Lab-6-Archosaurs - EEB 111 - Biology of Vertebrates Professor - Caitlin Brown Summary - Archosaurs PDF

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EEB 111 - Biology of Vertebrates
Professor - Caitlin Brown
Summary - Archosaurs...

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Lab 6: Non-Avian Archosaurs - Spring 2018 EEB 111 Week 7

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Introduction

There are three main groups of amniotes: reptiles, birds, and mammals. Reptiles, crocodylians and birds are often grouped together as Reptilomorpha or Diapsids, while Archosauria is a monophyletic clade that comprises crocs, birds, non-avian dinosaurs and their close relatives such as the pterosaurs. Many traits we associate with birds arose in archosaurs long before birds diverged from the archosaur lineage. The name Archosaur means ”ruling reptiles” for their long dominance during the Mesozoic era. Living archosaurs comprise two orders: Crocodilia, the alligators, crocodiles, and caimans and Aves, the only remaining dinosaurs. During the Mesozoic, the extant orders lived alongside more members of the clade Dinosauromorpha, the non-avian dinosaurs clade Sauropterygia, the fully aquatic icthyosaurs and plesiosaurs and Pterosaurs, a flighted class of archosaurs. The dinosaurian clades do not have formal rankings such as class and order, but are divided into two major lineages based on the morphologiy of their pelvic girdle. An antorbital fenestra, four-chambered heart, parental care, laterally compressed teeth and vocalizations unite the archosaurs despite considerable morphological and ecological variation. We will see in this lab how these traits are ascribed to the archosaurs only known from fossils. Like other diapsids, archosaurs retain the amniotic egg as their form of reproduction and they have scales (or feathers, a modified scale) in their epidermis. Modern crocodilians are a small remnant of a once large and successful group. All crocodilians have well-reinforced skulls with massive jaw musculature that provides a wide gape and rapid, powerful closure over prey. Their many sharp teeth are designed for gasping but not for chewing. A muscular gizzard finishes the job of grinding up food with the help of swallowed stones and other items called gastroliths. Crocodilia is composed of three surviving families (Alligatoridae, the alligators and caimans; Crocodylidae, the crocodiles, and Gavialidae, the gharials). All living representatives of this order have become secondarily aquatic, the driving force between some synapomorphies of the group: a medio-laterally compressed tail, webbed feet, ability to shunt blood through the Foramen of Panizza and pressure-sensitive receptors along the snout. →How does each synapomorphy aid crocodilians in their aquatic habitat? The 3 orders are easy to distinguish from one another as they have adapted to different habitats and have distinctive snout morphologies. Alligators and caimans are found in freshwater. Crocodiles can be found in freshwater rivers but may also live in brackish or saltwater habitats. The gharial is the only extant species in Family Gavialidae. Gharials have the narrowest snout of any crocodilian and once lived in large rivers from northern India to Burma.

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Fig. 1 Crocodilian skulls. (a-b) alligators (c) crocodile (d) false gharial crocodile (e) gharial Throughout their lives crocodilians are armored by Scutes, small plates of dermal bone. They are also defended by their many sharp teeth and extremely strong bites. One of the most important adaptations present in the crocodiles is the presence of a secondary palate dividing the nasal and oral cavities. The bony palate is covered by tissue in life, extending well into the pharynx. Compare the lizard and crocodile skulls to see the bony portion of the secondary palate. →The secondary palate evolved before crocodilians became aquatic. Mammals convergently evolved one as well. Why do you think the development of a secondary palate is an important adaptation?

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Fossil Species

Today we will examine replica specimens of several fossil species located in our lab and in the Geology building. The UCLA geology display also contains some fossil hominids, members of the mammalian lineage closely related to humans. Although our ancestors are certainly not archosaurs, we use similar techniques to infer their behaviors as we do the extinct archosaurs, and the hominids are thus included in this lab.

2.1

Phylogenetic Bracketing

One method of estimating behaviors in extinct lineages is to use a phylogenetic bracket. It is used to infer the likelihood of unknown traits in organisms based on their position in a phylogenetic tree. One of the main applications of phylogenetic bracketing is on extinct animals known only from fossils. By examining behavior in lineages that diverged before and after our extinct clade, we can infer whether a trait would have been ancestral to the extinct clade or not. Then, careful assessment of skeletal characters related to the behavior can be made using living and extinct species. For the trait parental care, we would expect most fossil lineages to share the trait with the more basal crocodilians and the more recent lineage, birds. The evolution of endothermy is more difficult to resolve, as the character appeared after the crocodilian lineage diverged from other archosaurs. Visit the phylogenetic bracketing station to see evidence for each behavior in extinct archosaur fossils.

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Fig. 2 Phylogeny of major diapsid clades and their relationship to mammals.

2.2

Ichnofossils

Ichnofossils, or trace fossils, are key to reconstructing behaviors of fossil species. Trackways can reveal details about biodiversity, social structure, speed of movement, migration patterns, predatory behavior and more. Two reproductions of fossil trackways are given on the supplementary ichnofossils handout. Several important clues to fossil vertebrate behavior are contained in the footprints.

2.3

Dietary Reconstruction

Stomach contents, tooth and claws and scars on bones are all potential sources of dietary information. Unlike modern archosaurs, non-avian dinosaurs had elaborate tooth structures specific to their diets. In contrast to mammals, dinosaurians were able to replace their teeth continually, ensuring that a sharp set was always at the ready (and hundreds of specimens fell out for paleontologists to find!). Examine the 3 specimens on the supplementary dietary reconstruction handout and reconstruct both what they are best adapted to eat and whether they broke down their food for easier digestion.

