Vertebrate Zoology Exam 3

Amniote Synapomorphies and Other Characters

Synapomorphies:

• Keratinized epidermis allowing variety of skin elaborations (scales, hair, feathers)

• Epidermally derived claws on digits

• Costal ventilation of lungs (use ribs to breathe)

• Elaborations of innervation of forelimbs

• Modification of 2nd neck vertebra into axis (lateral & rotational movements of head)

• Amniotic egg

Other Characters:

• Waterproof skin

  • Waterproofing produced by lipids in epidermis

  • Distinguishes living amniotes from living amphibians, but probably shared with various extinct non-amniotes (primitive for amniotes)

• Internal fertilization via a male intromittent organ (needed for amniotic egg)

  • I.e. so that egg can be fertilized before “packaged” in amniotic egg

Synapomorphies and characteristics of amniotes associated with occupying arid environments

• Impervious skin that reduces water loss

• Horny nails that, among other things, enable them to use their forelimbs to dig burrows into which they can retreat during the heat of the day

• Density of renal tubules in the metanephric kidney of amniotes, in the larger size of their water-resorbing large intestines

• Differentiation of the Harderian and lacrimal glands in the eye socket whose antibacterial secretions help to moisten and, along with a third eyelid (nictitans), to further protect the eye from desiccation.

• Extensive system of muscle stretch receptors that enables finer coordination and greater agility during locomotion

• Enlarged lungs (which are the only remaining organs of gas exchange owing to the loss of gills)

• Complete loss of the lateral line system other vertebrates use to detect motion in water. 

Skeletal characters unique to amniotes

• Have at least two pairs of sacral ribs, instead of just one pair = stronger ribcage with costal muscles for breathing

• Have an astragalus bone in the ankle, instead of separate tibiale, intermedium, and proximal centrale bones

• Have paired spinal accessory (11th) and hypoglossal (12th) cranial nerves incorporated into the skull, in addition to the ten pairs of cranial nerves present in amphibians

• Loss of labyrinthodont teeth

Amniotic egg

• Yolk sac surrounds yolk, transfers nutrients from yolk to embryo

• Amnion surrounds the embryo and forming a cavity filled with amniotic fluid (cushioning)

• Chorion – outermost membrane; forms primary respiratory surface (later in development aided by allantois)

• Allantois – lies within (and eventually fuses with) chorion. Creates allantoic cavity to store metabolic wastes

  • Becomes the urinary bladder in adult amniotes

In oviparous amniotes (most reptiles, all birds, monotreme mammals), entire egg is surrounded by leathery or hard shell

• Beneath shell is albumen (water and protein store)

• Shell and albumen secreted by oviduct

• Shell provides protection and support, but allows gas exchange

• Hatchling amniotes also possess an egg-tooth and horny caruncle on the snout tip to facilitate exit from their hardshelled eggs

• Some components of the amniotic egg have been variously modified within Amniota.

  • Placental mammals, for example, have suppressed the egg shell and yolk sac, and elaborated the amniotic membranes to enable nutrients and wastes to pass directly between mother and embryo

Adaptive significance of the amniotic egg

• Allow eggs to be laid on land with less reliance on moist conditions

• May allow the production of much larger embryos than possible with simpler eggs

• Membranes provided improved respiration (gas exchange for larger embryo)

• Shell may provide structural support for larger embryo

• The amniotic egg, together with a penis for internal fertilization, loss of a free-living larval stage in the life cycle, and the ability to bury their eggs, enabled amniotes to escape the bonds that confined their ancestors' reproductive activities to aquatic environments

Amniote skull

• Amniotes have more complex jaw musculature than non-amniotes:

  • Adductor muscle divided into two or more separate muscles that insert at different angles

  • Allows for different feeding mechanisms such as application of static pressure (biting and tearing instead of just snapping)

  • Switch from buccal to costal respiration probably allowed for diversification of jaw muscles

• Most amniote skulls have holes (fenestra) not found in non-amniotes

  • Probably associated w/ changes in jaw musculature

  • May function to provide:

    • Room for heavier jaw muscles

    • More secure attachment points for jaw muscles

Types of Amniote Skulls

• Anapsid – no additional holes (ancestral condition)

  • Modified in turtles with emarginations on back of skull (but probably derived from diapsids)

• Synapsid – single hole (lower temporal fenestra)

  • Merges with orbit in mammals

• Diapsid – two holes (lower and upper temporal fenestrae)

  • Modified in various ways (loss of bars that separate various openings; formation of additional fenestra)

Major groups of amniotes

• Synapsida – “mammal-like reptiles” and mammals

• Sauropsida – all other amniotes (reptiles, birds)

  • Parareptiles – extinct; formerly thought to include turtles

  • Eureptiles – diapsids & ancestral forms

    • Lepidosauria – tuatara, lizards, snakes, plesiosaurs & related groups

    • Archosauria – crocodilians, dinosaurs, birds, and related groups

Similarities in evolutionary trends between synapsids and sauropsids

• Parallel evolutionary trends in the two lineages illustrate the adaptive zones available to amniotes on land.

