and several blogs ago the large reptile tree nested the theropod dinosaurs Guanlong and Dilong (Fig. 1), not at the base of the Tyrannosaurus clade, but at the base of the Allosaurus and Sinocalliopteryx (Fig. 2) clade.
Prior to these taxon additions,
there were no long-snouted mid- to large-sized theropods in the large reptile tree other than Sinocalliopteryx, a feathered theropod with a kinked snout. Now we add two more.
long-snouted Suchomimus (Fig. 3) and the short-legged, aquatic, giant Spinosaurus (Fig. 3) nests them both unambiguously with Sinocalliopteryx, The naris had not yet migrated posteriorly in Sinocalliopteryx, but the snout was already filled with long strongly curved teeth. And we see a hint of a nasal crest, narrow cranium, etc.
When we first discussed Sinocalliopteryx
in the summer of 2014, its feathers and a lack of many other theropodss allied it with birds. Now with the addition of more theropods, like Suchomimus and Spinosaurus (Fig. 4), along with the realization that feathers were plesiomorphic for dinosaurs, a new phylogenetic picture comes into focus.
the earliest known members of the spinosaur clade are represented by Late Jurassic teeth. Sinocalliopteryx and Dilong help fill that gap with more bones.
Speaking of more bones
Neither Suchomimus nor Spinosaurus preserve the circumorbital region and cranium. The related taxon, Irritator (Fig. 5) does preserve these regions and little else, and so is often used in chimaera restorations of spinosaurids.
Several traits set this clade (spinosaurids + Sinocalliopteryx) apart from other theropods:
- narrow cranium with wide jaws
- pmx/mx notch for dentary fangs
- cranium descends posteriorly
- snout tends to lengthen
- skull not < 1.2 x wider than tall at orbit
- posterior maxilla ascends lacrimal
- nasal develops parasagittal crest (smaller than in Guanlong)
Nest spinosaurids with
- Baryonyx derived from the abelosaurid Berberosaurus + Coelophysis (Cau et al. 2015).
- Megalosauroidea derived from a sister to Monolophosaurus (Wikipedia) which likewise as a median nasal crest, like Guanlong.
which are all still in the same branch of the theropod tree.
Ibrahim N et al. 2014. Semiaquatic adaptations in a giant predatory dinosaur. Science 345 (6204): 1613–6.
Ji S, Ji Q, Lu J and Yuan C 2007. A new giant compsognathid dinosaur with long filamentous integuments from Lower Cretaceous of Northeastern China. Acta Geologica Sinica, 81(1): 8-15.
Sereno PC, et al. 1998. A long-snouted predatory dinosaur from Africa and the evolution of spinosaurids. Science 282 (5392): 1298–1302.