Lecture 6: Saurischia and Theropoda

Transcription

1 Lecture 6: Saurischia and Theropoda Saurischia Term coined by Seeley (an employee of the Cambridge Museum) in 1887 Name is based on the anteriorlly-oriented pubis of most (but not all saurischians). This is not a shared derived trait; it is a primitive trait from deep in Reptilia Shared derived characters of Saurischia: Twisted thumb (digit 1) Reduced pinky (digit 5) New articulation points in dorsal vertebrae (hyposphene-hypantrum articulation) Elongation of forward neck (cervical) vertebrae Controversy over placement of earliest, carnivorous dinosaurs (Eoraptor, Herrerasaurus, etc.). 1) Basal Dinosaur 2) Basal Theropod (this is what I told you in class) 3) Basal Saurischian OUTLINE OF THEROPOD LECTURE 1. Defining characteristics of clades 2. Distribution in time and place 3. Lifestyle issues Theropods 1. Defining characteristics of clades Theropoda ( beast foot, really bird-footed) General characters: bipedal, carnivores (more or less), less abundant than herbivorous dinosaurs (only ½ to 1/11 as numerous), less diverse than herbivores (only 40%), classification nightmares, ranged in size from <2 feet/10 lbs. to 50 feet or more and 5-7 tons. Shared derived characters: big eyes, extra antorbital fenestra, broad lachrymal ( tear-duct ) bone, 3 elongate fingers (vestigal 4 th and 5 th digits) with grasping capability, narrow hind foot with three functional digits, hollow bones, bowed femur, sacrum with at least 5 vertebrae, pubic boot, tails stiff distally used as counterbalances, loosely-jointed kinetic skulls Long bones are hollow - have an outer layer of dense, compact bone that is far thicker than that of other dinosaurs; spongy bone confined to the articulation zones and edge of the marrow cavity. 1

2 Ceratosauria Shared derived characters: Reduction and fusion of skeleton, specifically the upper ankle bones and the sacral ribs with the ilium, ridges around hip socket for support, in most - notch between maxilla and premaxilla on upper jaw to accept large lower tooth Representative genera: Ceratosaurus (7 m), Coelophysis (2-3 m), Syntarsus, Dilophosaurus Ceratosaurs existed from the Late Triassic through the Late Jurassic, global distribution Tetanurae Shared derived characters: Overall, more bird-like than ceratosaurs: Large portions of the tail stiffened by extended bony struts (zygopophyses); tooth row doesn t extend behind antorbital opening) Spinosauroidea Shared derived characters: Large snouts, strong shoulders, long arms and clawed hands; probable fish eaters Representative genera: Afrovenator (10 m), Megalosaurus (10 m), Spinosaurus (~18 m), Baryonyx (7 m) + see below, and including Masiakasaurus knopfleri (2 m spinosauroid from Madagascar named after Dire Straits guitarist Mark Knopfler) Late Jurassic Early Cretaceous, global distribution *Note: Spinosaurus was first discovered in Egypt in 1915; specimens in Munich were destroyed during the allied bombing in WW II. More remains from the Bahariya oasis have recently been found by grad students from Penn Carnosauria Shared derived characters: Big nostrils with elaborate sinuses, big heads, all more than 5 m long Representative genera: Allosaurus, Gigantosaurus Late Jurassic Early Cretaceous, global distribution Coelurosauria Shared derived characters: Big brains, elongate forelimb, arctometatarsal ankle, very bird-like Tyrannosauroidea Shared derived characters: Very reduced forelimbs with only 2 functional fingers, m long Ornithomimosauria Shared derived characters: Small, lightly built skulls with very large orbits, no upper teeth, few lower teeth, long arms Very ostrich-like 2

3 Oviraptosauria Shared derived characters: Short skulls with many fenestrae, no teeth, wishbone Therizinosauroidea The giant claw Shared derived characters: very long claws, teeth adapted for plant-eating Troodontidae Shared derived characters: Huge front-facing eyes, very large brain Dromaeosauridae Shared derived characters: Enlarged claw on 2 nd toe, rear-facing pubis One spectacular fossil of Velociraptor captures it in a struggle with a Protoceratops 3