  • Both lineages developed fast-moving predators and prey

  • Both lineages evolved powered flight

    • Bats

    • Pterosaurs and modern birds

  • Both lineages evolved parental care and complex social systems

  • Both lineages had species that became endotherms

Early tetrapod locomotion

• Early tetrapods moved with lateral undulations of the trunk

  • Similar to salamanders and lizards

  • Called axial bending

  • Limbs and feet are used in alternate pairs to grip and push on the substrate

  • Problem- axial muscles are responsible for two essential functions

    • Bending the trunk unilaterally for locomotion

    • Compressing the rib cage bilaterally to ventilate the lungs

    • Two process cannot occur simultaneously

  • As a consequence, only short bursts of speed are possible

    • Quickly go into anaerobic metabolism

Locomotion and respiration in synapsids

• Upright posture with limbs held more beneath the trunk. Limbs in this position can move fore and aft without bending the trunk

• Evolution of the diaphragm

  • Coupled with movements of the viscera while running

Locomotion and respiration in sauropsids

• Bipedal locomotion (Velociraptor and birds)

• Incorporated pelvic movements and the ventral ribs (gastralia) in lung ventilation

• Crocodiles have axial bending, but use three methods to increase respiration

  • Movement of ribs

  • Movement of the liver

  • Rotation of the pubic bones

What kind of lungs do synapsids have?

Alveolar lungs (alveoli fill with air, then they are emptied)

What kind of lungs do sauropsids have?

• Faveolar (septate) lungs

  • Faveoli - small chambers that open from a common space

  • Limits dead air space

  • The large air sacs are poorly vascularized and do not participate in gas exchange

    • Instead, function as reservoirs that store air during parts of the respiratory cycle to create a one-way flow of air through the lung

  • Trachea branches into a pair of primary bronchi, one passing into each lung

    • Secondary bronchi branch from each primary bronchus and several parabronchi open from each secondary bronchus

    • Millions of short interconnected chambers called air capillaries branch from each parabronchus

    • The air capillaries intertwine closely with vascular capillaries

    • Airflow and bloodflow pass in opposite directions, but they are not exactly parallel

    • Crosscurrent exchange system

Renal systems in amniotes

• Urea synthesis is the ancestral condition in amniotes (and probably all gnathostomes)

  • Synapsids excrete urea and developed a kidney that is extraordinarily effective in producing concentrated urine

  • Sauropsids synthesize and excrete uric acid and recover the water that is released when it precipitates

    • Precipitates as salt or potassium urate

    • Sauropsids can also have salt-secreting glands to remove excess salt

Terminology about temperature regulation

• Classification based on the Source of Body Heat

  • Endothermy - internal heat source

  • Ectothermy - external heat source

• Classification based on Constancy of Body Temperature

  • Homeothermy - constant within a range of ambient environmental temperatures

  • Heterothermy – regulates body temperature when active, but allows body temp. to fluctuate with the environment when inactive

  • Poikilothermy – body temperature approximates the ambient temperature. Not capable of controlling body temp. as ambient temp. varies

• Classification based on Metabolic Rate (i.e., how much food energy is used up over time?)

  • In "Cold-blooded" animals, rate of fuel usage is low: called bradymetabolic ("slow metabolism")

  • In "Warm-blooded" animals, rate of fuel usage is high: called tachymetabolic ("fast metabolism")

Advantages and drawbacks of endothermy

Advantages:

• Behavioral and physiological processes are freed from temp dependence

• Allows more active lifestyle and longer movements

Costs:

• Endotherms require more food than ectotherms

• A given environment can support fewer endotherms

• Spend more time finding food

Why evolve endothermy?

• Aerobic Scope Model

  • Increased aerobic capacity, allowing for greater total activity levels and greater ability to recover from sustained activity

• Thermogenic Opportunity Model

  • Greater environmental tolerance – endotherms can live in wider range of latitudes and altitudes and be nocturnal

• Warmer is Better Model

  • Increased metabolic efficiency due to homeothermy – can "fine-tune" physiological systems to a narrow range of temperatures

• Parental Care Model

  • Increased ability for parental care – both brooding/gestating at constant temperature, and increased ability to watch over young

History of dinosaur endothermy

• 1800s paleontologists thought dinosaurs were warmblooded due to their upright posture

• 1900s paleontologists thought dinosaurs were more lizard-like and therefore coldblooded

Evidence supporting dinosaur endothermy

• High blood pressure is necessary to pump blood into brains of tall theropods, ornithopods, and (most especially) sauropods – but this requires powerful, active heart

• Origin of birds from coelurosaurs – birds are known to be warm-blooded, so their immediate relatives might have been, too

  • Problem – some argue that early birds themselves could have been ectothermic, with endothermy evolving AFTER modern birds diverged from non-avian dinosaurs

    • Not a good argument though

• Bone microstructure – lots of signs of reworking (bone being resorbed as mineral source in metabolism, and redeposited), lots of Haversian canals

  • Typical bradymetabolic animals have little reworking and few Haversian canals; typical tachymetabolic animals have lots. Dinosaurs resembled tachymetabolics.

    • Problem – suggestion that very old bradymetabolic animals might develop tissues similar to younger tachymetabolic animals.

      • But, even baby dinosaurs show endothermic-style bone tissues!