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Specimens

3.1

Archosaurs, Order Crocodilia: Crocodilians

3.1.1 Family

American alligator Alligatoridae

Sci. Name Alligator mississippiensis Habitat

Coastal freshwater, swamps, lakes, and marshes

Range

Southeastern USA

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Notes

An oviparous species that builds large nests of rotting vegetation to incubate eggs. Females provide maternal care, staying with the eggs from the time of laying to the time of hatching, and guarding the nest against scavengers looking for a quick omelet.

3.1.2

Spectacled caiman

Family

Alligatoridae

Sci. Name Caiman crocodilus Habitat

Shallow freshwater lakes, streams, and ponds. Swamps and marshes

Range

Tropical South America

Notes

Caimans range from 4 to 15 feet in length as adults, and are the major South American crocodilians. They are effective predators, feeding on a wide variety of vertebrate prey from fish to small mammals.

3.1.3

African dwarf crocodile

Family

Crocodylidae

Sci. Name Osteolaemus tetraspis Habitat

Freshwater swamps and swamp forests

Range

West and West-Central Africa

Notes

The african dwarf crocodile is the smallest crocodilian species, usually no more than 1.6m in length. Living in monsoon habitats, they feed mainly on fish during the wet season and crustaceans in the dry season. Due to their relatively docile nature, they are frequently caught for meat and are now listed as vulnerable by the IUCN.

3.2

Dinosauria: Clade Ornithischia

3.2.1 Family

Edmontosaurus- Geology Building Hadrosauridae

Sci. Name Edmontosaurus annectens Habitat

Along the coastal plain that bisected Cretaceous North America.

Range

From Colorado to Canada

Notes

Named after Edmonton, Canada. Bone beds contain many individuals of different sizes, indicating this animal may have traveled in herds. They are typically in association with Triceratops. Some bones have scars consistent with Tyrannosaur attacks.

3.2.2 Family

Diplodocus- Geology Building Diplodocidae

Sci. Name Diplodocus sp. Habitat

Inland, vegetated areas of western North America.

Range

Central western North America 4

Notes

Named after the ”double beam” structure of the tail vertebrae. One of the longest dinosaurs at 82ft, its size may have deterred predation from the associated predators Allosaurus and Ceratosaurus. It is though to have had a derived respiratory and circulatory system to survive with such an elongate neck.

3.2.3

Stegosaurus- Geology Building

Family

Stegosauridae

Sci. Name Stegosaurus sp. Habitat

Semiarid areas of western North America

Range

Central western North America

Notes

”Roof lizard”. A Jurassic herbivorous dinosaur with a short neck and legs, inidcating a diet of bushes and other low-growing plants. The anterior rostrum appears to have supported a turtle-like beak and the teeth are small, yet no gastroliths have been found in fossils.

3.3

Dinosauria: Clade Saurischia

3.3.1

T. rex (Stan)- Geology Building

Family

Tyrannosauridae

Sci. Name Tyrannosaurus rex Habitat

Semiarid areas of western North America

Range

Throughout western North America, east of the sierras

Notes

”Terrible lizard king”. Whether this animal was an apex predator or scavenger is one of the fiercest debates in paleontology. The robust, U-shaped front of the rostrum and densely packed anterior teeth indicates considerable force was exerted on this part of the jaw.

3.3.2

Compsognathus- Geology Building

Family

Compsognathidae

Sci. Name Compsognathus longipes Habitat Range

SW Canada to the equator; Rocky Mountains to Atlantic coast

Notes

”Dainty jaw”. One of the smallest known non-avian dinosaurs. They appeared in the Jurassic Park films and were referred to as ”compies”.

3.3.3

Deinonychus- Geology Building

Family

Dromaeosauridae

Sci. Name Deinonychus antirrhopus Habitat:

Swamps or floodplains

Range:

Central North America

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Notes:

”Terrible claw” Like other members of the family, the hindfeet bear three claws. Two support weight and one is held flexed to maintain an enlarged, sharp claw. In smaller species the claw is inferred to be an aid in climbing, but in large species such as Deinonychus it was likely used to restrain prey in a manner similar to modern flightless, carnivorous birds.

3.3.4

Velociraptor

Family

Dromaeosauridae

Sci. Name Velociraptor mongolensis Habitat

Arid environments with sand dunes

Range

Discovered in the Gobi desert of inner Mongolia and nothern China

Notes

”Swift robber”. Forearm skin impressions from this species bear bumps believed to be the knobs found at the base of feathers. Many dromaeosaurs have been found with downy feathers on their trunks and stiff vanes on the forelimbs and distal portion of the tail.

3.3.5

Archaeopteryx

Family

Avialae

Sci. Name Archaeopteryx lithographica Habitat

Near-shore habitats

Range

Fossils are known from continental Europe

Notes

”Ancient wing”. The first identified avian dinosaur. The exceptional preservation of the feathers surrounding the fossil gives us resonable confidence that this animal could fly. Modeling supports this hypothesis.

3.4

FOR FUN Hominids: Class Mammalia)

Several hominid skulls reside with the collections here, despire these being mammals from the Cenozoic era. They are not part of our lab materials but are still interesting to study. 3.4.1 Family

Austalopithecus (Lucy) Elapidae

Sci. Name Australopithecus afarensis Habitat

Open and riparian habitats

Range

Modern day Kenya and Tanzania

Notes

The most likely creator of the Laetoli trackways. Their cranial size and leg lengths relative to body size show that bipdeal locomotion preceded large brains or long-distance dispersal capabilities.

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3.5

FOR FUN: UCLA Meteorite Galley)

Just around the corner from the fossil display is the UCLA meteroite gallery, the 5th largest collection of meteorites in the United States. Much smaller than asteroids, meteorites do not have the mass to trigger a cataclysmic extinction. Nonetheless, they are a reminder that our planet is constantly bombarded by extraplanetary material and could face a large bolide impact again someday...

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