4 2. Distribution in time and place Ceratosauria: Late Triassic through the Late Jurassic, global distribution Spinosauroidea: Late Jurassic Early Cretaceous, global distribution Carnosauria: Late Jurassic Early Cretaceous, global distribution Coelurosauria: Early - Late Cretaceous, largely Northern Hemisphere 3. Lifestyle issues a. Habitats Theropods lived in all habitats! b. Preservation and taphonomy Theropod fossils are relatively rare (delicate bones), however some spectacular nearly complete fossils have been found and there are at least two theropod bone beds: the Coelophysis quarry at Ghost Ranch (where 5-10,000 skeletons and partial skeletons of the dinosaur Coelophysis bauri have been found) and the Cleveland-Lloyd quarry in Utah where the remains of at least 44 Allosaurus have been found. It is unusual for large numbers of carnivores to amass, so the sites above must be anomalous. c. Diet i. The "Great" Tyrannosaurus Debate: was T. rex a predator (using a chasing or land shark ambush approach) or a scavenger? Evidence for predation May have had pockets on teeth that housed bacteria (for poisoning victims; Komodo dragons have these too) Analysis of Triceratops bone with bite marks indicates a very strong bite potential (13,000 newtons, like that of an alligator) Skull analysis shows that tyrannosaur had vertically oriented orbits, like other carnivores with very strong bites **The Truth? T. rex probably did both Evidence for scavenging Had rounded (crushing) rather than flattened (slicing) teeth Teeth typically found in mass death deposits Arms too small to hold on to struggling prey Large olfactory lobes in brains (for detecting rotting meat) T. rex couldn t run fast enough to catch prey Tyrannosaurs were clearly alpha carnivores (bone-filled coprolites are direct evidence). ii. Oviraptors used tough beaks for eating shellfish, seeds, or eggs? iii. Spinosauroids fish eaters? (narrow snouts, long claws) 4

5 iv. Ornithomimids some are found with gastroliths and one species has a mouth full of baleen they were almost certainly not carnivores d. Senses Smell: tyrannosaurs had huge olfactory lobes Vision: based on the positions of their orbits, tyrannosaurs also had excellent stereoscopic vision (over a 50 range) and depth perception; the troodon Saurornithoides had an even larger range of stereoscopic view (~130, the same as modern cats), but many theropods had a much more restricted range for excellent depth perception (e.g. Carcharadontosaurus = 20 ) Hearing: Theropods all had large auditory canals, which probably equated to excellent hearing; troodons had an extra feature: an aural canal that connected the two ears and allowed for outstanding sound location e. Living arrangements Several fossil sites contains groups of one theropod species (e.g. Albertosaurus), suggesting that theropods, like many modern carnivores, lived and hunted in packs or prides f. Nesting Little evidence of theropod parental skills or strategies. However, one exceptional nest in Mongolia preserves an Oviraptor brooding its eggs and an Albertasaurus site in Canada includes both adults and sub-adults. These suggest significant parental care. g. Moving around All theropods were obligate bipeds (very long legs, trackways always narrow). Estimates based on trackways, leg lengths and leg proportions suggest that large theropods like T. rex might have been able to run as fast as 50 km/h or more Functional analysis of the T. rex skeleton suggest that it couldn t run faster than about 30 km/h because of the unrealistic amount of muscle mass needed to move faster and because of potential damage to the skull if it fell Tails would have been important counterbalances in large running theropods! Ornithomimids could probably run fast up to 80 km/hr (based on leg length, leg proportions, trackway evidence) h. Brains and behavior Theropods had the largest EQ s of any dinosaur clade, and the EQ of troodontids and dromaeosaurs was well above that of any living reptile, and very much like that of some modern birds and mammals. 5