• Computer models of the required metabolic rates required for even walking and slow running for large bipedal dinosaurs exceeds the metabolic rate of ecotherms

  • So if they actually moved, big theropods HAD to have been endotherms

  • Also data for small dinosaurs are at least consistent with endothermy for walking, and require endothermy if they were even moderate runners (as their skeletons and foot print evidence shows)

• Examining the nutrient foramina (holes for blood vessels into and out of bones) in dinosaurs shows that the amount of blood flowing through dinosaurs greatly exceeded modern ectotherms, and even modern mammals

  • This suggests that the metabolic rate of dinosaurs was indeed very high

Evidence supporting dinosaur endothermy

• Small brain size – most dinosaurs characterized by brain sizes expected in crocs or lizards of that size; modern endotherms all have much larger brains!

  • Problem - no causal link established between brain size and metabolism. Also, coelurosaurs have larger brains than typical dinosaurs, and proportionately larger brains than contemporary mammals

• Lack of specialized teeth in most herbivorous dinosaurs – non-hadrosaurid, non-ceratopsid ornithischians and sauropods lack sophisticated chewing or shearing teeth

  • Problem – these other dinosaurs are known to have gizzards, which could process the food

Summary of the dinosaur endothermy debate

• If you want to evolve endothermy, need to: increase glucose intake, increase oxygen intake, increase the speed of distribution of glucose and oxygen throughout the body, and deal with excess carbon dioxide, water, and heat

  • All living dinosaurs (Aves) are endothermic tachymetabolic homeotherms

  • The living outgroups (crocodilians, lepidosaurs, turtles) are all ectothermic bradymetabolic heterotherms

  • Non-avian dinosaurs show many anatomical features suggesting levels of activity higher and/or more continuous than that seen in modern "cold-blooded" animals

  • Non-avian dinosaurs show growth patterns comparable to those of modern endotherms, and unlike those of modern and extinct ectotherms

• What evidence is needed to support previous statements?

  • Non-avian dinosaurs would need active ventilation (breathing) to power the muscles and to fuel the growing tissue 

  • Non-avian dinosaurs would need strong, active heart to get the oxygen to the muscles and tissues

  • Non-avian dinosaurs would need structures to control heat

  • Evidence for these structures in dinosaurs? Yes!

Gigantothermy in dinosaurs

• As size increases, surface area to volume ratio decreases.

• The mass of an animal, and the heat it produces, is based on its volume.

• The rate at which an animal can gain and lose heat is based on its surface area.

• Therefore, with bigger body size it takes longer and longer for heat to be lost or gained:

  • Become homeothermic without having the energy costs of endothermy!

• Problem - no good living models (elephants once thought to be partial gigantotherms, but does not now seem true; marine leatherback turtles are, but are not in same type of environment)

  • Also, gigantothermy might apply to large dinosaurs, but would not apply to small species or to babies.

Mesothermy in dinosaurs

• Maybe dinosaurs were intermediate between ectothermy and endothermy…

  • Grady et al. interpreted this data to mean that dinosaurs had the ability to generate internal heat, but did not greatly regulate their body temperature

  • So in fact, what they call "mesothermy" is technically not intermediate between endothermy and ectothermy, but between homeothermy and poikiliothermy

    • And thus dinosaurs in their interpretation would be endothermic mesotherms

Derived traits of testudines

• Body covered in a bony shell

  • Dorsal carapace – usually domed

  • Ventral plastron – usually flat

    • The plastron of many species (including box, mud, and musk turtles) has a hinge. The hinge allows the turtle to tightly clamp the plastron and carapace together, protecting the turtle from predators.

• Shell composed of expanded vertebrae and ribs (carapace) and clavicles and gastralia (plastron)

• Ribs are external to limb girdles

• Bony shell covered with epidermal keratinized (horny) scutes

• No teeth; mouth lined with horny beak

• Neck vertebrae modified to allow greater flexibility

  • Many species can withdraw head entirely within shell

Testudine Fossils

• Oldest fully-shelled fossil dates to late Triassic, 205 mya – Proganochelys (Germany)

  • Proganochelys was similar to modern turtles, but had extensive dermal armor on neck, tail and could not withdraw head into shell

  • Probably terrestrial

• Odontochelys (late Triassic, 220 mya) had welldeveloped plastron, but no carapace except for neural plates

  • Ribs were thickened, but separate

  • Small peg-like teeth

  • Long tail

  • Probably aquatic

Testudine respiration

• Rigid shell prevents respiration by expansion of ribs

• Exhale by contracting muscles that force viscera upward into dorsally located lungs

• Inhalation involves relaxation of muscles; viscera move downward, pulling on lungs

• Most turtles can respire independently of locomotion (unlike lizards)

• Respiration is limited or absent while limbs are withdrawn

• Some aquatic turtles obtain oxygen through their pharynx or their cloaca while submerged

Testudine circulation

• Double circulatory system involves a 3-chambered heart (2 atria, 1 ventricle)

• Ventricle is partially divided, but valves minimize mixing

• Increasing resistance in pulmonary circulation causes deoxygenated blood to backwash to mix with systemic circulation

• Intercardiac shunt shifts deoxygenated blood to the systemic circuit

  • Matches patterns of lung ventilation with pulmonary gas flow

  • Stabilizes O2 concentration in the blood during alternating periods of apnea (no breathing) and breathing

