Guanlong and Dilong: basal allosaurs, not basal tyrannosaurs
I thought more taxa
would help the present shifting of taxa from their original designations, seen here, here and here.
Figure 1. Guanlong reconstructed by moving elements tracing Xu et al. 2006. The robust foot and small skull are notable.
Giuanlong wucaii (Xu et al. 2006, Late Jurassic, China, 3m long) was originally considered a basal tyrannosauroid. An adult (IVPP V14531) and a more complete juvenile (IVPP V14532) are known. The adult had an elaborate medial nasal head crest. The juvenile had a smaller crest restricted to the anterior.
Figure 2. Theropod cladogram. Guanlong nests with two other putative tyrannosaurs, Dilong and Yutyrannus, along with feathered Sinocalliopteryx and Allosaurus.
Here Guanlong nests
in the large reptile tree (Fig. 1) at the base of the Allosaurus clade, not the T-rex clade. But note the Allosaurus clade nests at the base of the T-rex clade. Remember I’m using generalized traits to lump and separate my taxa, not traits specific to theropods.
According to Xu et al. 2006
traits Guanlong shares with tyrannosauroids largely exclusive of other theropods include:
- large foramina on the lateral surface of the premaxilla
- tall premaxillary body
- fused nasals
- a large frontal contribution to the supratemporal fossa
- a pneumatic jugal foramen in the posterior rim of the antorbital fossa
- a deep basisphenoidal sinus with large foramina
- a subcondylar recess on the basisphenoid
- the supraoccipital excluded from the foramen magnum
- the short retroarticular process
- the relatively small, U-shaped premaxillary teeth that are arranged in a row more transversely than anteroposteriorly oriented
- and labio-lingually thick maxillary and dentary teeth
- Striking tyrannosauroid pelvic features include an ilium subequal to femoral length, a distinctive dorsal concavity on the pre-acetabular process, a supracetabular crest that is straight in dorsal view, a prominent median vertical crest on the lateral surface of the ilium, a concave anterior margin of the pubic peduncle, a pubic tubercle close to the dorsal part of the pubic shaft, an extremely large pubic boot (55% of pubic length), and a thin sheet of bone extending from the obturator process down the ischial shaft
None of these are character traits listed in the large reptile tree. The authors note, “Guanlong represents a specialized lineage in the early evolution of tyrannosauroids.”
Figure 3. The CM 31374 specimen of Coelophysis. The long snout becomes progressively shortened in the Guanlong/Dilong clade.
The unusually long snout of Guanlong
might appears to be elongated in order to support a larger crest. The authors note, “Cranial horns, bosses and crests are present in many non-avian theropods and are best exemplified by Dilophosaurus, Monolophosaurus and oviraptorids, among others.” However, Guanlong is also derived from a sister to Coelophysis, which also had a longer rostrum and similar proportions.
Figure 4. Dilong paradoxus. Images from Xu et al. 2004. Colors added.
Dilong paradoxus (Xu et al., 2004 TNP01109) Early Cretaceous ~125 mya, 2.75 m in length, was also originally considered a basal tyrannosauroid. In the large reptile tree (Fig. 2) it is derived from a sister to Guanlong at the base of the Allosaurus/Sinocalliopteryx clade. Dilong was larger than Guanlong and covered with feathers or protofeathers. Here the snout is shorter still.
None of these taxa
are known to have stiff wing feathers as seen in the tyrannosaurs, like Zhenyuanlong, and later clades leading to birds.
References
Xu X, Clark JM, Forster CA, Norell MA, Erickson GM, Eberth DA, Jia C and Zhao Q 2006. A basal tyrannosauroid dinosaur from the Late Jurassic of China” (PDF). Nature 439 (7077): 715–718.
Xu X, Norell MA, Kuang X, Wang X, Zhao Q, Jia C 2004. Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroid (PDF). Nature 431 7009: 680–684.
Source: https://pterosaurheresies.wordpress.com/2015/12/27/guanlong-and-dilong-basal-allosaurs-not-basal-tyrannosaurs/
