Sunday 23 June 2013

Tyrannosaurus rex Part 2 - Systematics


Last time, we looked at the anatomy of Tyrannosaurus rex. This week, we will look at where it fits in relation to other dinosaurs.

The following cladograms (adapted from Tweet, 2013) illustrate Tyrannosaurus rex's position. The groups to which Tyrannosaurus rex belongs will be placed at the bottom.

Cladogram of Dinosauria (Owen, 1842)

O - Dinosauria
 |---o Ornithischia (Seeley, 1888)
 `---o Saurischia (Seeley, 1888)

The Ornithischia contains the horned dinosaurs, the armoured dinosaurs and the duck-billed dinosaurs. The Saurischia contains the meat-eating and the long-necked dinosaurs.

O - Saurischia
 |--- o Herrerasauridae (Reig, 1963)
 |--- o Sauropodomorpha (Huene, 1932)
 `---o Theropoda (Marsh, 1881)

The Herrerasauridae contain a small group of dinosaurs that have bounced between the Sauropodomorpha and the Theropoda since...well, forever really (and have sometimes believed to be non-dinosaurs). The Sauropodomorpha contains the Sauropods and their relatives. Theropoda contains all the meat-eating dinosaurs.

O - Theropoda
 |---o Daemonosaurus (Sues et al., 2011)
 `---+---o Tawa (Nesbitt et al., 2009)
        `---o Neotheropoda (Bakker, 1986)

Daemonosaurus and Tawa are the most basal ("primitive") well-established Theropods (if you discount the Herrerasaurids that is). The Neotheropoda contains all the other Theropods, being characterised by only four fingers (as opposed to five), a small fourth finger (and in some, no fourth finger at all), and three weight-bearing toes (sometimes with a fourth toe higher up that is not in contact with the ground).

O - Neotheropoda
 | ---o Coelophysoidea (Holtz, 1994)
 `---+---o Zupaysaurus (Arcucci and Coria, 2003)
        `---+---o Dilophosauridae (Madsen and Welles, 2000)
               `---o Averostra (Paul, 2002)

The Coelophysoids were basal theropods that were typically small with long, low bodies, long necks and heads and short hands. Zupaysaurus is somewhere in between the Coelophysoids and the Dilophosaurids. The Dilophosaurids were effectively larger versions of Coelophysoids with shorter necks and they had a sort of kink in their jaws like crocodiles do. The Averostra consists of all other theropods which, unlike the Coelophysoids and the Dilophosaurids have an extra hole in their upper jaw called the promaxillary fenestra.

O - Averostra
 |---o Ceratosauria (Marsh, 1884)
 `---o Tetanurae (Gauthier, 1986)

The Ceratosaurians are an arbitrary group of Averostrans that aren't Tetanurans but no-one can agree on their characteristics or members. The Tetanurae consists of theropods that lack a fourth digit on their hand, have all their upper jaw teeth situated in front of the eyes and have a strap-like shoulder-blade.

O - Tetanurae
 |---o Chuandongocoelurus (He, 1984)
 |---o Monolophosaurus (Zhao and Currie, 1993)
 `---+---o Megalosauroidea (Huxley, 1869)
        `---o Avetheropoda (Paul, 1988)

The Megalosauroids have long, low builds and short, stout arms (being used as fishing implements by some). The Avetheropods, as their name would suggest, contain the bird-like theropods.

O - Avetheropoda
|---o Carnosauria (Huene, 1920)
`---o Coelurosauria (Huene, 1914)

Traditionally, the big theropods were in the Carnosauria and the small theropods in Coelurosauria. This has now been abandoned. In general the Carnosaurs had large eyes, long, narrow skulls and thighs longer than their shins. The Coelurosaurs have a long sacrum, a stiffened tail (especially at the tip), a bowed ulna (one the forearm bones) and shins longer than their thighs.

O - Coelurosauria
|---o Compsognathidae (Cope, 1871)
`---+---o "Bunch of well-known coelurosaurs that don't fit anywhere"
      `---o Tyrannoraptora (Sereno, 1999)

The Compsognathids were tiny coelurosaurs. The Tyrannoraptora are all the coelurosaurs closer to birds than the Compsognathids. In between, are a large number of dinosaurs that don't fit anywhere else. In fact, some have suggested that the classification of basal coelurosaurs needs to be completely re-written given the number of taxa that don't have a home and the fact that neither the Compsognathids nor the Tyrannoraptorans have an agreed-upon set of characteristics.

O Tyrannoraptora
 |---+---o Coelurus (Marsh, 1879)
 |     `---o Maniraptoriformes (Holtz, 1995)
 `---o Tyrannosauroidea (Walker, 1964)

Coelurus is another critter that changes address every few years. Maniraptoriformes contains all the theropods closer to birds than to Tyrannosaurus. They are generally characterised by having bird-like shoulders and breastbones. The Tyrannosauroids are what we are interested in.

O Tyrannosauroidea
|--? Bagaraatan (Osmolska, 1996)
`---+---o Proceratosauridae (Rauhut et al., 2010)
      |---o Dilong (Xu et al., 2004)
      `---+---o Yutyrannus (Xu et al., 2012)
            `---+---+---o Eotyrannus (Hutt et al., 2001)
                   |      |---o Juratyrant (Brusatte and Benson, 2012)
                   |      `---o Stokesosaurus (Madsen, 1974)
                   |---o Xiongguanlong (Li et al., 2010)
                   `---+---o Dryptosaurus (Marsh, 1877)
                         `---+---o Appalachiosaurus (Carr et al., 2005)
                                `---+---o Bistahieversor (Carr and Williamson, 2010)
                                       `---o Tyrannosauridae (Osborn, 1905)

Bagaraatan might actually be a basal coelurosaurian instead (Unfortunately, because Tyrannosauroids branched off from the family tree so early on, basal coelurosaurians and basal tyrannosauroids are very similar). The Proceratosaurids had crests. The dinosaurs between the Proceratosaurids and the Tyrannosaurids, possessed three fingers on their hands and were relatively small (with the notable exception of Yutyrannus being 9m long). Quite a few of them also had feather-like coatings. The Tyrannosaurids were big and bad with the teeth at the front of the jaws D-shaped in cross-section and two fingers on their hands.

O Tyrannosauridae
 |---o Albertosaurinae (Currie et al., 2003)
 `---o Tyrannosaurinae (Matthew and Brown, 1922)

The Albertosaurines were generally smaller and possessed small horns in front of their eyes. The Tyrannosaurines were generally larger and possessed large bumps (called bosses) behind their eyes. The next tree is the last one, I promise.

O Tyrannosaurinae
|---o Alioramus (Kurzanov, 1976)
`---+---o Teratophoneus (Carr et al., 2011)
       `---+---o Daspletosaurus (Russell, 1970)
             `---+---o Tarbosaurus (Maleev, 1955)
                    `---o Tyrannosaurus (Osborn, 1905)

There are two types of Tyrannosaur researcher: the lumpers and the splitters. The lumpers regard all of these taxa to be synonymous with Tyrannosaurus whereas the splitters split them into into multiple genera and species (some of which we will look at in Part 3). I was hoping to cover them in this post, but considering this took me two weeks to do, I decided to leave it here. Next time, we'll look at life before the dinosaurs.

See also:
More dinosaurs
More Tyrannosaurus rex

References:
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