Testudine reproduction

• All turtles are oviparous

  • Eggs may be soft or hard-shelled

• Females dig nest hole in sand/soil and bury eggs

  • No parental care after eggs are laid, except Giant Amazon turtles

• In sea turtles, simultaneous emergence of hatchlings swamps predators, allowing many to escape

• Most turtles have temperature-dependent sex determination (also in crocodilians, tuataras, and a few lizards)

Temperature-dependent Sex Determination

• Critical period is during middle of incubation period

• Three types of TSD

  • 1a – produces males at low temps and females at high temps

  • 1b – produces females at low temps and males at high temps

  • 2 – produces females at both low and high temps and males at intermediate temps

• Individual nests may produce all males or all females depending on where/when laid

• Conservation implications:

  • Loss of nesting sites of loggerhead turtles on southern beaches results in loss of females; northern beaches produce mostly males

  • Artificial incubation programs for some sea turtles produced only one or other sex

Testudine Lineages

• Pleurodira (“side-neck turtles”)

  • Retract the head sideways into the shell

  • Found in Southern Hemisphere

• Cryptodira

  • Have a specialized, hinge-like vertebral joint that allows the neck to double back in an S-shaped vertical curve

Diapsida synapomorphies

• Infratemporal and supratemporal fenestra

• Snout longer than temporal region of skull

• Limbs long and slender

• Complex joint between tibia and astragalus (a proximal tarsal) that creates a relatively solid immobile articulation between the two bones

There are two general evolutionary patterns here:

• The jaws are adapted to a faster but weaker bite

• The creatures are adapted for faster locomotion

Two lineages within diapsida

• Archosauria (crocodiles, pterosaurs, dinosaurs, their common ancestor and all of its descendants, including birds) developed additional openings

  • Antorbital fenestra (in front of the eye)

  • Mandibular fenestra (on the side of the lower jaw)

  • They have teeth that are set in sockets (convergent with mammals in this respect)

• Lepidosauria - represented in the recent by lizards, snakes, and the tuatara

  • The tuatara primitively retains two well defined temporal fenestrae.

  • Lizards and snakes share a common ancestry in which the lower temporal bar has been lost

  • Snakes have further lost the upper temporal bar that primitively separates the upper and lower fenestrae, so that the two openings are confluent in snake skulls

    • This loss of bone comprises part of a trend towards increased skull flexibility in lepidosaurs; snakes are especially talented at swallowing relatively large and difficult prey

    • In contrast, archosaur skulls are typically more massive

Lepidosauria synapomorphies and characteristics

Synapomorphies:

• Overlapping scales; scale-covered skin that is relatively impermeable to water; outer layer is shed at intervals

• Modified mid-dorsal scale row

• Only tetrapod group with a transverse cloacal slit

Characteristics:

• Caudal autotomy

  • Ability to voluntarily lose the tail by means of a fracture plane within caudal vertebrae

    • Characteristic of many squamate groups

Squamata

• Snakes, lizards, and amphisbaenians (burrowing lizards)

• Unlike crocodilians or turtles, squamates exhibit determinate growth

  • Stop growing after reaching adulthood

• Two major groups: Iguania and Scleroglossa

Squamata synapomorphy

• Hemipenes - paired copulatory organs (unique feature of squamates)

  • This organ is tucked away in the tail, emerging from the body through the vent, generally during mating when it is inserted into the female's vent. Some males, such as green iguanas, may ejaculate during breeding season outside of mating attempts.

  • Grooves in surface for transport of sperm

  • Often ornamented with spines and ridges

Evolutionary trends in Squamata

• Limb reduction

  • Prevalent during squamate evolution

  • >60 clades have experienced limb reduction

  • Serpentes, Amphisbaenia, and many groups of “lizards” (especially skinks)

  • Limb reduction is generally associated with habitats such as dense grass or shrubs, where a long, slim body is more maneuverable than a short one with functional legs

• Venom production

  • Present in two major squamate groups

    • Serpentes (rattlesnakes, cobras, etc.)

    • Helodermatidae (Gila monster and beaded lizards)

  • Prey incapacitation

• Highly kinetic skulls

  • Lower temporal bar lost

  • Streptostylic jaw articulation (quadrate free from lower temporal arch as a result of loss of the quadratojugal)

  • Mesokinetic joint between frontal and parietal

Iguania

• Contains iguanas plus a great diversity of smaller groups such as anoles, fence lizards, horned lizards, agamids, and chameleons 

  • Many synapomorphies

    • Use of the tongue in prey capture

      • MOST iguanians use the tongue to pick up prey (obviously, secondarily derived plant eaters, such as the green iguana, don't do this)

      • The extreme case is with the chameleons, in which the tongue can be projected more than the length of the torso

        • Tongue uses suction to attach to prey

Scleroglossa synapomorphies

• A scaled tongue

• Loss of modified middorsal scale row

Scleroglossan adaptations

• Within the group we see a variety of different evolutionary adaptations:

  • Limblessness – loss of limbs has occurred at least four separate times in Scleroglossa, whereas it is unknown in Iguania

  • Marine adaptations – mosasaurs appeared and thrived in the second half of Cretaceous, but were extinguished by K-T extinction

Serpentes evolutionary trends

• Evolutionary trends:

  • Limbs reduced or absent

  • Vestigial hind limbs in male Boidae

  • Kinetic skull

  • Bones of the dermal skull roof and braincase fuse into a solid cylindrical unit

  • The quadrates, palate bones, maxillae and premaxillae become very loose and mobile, allowing manipulation and swallowing of large food items

  • The supratemporal, the skull bone to which the quadrate attaches, also becomes mobile

  • The jaws are jointed and flexible, and in many cases, the two sides do not connect in front, allowing swallowing of even larger objects

Kinds of serpentes teeth and venom delivery

• Venom glands in snakes have a maxillary location

• Classification of dentition and venom delivery

  • Aglyphous dentition – associated with non-venomous snakes. These snakes possess solid fixed teeth, usually uniform in size. The teeth are extremely sharp and are recurved backwards to grip prey more effectively

  • Opisthoglyphous dentition – one or more enlarged teeth near the rear of the maxilla, with smaller teeth in front. In some species, the fangs are solid; in others there is a groove on the surface of the fang that may help to conduct saliva into the wound

  • Proteroglyphous dentition – hollow fangs located at the front of the maxilla; the fangs are permanently erect and relatively short

  • Solenoglyphous dentition – hollow fangs are the only teeth on the maxilla, which rotates so that the fangs are folded against the roof of the mouth when the jaws are closed. Permits solenoglyphous snakes to have longs fangs that inject venom deep into the tissues of the prey

Types of serpentes locomotion

• Lateral undulation - S-shaped movement by exerting force against surface irregularities such as rocks, plants, and other features of the terrain

• Concertina movement - enables some snakes to move through a narrow passage, such as burrows or when climbing trees by moving within and using irregular channels in the bark

  • Anchors posterior part of body then extends head, then anchors anterior part of body and draws body up forming new loops then anchors posteriorly

• Rectilinear movement – in a straight line, such as when stalking prey; use ventral scales in an alternating pattern of movement, contraction, fixation, and stretching to achieve movement

• Sidewinding – used by desert vipers to move across loose sandy surfaces with minimal surface contact; raises body in loops, touches ground at only 2 or 3 points

Amphisbaenians

• Fossorial; difficult to study

• Skulls are rigid, used for tunneling

• Most are legless, but species in the genus Bipes have well developed forelegs used for digging

• Distinct dental structure

  • Single median tooth in the upper jaw, which is unique to this group of vertebrates

  • Tooth aids in capturing invertebrate prey

• Skin has annuli (note earthworm-like appearance)

• Relationships of Amphisbaenia within the Scleroglossa are uncertain

Lizards vs. Snakes

• Snakes do not have eyelids

• Snakes do not have external ear apertures

• Snakes have only one functional lung; lizards have two functional lungs

• Snakes have a single row of ventral scales; lizards have several rows of ventral scales

Reproduction in lizards and snakes

• Variety of reproductive strategies from oviparity to viviparity

  • Viviparous species tend to produce small numbers of large young

  • Oviparous species tend to produce large numbers of small young

  • Large species tend to produce more eggs/fetuses than do small species

• Parthenogenesis

  • Only one sex - all females

  • Reported in six lizard families and one snake species, but may be more common

  • Females produce fertilized eggs

  • Progeny are genetically identical to mother

  • May have evolved from hybrids

• Associated vs. Dissociated Reproductive cycles

  • Associated

    • Maturation of gametes/hormones associated with mating and fertilization

    • Most common system

  • Dissociated

    • Maturation of gametes/hormones not associated with mating and fertilization

    • Involves storage of sperm in special tubules of the oviducts

Sphenodontidae

• Tuataras; 2 species

• Lizard-like in appearance, but skull is heavier, vertebrate exhibit a more primitive form, and rib morphology differs

• Nocturnal, live in cool habitats so body temps of active tuataras are lower than those of most lizards (6-16°C).

• Well developed parietal eye on the top of the head

  • Parietal or pineal median eye has elements of a cornea, retina, and lens, but is covered with scales and only detects changes in light intensity

  • It is speculated that it functions as a light sensor to influence the amount of time they send basking

• Lowest active body temperature of any reptile = 13 C (6-16 C)

Archosauromorpha synapomorphies

• Two new skull openings, the mandibular fenestra and antorbital fenestra

• Orbit of eye is shaped like an inverted triangle

• Serrated (saw-edged), laterally- compressed teeth set in sockets (a condition called thecodonty)

• A large process on the shaft of the femur – the fourth trochanter. It served as the attachment point for major tail muscles, the caudofemoralis group of thigh retracting muscles

Features evolved in Pseudosuchia and Ornithodira (Archosaur stem groups)

• The fifth toe in the foot, homologous with your "pinky toe" was reduced in size.

• In earlier vertebrates, the palate (roof of the mouth) bore at least one row of accessory teeth, but the archosaurs appear to have lost this feature, as did many other lineages of tetrapods.

