predator

Ankylosaurids had a massive tail club

Front half of tail was flexible, but the rear half was stiffened by modified vertebrae and longitudinally running tendons

Potent threat to the legs of a T. rex

Brains

Stegosaurs had small brains based on brain casts - brains were only an estimated 0.001% of body weight

Relatively large olfactory bulbs suggest Stegosaurs had a good sense of smell

Stegosaurs had an enlargement of the spinal cord canal in hip vertebrae

Marsh and others suggest this enlargement housed a second brain (to control the back legs and tail)

Giffin has recently shown that the enlargement was probably used for a glycogen body, as occurs in some living birds

Stegosaur Plates and Spines

Stegosaurus may have been able to rotate its plates from a folded down to erect position according to Bakker

Importance of spines and plates

Protection and defense

Particularly tail spikes and parascapular spines

Display

Shapes and patterns of plates and spines are nearly always species specific

Thermoregulation

Stegosaurus plates are covered with an extensive pattern of grooves, and the insides are filled with a honeycomb of chennels, probably for conveying blood vessels

Plates were probably arranged in alternating pairs as indicated by wind tunnel experiments

NOTE - Juveniles may not have developed plates, presumably because they did not need them for display or thermoregulation

Spines probably developed for protection & defense initially - Display & Thermoregulation would have developed later

Ankylosaur Feeding

Ankylosaurs were low-browsers (less than 1 meter)

Ankylosaurs, like Stegosaurs, have a mix of characteristics, some of which suggest sophisticated oral food processing and others of which suggest simple oral food processing combined with internal food breakup

Sophisticated oral food processing

Scoop-shaped beaks

Narrower in nodosaurids; wider in ankylosaurids

Nodosaurids may have been more selective in foliage or fruit choice

Well-developed, deep cheek pouches

Long, flexible tongues

Large hyoid bones

Jaw bones that were large and strong

Jaw muscles seem to be weak, however

had secondary palates that apparently allowed them to breath & chew at the same time

Simple oral food processing combined with internal food breakup

Teeth are relatively small, simple, and triangular and apparently did little grinding (lack well-developed wear surfaces on crowns) and

Ankylosaurs had a huge abdominal cavity

May have housed differentiated fermentation chamber(s)

Note - Ankylosaurids had curved thin bones within their nasal passages similar to those of birds & mammals (lined with membranes to filter, warm & moisten incoming air and retain moisture & warm from outgoing air)

Stegosaur Feeding

Stegosaurs have a mix of characteristics, some of which suggest sophisticated oral food processing and others of which suggest simple oral food processing combined with internal food breakup

Sophisticated oral food processing

A horn-covered beak (rhamphotheca) covered the fronts of the upper and lower jaws and was used to crop or strip foliage from plants

Stegosaurs seem to have had cheeks

Simple oral food processing combined with internal food breakup

Teeth are relatively small, simple, and triangular and apparently did little grinding (lack well-developed wear surfaces on crowns) and

Jaw muscles seem to be weak

So perhaps Stegosaurs had a gizzard packed with stones to grind food (like some birds, crocodiles, sauropods and psittacosaurs)

Gastroliths have not been found associated with Stegosaur skeletons, however

Stegosaurs have a narrow snout, suggesting a fair degree of food selectivity

Posture suggests Stegosaurs were principally low-browsers (less than 1 meter)

Bakker argues that some Stegosaurs could have reared up on their hind legs, using their tail as a third “leg” to from a tripod, to feed higher up in trees (2 - 4 meters)

Center of gravity near hips and hind limbs supported nearly 80% of body weight