Analysis of skeletal features has led to the following estimation of the order in which the lineages of higher theropods diverged from the ancestral branch. The first branch led to the family Tyrannosauridae (with Tyrannosaurus and Albertosaurus) followed by the bird mimic group Ornithomimosauria, Ornitholestes, the group Alvarezauridae which was originally classified as a group of flightless birds (and includes Patagonykus, Mononykus, and Shuvuuia), Dromaeosauridae (including Velociraptor, Dromaeosaurus, Deinonychus, Unenlagia, Adasaurus, Utaraptor, Saurornitholestes, Achillobator, and Sinornithosaurs), Avialae (a branch of the dromaeosaurs with Rahonavis representing a more primitive branch than Archaeopteryx), Troodontidae (including Troodon, Saurornithoides, and Byronosaurus), the segnosaur group Therizinosauroidea, and finally Oviraptosauria (including Chirostenotes, Avimimus, Microvenator, Caudipteryx, Ingenia, Oviraptor, and Concharaptor). The term coelurosaurs is used for all members of the lineage after Tyrannosauridae diverged and the term Maniraptoran is used for all lineages after the bird mimic lineage diverged (Norell, 2001).

The family Tyrannosauridae includes some of the largest terrestrial predators that have ever existed.  The earliest tyrannosaurids, however, were much smaller such as Aviatyrannis and Siamotyrannis.  The first tyrannosaurids might have evolved from small maniraptorans such as Stokesosaurus, given similarities in their hips.

The earliest known tyrannosauroid is a 3 meter theropod named Guanlong from the Late Jurassic. It possessed 3 fingered hands, a primitive pelvis, and a large, fragile crest on its head which may have functioned in courtship (Xu, 2006). An early tyrannosaur named Eotyrannus lived 132 million years ago, was only 5 m long, and possessed long, grasping arms (Stokstad, 2001).

Dilong paradoxus is a primitive, emu-sized tyrannosaur which was covered in feathers. It possessed three fingers on its hands.   Growth ring studies indicate that tyrannosaurs grew very quickly when young, supporting warm-bloodedness (Lemonick, 2004).

The skull of Nannotyranuus measured 60 cm in length (Czerkas, 1990).

The forelimbs of tyrannosaurids reduced were reduced in size and the third finger was absent.  Given the enormous size of the head, this may have been an adaptation to maintain balance by reducing the weight of the front end of the animal.  These short arms were not necessarily useless, despite their inability to even reach the animal’s mouth.  In Tyrannosaurus,  the “tyrant lizard”, the short arms were very muscular and were perhaps used to stand after the animal had been laying down (Lambert, 1990, Fastovsky, 1996).

Tyrannosaurus had a small crest on its skull and the position of its orbits would have made binocular vision possible.  Tyrannosaurus rex was the largest known terrestrial predator for about 90 years but a number of tetanurans are now known which reached similar (and perhaps even greater) sizes.

    Some researchers have questioned whether Tyrannosaurus was an active hunter.  A Triceratops pelvis with T. rex toothmarks allowed a calculation of the strength of the bite and of the teeth.  T. rex had a stronger bite than anything alive today and thus was strong enough for an active hunter.  However, given that the bite was on the pelvis, it also raised the possibility that T. rex was a scavenger.  The pelvis is not a prime area for meat, often left for scavengers, and increased strength in a bite and wider teeth are seen in some modern scavengers to allow them to crush bone.  Answering this question is difficult given that many modern hunters can also scavenge and that no large modern terrestrial carnivore lives only by scavenging (Erikson, 1996).

     All theropods other than Tyrannosaurus had a femur which was longer than the tibia while in T. rex the two bones were almost equal in size.  This would have made tyrannosaurs slower but capable of greater endurance.  Some have suggested that this would have made them less likely to actively hunt but given them greater stamina in tracking down carrion.  Although this may be true, it does not prove that T. rex could not have hunted in other ways, such as through ambush.

COELUROSAURIA

Coelurosaurs evolved a larger brain (particularly the cerebrum) (Chatterjee, 1997). The basal coelurosaur Juravenator from the Late Jurassic did not possess feathers on its tail (Gohlich, 2006).

Ornithomimids are named the “bird mimics”.  They were ostrich-like in build and, like ostriches, were adapted for speedy running.   Their belly ribs (gastralia) and muscle attachments on vertebrae indicate that the abdomen was held forward and horizontally while running.  The tail was stiff with large muscle attachments, being used for balance.  The brain was large, approaching bird proportions.  There were no teeth in the beak of some species (Lambert, 1990, Fastovsky, 1996). 

  Pelicanimimus is an ornithomimid which had over 200 teeth in its mouth (the highest number for a theropod).  It also seems to have had a small crest on its head and some anatomical traits suggest that the ornithomimids were related to Troodon (Perez-Moreno, 1994).

     The garudimimids “Garuda mimics” were similar to and related to the ornithomimids.  Harpymimus possessed 10 small teeth in tip of lower jaw.  Garudimimus still retained an inner toe that ornithomimids lost.

Two enormous theropods, the possible ornithomimid Deinocheirus and the possible segnosaur Therizinosaurus, are known only from their arms which were than 2.5 meters long (Czerkas, 1990).

MANIRAPTORANS

    The members of the Coeluridae ranged from 1.2 to 2.4 meters in length.  They had long limbs and tail and their lightweight build would have made them fast runners.  Some possessed a nasal horn and some completely lost traces of the 4^th finger.   This group includes Coelurus, “hollow tail”, and Ornitholestes, “bird robber”.  In Ornitholestes, depressions on lateral surface of vertebrae (pleurocoels) decrease the weight of vertebrae but not their strength (Lambert, 1990, Fastovsky, 1996).

Nqwebasaurus is a coelurosaur from Africa that possessed 3 partially opposable fingers, a highly reduced 4th metatarsal, and swallowed stones (gastroliths) to help it with digestion.  It is significant because few Cretaceous dinosaurs are known from sub-Saharan Africa (DeKlerk, 2000).

The group Alvarezsauridae was originally classified as a primitive family of flightless birds and included the fossils Alvarezsaurus, Mononychus, Patagonykus, Parvicursor, and Shuvuuia. It is now classified as a group of dinosaurs close to the dromaeosaur lineage which gave rise to birds. Like dromaeosaurs and birds, Mononychus possessed a pubis which projected backwards (Sereno, 2001; Rayner, 2001). Alvarezsaurids may be the sister group of birds with Caudipteryx, Protoarchaeopteryx, dromaeosaurs, and oviraptorids forming a descending order of related lineages (Rayner, 2001).

     Mononychus olecranus had teeth, a keeled sternum, and a long, reptilian tail.  It had one claw on its hands, perhaps to help it in digging or tearing up the nests of insects. Shuvuvuia, Patagonykus, Alvarezsaurus, and Parvicursor also belong to this family.  Shuvuvuia had an incomplete postorbital bar behind its orbit.  (Altangerel, 1993; Chiappe, 1995; Chiappe, 1998).  Analysis of the feathery structures which covered the fossil Shuvuuia through electron microscopy and immunological techniques indicate that they should be considered as true feathers (Schweitzer, 2001).