• Specialized ankle joints (crurotarsal and mesotarsal)

Crurotarsal vs. Mesotarsal ankle

Pseudosuchia have a crurotarsal ankle joint

• Very flexible

• The Astragalus has a peg that fits into a socket in the calcaneum

• Permits either a “high walk” stance (for burst speed) or a sprawling posture (like early tetrapods)

Ornithodira have a mesotarsal ankle joint

• Simple hinge joint between the lower leg and the astragalus and the calcaneum, and the distal ankle bones

• Restricts posture to an erect orientation

• Gait is called parasagittal (the limbs move parallel to the vertebral column)

  • Birds and most mammals have parasagittal gait. Birds inherited it from their dinosaurian ancestors, while mammals evolved it independently

  • Advantage of a parasagittal gait – it improves maneuverability/agility.

  • Disadvantage – reduces stability. It's easier to tip over an erect cow than it is to tip over a more sprawling crocodile of similar size; the crocodile has a wider base of support, and thus can be said to be more stable.

Crocodylia

Alligatoridae, Crocodylidae, Gavialidae

• Gavialidae were almost exclusively fish-eaters

Crocodylia history

Today's crocodiles are mostly large reptiles adapted to a semi-aquatic existence. This was not always the case.

• During the late Triassic and early Jurassic these animals were mostly small, active terrestrial forms, quite unlike crocodiles today

• In fact the crocodilians began as small, gracile (slender and graceful) terrestrial predators

• After flourishing for some 40 or 50 million years - almost as long as the age of mammals - these small active animals died out, either through competition with small theropod dinosaurs, or perhaps through a mass extinction

• Surviving crocodilians were larger animals, more like the crocs we know today, but mostly marine

• Later in the Jurassic, these marine forms invaded fresh water and swamp environments as large semi-aquatic predators, and finally returned to the land again as small forms

Gavialidae

Gharial or Gavial

• Have a very slender snout, with very small nasal bones, which reduce resistance in the water

• A change in the set of jaw muscles emphasized in this lineage.

• For these fish eaters, there is little need for the crushing blow the heavy crocodylid skull could potentially produce. A fast, snapping action is far more important. This tendency developed independently in a variety of crocodilian lineages.

• Only one species of this lineage survives, the gavial or gharial, Gavialis gangeticus, also known as the ‘true’ or Indian gavial. 

  • Lives in the Indus, Ganges and Brahmaputra river systems, feeding on fish. Despite its large size (up to 8 meters in length) it is harmless to man.

Alligatoridae

• This group includes the alligators and their more primitive cousins, the caimans

• The snout is broad and only moderately elongated, and not demarcated from the rear of the skull. There are 17-22 teeth on either side of each jaw. The fourth tooth of the lower jaw fits into a pit in the upper jaw and is invisible when the mouth is closed (distinguishes it from crocodiles)

• During the past this group ranged widely through Europe, Asia, North and South America.

• 7 living species, most of which are endangered

Crocodylidae

• Includes 12 living species

• These crocodiles have 14 or 15 teeth on each side of the mouth.

• The fourth tooth fits into a pit in the upper jaw, but remains visible when the mouth is closed

• False gavials are so called because these long-snouted forms resemble Gavials, to which they are only distantly related.

• Tomistoma has 20 or 21 teeth on each side, and, as with the Crocodylinae, the fourth tooth remains visible when the mouth is closed

Alligatoridae vs. Crocodylidae

Crocodiles have 14 or 15 teeth on each side of the mouth, and the fourth tooth remains visible when the mouth is closed.

Alligators have 17-22 teeth on each side, and the fourth tooth is invisible when the mouth is closed.

Pterosauria

• Some paleontologists thought that because pterosaurs were reptiles, they must have been cold-blooded, slow-moving, and poor flyers.

• Some pterosaurs may have had some sort of hair-like body covering, which could very well mean that they were endothermic; they were active creatures with powerful flight muscles, hollow bones, and other birdlike features.

• Some were probably even bipedal, walking on their hind legs like birds. Unlike birds, they had papery wing membranes

• Two major lineages:

  • Rhamphorhynchoids, more properly termed the basal Pterosauria, had long tails and teeth; dominated skies of the Jurassic period; all went extinct at the end of the Jurassic

  • Pterodactyloids had shorter tails and, in some cases, toothless beaks

• Pterosaurs had a wide range of feeding adaptations and held similar niches to birds.

  • Diverse range of head types. Ability to fly allowed them to evolve into many niches, taking advantage of many different food sources, which would explain the range of skull morphology seen

Pteranodon

• Several species of large pterosaurs from the Cretaceous period in North America.

• It had a large crested head, a huge wingspan (some 20-25 feet), and a comparatively small body.

• This is deceiving; it looks like the head and wing bones were too bulky, and the hindlimbs appear small and weak. Not so; the bones of Pteranodon are actually completely hollow (about 1 millimeter thick!), and were quite light. The whole animal probably weighed about 25 pounds, only slightly heavier than the largest flying birds

• Almost certainly a soaring animal; it used rising warm air to maintain altitude; a common strategy among large winged animals (among birds, albatrosses and vultures are adept at soaring).

  • Its scooplike beak was used for snapping up fish as it soared over the oceans that it nested by.

    • A good modern analog for Pteranodon would be the pelican

Lagosuchians

• Lagosuchians like Marasuchus were not dinosaurs, but rather dinosauramorphs, probably fairly close to looking like primitive dinosaurs

  • The literal meaning of "lagosuchid" is "rabbit-croc."

  • Thought that Lagosuchids were accomplished jumpers, although perhaps not as specialized for this locomotor style as rabbits. A reasonable speculation might be that they were ambush predators who used a leaping attack, but did not use repeated jumping in routine locomotion

Ornithischia

• “Bird-hipped” dinosaurs

• Have a very similar hip structure to living birds

  • In this type of hip the pubis points backward parallel with the ischium

• Includes Hypsilophodon, Maiasaura, Triceratops, Stegosaurus, and Ankylosaurus

Thyreophora

Stegosauria and Ankylosauria

Stegosauria

• Characterized by the armored plates or spines on back

• Recent work suggests that they may have served a thermoregulatory function; the plates are riddled with blood vessel canals

Ankylosauria

• “Armored-plated dinosaurs”

• First appeared in the Early Jurassic (~208-204 my) and became extinct at the end of the Cretaceous (65 my).

• Their fossils are known from every continent, including Antarctica (which was not ice covered during the Mesozoic)

Genasauria lineages

Pachycephalosauria, Ceratopsia, Hadrosauridae

Pachycephalosauria

Thick dome on skull used for male-male competition

Ceratopsia

• Characterized by a giant bony frill on the head

• Recent work suggests that the frill may have served a thermoregulatory function

Hadrosauridae

• “Duck-billed” dinosaurs

• Common in the Upper Cretaceous of Europe, Asia, and North America

• From fossil skin impressions it is known that hadrosaur hands were webbed But…stiff tails (supported by ossified tendons), sturdy bones, and rapidly- replaced teeth suggest that hadrosaurs spent most of their time on land, though close to bodies of water, feeding on tough terrestrial plants.

• Discovery of spectacularly preserved hadrosaur nests and young shows that hadrosaurs migrated to higher ground to reproduce.

• Fossil finds indicate nesting behavior and parental care of young

• Two subfamilies of hadrosaurs

  • Lambeosaurinae: have a crest on their skull

  • Hadrosaurinae: lack the crest

Lambeosaurinae crest function

• Contains the nasal passages, which "looped" through the crest and often formed sizeable chambers before passing into the airway

• Most accepted theory today of the function of the crest is that it served as a resonating chamber, allowing lambeosaurs to make deep, loud sounds. 

• Perhaps these calls warned of predators, or kept a herd together, or attracted potential mates, or did all these things.

• Crest may have also functioned as a visual display device; also, perhaps large and odd-shaped crests attracted mates

Saurischia

• “Reptile-hipped” dinosaurs

• Have the more primitive hip structure, as seen in the archosaurs.

  • In this type of hip, one of the three main bones (the pubis) points forward.

• Includes Velociraptor, Diplodocus, and Seismosaurus

• The saurischian dinosaurs include the largest land animals ever to have lived, such as Seismosaurus, which is estimated to have weighed up to 100 tons.

• Two major lineages:

  • Sauropodamorpha

  • Theropoda

Saurischia Synapomorphies

• Modifications of the “hand” including a large thumb (in those lineages retaining a “hand-like forelimb)

• Elongation of posterior cervicals

Sauropod vertebrae

• Contain large pleurocoels: holes that reduce weight and allow muscles/blood vessels to pass through

• Contain a large ligament to provide support to neck

Sauropodomorpha

• The largest animals ever to walk on land.

• But…no sauropod ever equaled in size the greatest of the baleen whales

• It was once thought (on the basis of the North American fossil record) that the sauropods became rare after the Late Jurassic, but in most parts of the world they continued as a common element of the megafauna until the very end of the Cretaceous. Throughout their long history they continued to evolve and branch out new species and families.

• There are at least three peaks of sauropod diversity in the Late Jurassic, late Early Cretaceous and latest Cretaceous, corresponding to high sea levels and increased speciation resulting from geographical isolation

• Sauropods were long believed to be semi-aquatic swamp wallowers, relying on the buoyancy of water to support their massive bodies. But analysis of their skeletons, in comparison with those of large terrestrial and semi-aquatic animals, and of sedimentation where their fossils have been found, show that sauropods were fully terrestrial

• Not only were sauropods as terrestrial as elephants, but fossil trackways indicate that they lived in herds, again like elephants today

• Sauropods were superbly adapted creatures, the Mesozoic equivalent of the elephant.

Sauropod Endothermy

• It has been suggested that they were ectotherms – that they could not possibly have been endothermic, because they were too big to eat enough food to fuel their bodies. 

• This argument falls apart when you realize most weight estimates have been overstated.

  • An analysis of the load-bearing capacity of Giraffatitan ("Brachiosaurus") brancai limb bones show that this creature could only have weighed 15 tons, not the 50 to 80 tons that had previously been estimated

  • Combine that with a huge stomach full of gastric stones (giving a food-grinding ability greater than any elephant's) and a gut full of symbiotic bacteria, and the possibility they were at least partially endothermic is not too unreasonable

• Whether Sauropods had the full endothermy of birds, mammals, or small theropod dinosaurs is another question.

  • The most likely explanation is that (and this would be true of most large dinosaurs) they started out as active fully endothermic youngsters and settled down into a gigantothermic metabolism as they approached adult size.

  • Gigantothermy

Macronaria

• Containing brachiosaurs, camarasaurs and titanosaurs

• These dinosaurs were massive

• Synapomorphy

  • Front legs as long as or longer than the hind legs. Thus these dinosaurs had a giraffe-like posture.

• Other characters

  • “Shortish” tails

  • Short, high skulls

  • More and broader teeth

• Sauroposeidon

  • Means "earthquake god lizard” 

  • The colossal creature would have weighed 60 tons and stood 18 meters (60 feet) tall. 

  • The massive load was made lighter by the bones being filled with tiny air cells, revealed by CAT scan

Diplodocomorpha

• Containing Diplodocus and Seismosaurus

• Characterized by having whip-like tails, low skulls and teeth restricted to the front of the jaws

Diplodocus

• Diplodocus was among the longest of the land animals but not the heaviest; much of its length was taken up with its strong whip-like tail.

• It had a brain the size of a fist; a concentration of nerves in the base of its spine helped it to cope with its enormous size, and control its hind legs and tail. 

• The largest Diplodocus is usually noted as being 15 tons and 27 meters long

Seismosaurus

• Found in New Mexico in 1986

• May have weighed 80 to 100 tons and might have been 120 feet long and stood 18 feet tall at the shoulders.

• If correct, this was the longest dinosaur known.

• But…might have been a really old Diplodocus. Still not clear

Theropoda

• Bipedal predatory dinosaurs, including birds

• Name means “beast foot”, after the curved claws 

• Characterized by extremely hollow limb bones

• Most retain the primitive condition of bladelike, serrated teeth, indicating they were carnivores

• All theropods were obligate bipeds

• Became the largest terrestrial carnivores of all time, but began at about 1 m long, and even by end of Late Triassic were dwarfed by other dinosaur lineages

Tetanurae

• (“Stiff tails”)

• Synapomorphies

  • Teeth restricted to front of jaws in front of orbit

  • Larger hands for clutching prey

  • Interlocking caudal vertebrae to make tail rigid for counterbalance while running

• The Tetanurae ("stiff tails") consist of a number of parallel evolutionary lines, all of which seemed to have evolved increasingly bird-like features.

  • For example, their rib-cages indicate they had a sophisticated air-sac-ventilated lung system, which exists today only in birds.

  • Such an advanced respiratory system would have been accompanied by an advanced circulatory system

    • Even ectothermic crocodiles have an efficient four-chambered heart, like mammals and birds, rather than the threechambered reptile heart

  • All this indicates a high metabolic rate; like birds, advanced theropods were certainly endothermic (warm-blooded).

Avetheropoda

• Synapomorphies and other characters

  • More complex air chambers, indicating a more advanced air sac system

  • Semilunate carpal block, a specialized half-moon shaped carpal bone structure which let the hand fold up against the body while running

  • Greatly reduced or lost manual digit IV (present in primitive forms, but most avetheropods have only three fingers, namely I-III)

• Divided into two clades: Carnosauria and Coelurosauria

Carnosauria

• The dominant predators from the Middle Jurassic until the beginning of the Late Cretaceous

• Most of them were large; in fact, the largest known theropods are carnosaurs

• Characterized by: Enlarged nares (better respiration) and extra openings in snout (additional muscle attachments for jaws)

• Best known from Allosaurus of the Late Jurassic of western North America and Europe (and maybe Africa) and Giganotosaurus (the largest known theropod) from the earliest Late Cretaceous of Argentina

• In the Late Cretaceous carnosaurs are replaced by ceratosaurs in the southern continents and Europe, and by coelurosauroid tyrannosaurids in Asia and North America

Coelurosauria

• Clade containing all theropods more closely related to birds than to carnosaurs (Velociraptor, Tryannosaurus, Albertosaurus)

• Characteristics include elongated arms with narrow, three-fingered hand, well-developed hinge-like ankles (possible rotation of the ankle is reduced, which is helpful during locomotion), and “protofeathers” in some (filament-like structures covering much of the body)

• These features may be lost or modified by later coelurosaurs (birds, for example)

Maniraptora Synapomorphies

• Enlarged brain size compared to other dinosaurs

• Bony secondary palate (roof of mouth) to absorb additional stresses

• Enlarged sternal plates

• Increased forelimb length

• Enlarged semilunate carpals, perhaps to compensate for enlarged forelimb and hand

• Shoulder joints that faced sideways rather than backwards (as in most dinosaurs)

Eumaniraptora

• Synapomorphies

  • Backwards-pointing pubis (in this case, associated with change in leg muscles)

  • Tails which were very mobile proximally but very stiff distally

  • Retractable pedal digit II, often tipped with a big sickle claw

• Eumaniraptorans include Avialae (Birds) and Dromaeosauridae

  • Note: the term "Avialae" is used for the taxon comprised of Archaeopteryx, modern birds (true Aves), their most recent common ancestor, and all of its descendants

Dromaeosauridae

• Dromaeosaurids, or "raptors", were carnivorous, had tails stiffened by long rods and extremely large sickle claws

• Dromaeosaurids seem to have shorter, stockier legs than a lot of other predator theropods: may have been more cat-like (agile ambush predators) than dog-like (fast pursuit predators)

• Best known from 4 m long Deinonychus of the late Early Cretaceous of North America and 2 m long (at most) Velociraptor of the Late Cretaceous of Mongolia 

• Dromaeosaurids may have hunted in groups