Sciurumimus: a juvenile ornitholestid in the LRT

We looked at tiny,
feathered Sciurumimus albersdoerferi (Germany, Rauhut et al. 2012; BMMS BK 11) and larger bones-only Ornitholestes (North America) earlier as Late Jurassic sisters in the large reptile tree (LRT, 1659+ taxa). After a recent review, these two continue to nest as sisters at the base of the Microraptor (Fig. 3) + Sinornithosaurus clade. So no news here… except now let’s combine the extraordinary size difference between the two and the widely accepted observation that Sciurumimus is a juvenile.

That brings to mind: a juvenile of what?
The LRT indicates a juvenile ornitholestid (Fig. 1). The overall morphologies are strikingly similar and the size difference is appropriate. Other published studies recover other nestings.

Rauhut, et al. 2012
(Suppdata) nested Ornitholestes between ornithomimosaurs and deinonychosaurs, far from Sciurumimus, which Rauhut et al. nested Sciurumimus between an unresolved clade of giant spinosaurs + megalosaurs and giant Monolophosaurus. Like Rauhut et al., the LRT nests also nests Ornitholestes between ornithomimosaurs (+ tyrannosaurs + oviraptors + therizinosaurs) and deinonychosaurs.

Key differences in the LRT include

  1. the use of two Compsognathus specimens. The each nest at the base of their own clade, a hypothesis of interrelationships overlooked by Rauhut et al.
  2. the inclusion of three Microraptor specimens and two Sinornithosaurus specimens, adults of which are closer in size and morphology to Sciurumimus. This brings to mind the possibility that phylogenetic miniaturization and neotony played a part in the evolution of these bird-mimics. These closely related taxa were omitted by the Rauhut et al. selection process.
Figure 1. Sciurumimus compared to Ornitholestes and Microraptor to scale.

Figure 1. Sciurumimus compared to Ornitholestes and Microraptor to scale.

In their study of the wonderfully preserved
anchiornithid, Aurornis, Godefroit et al. nested Sciurumimus between Monolophosaurus + Sinraptor and Zuolong, all more primitive taxa in the LRT. In Godefroit et al. these taxa are far from Ornitholestes, which nested with another small compsognathid, Juravenator. Juravenator nests with equally small, but shorter limbed Sinosauropteryx in the LRT. Evidently few theropod studies agree with one another in the details.

Rauhut et al. 2012 reported,
“Our analysis confirms Sciurumimus as the basalmost known theropod with evidence of feather-like integument.” By contrast, in the LRT, Tawa-like, feathered Sincalliopteryx (Fig. 2) is more primitive, despite its late appearance (Early Cretaceous) in the fossil record.

Figure 4. Sinocalliopteryx currently nests as a provisional sister to Deinocheirus, awaiting the discovery of transitional sister taxa.

Figure 2. Late surviving Sinocalliopteryx currently nests basal to Late Triassic Coelophysis, derived from Late Triassic Tawa. It has the most primitive presence of feathers despite its late appearance.

Sinocalliopteryx
currently nests basal to Late Triassic Coelophysis, and was derived from Late Triassic Tawa. In the LRT, Sinocalliopteryx has the most primitive presence of feathers among theropods despite its appearance tens of millions of years later than its phylogenetic genesis.

Figure 2. Microraptor gui (IVPP V 13352) reconstructed from tracings in figure 1. There are no surprises here, except a provisional closer relationship with Compsognathus than with Velociraptor. Microraptor has a large pedal claw two, but it is not quite the killing claw seen in droamaeosaurs.

Figure 3. Microraptor gui (IVPP V 13352) reconstructed from tracings in figure 1. There are no surprises here, except a provisional closer relationship with Compsognathus than with Velociraptor. Microraptor has a large pedal claw two, but it is not quite the killing claw seen in droamaeosaurs.

The Ornitholestes + Sciurumimus + Microraptor + Sinornithosaurus clade
were bird-mimics and bird-mimic ancestors not directly related to birds or bird ancestors in the LRT.


References
Godefroit P, Cau A, Hu D-Y, Escuillié F, Wu, W and Dyke G 2013. A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds. Nature. 498 (7454): 359–362.
Rauhut OWM, Foth C, Tischlinger H and Norell MA 2012.
 Exceptionally preserved juvenile megalosauroid theropod dinosaur with filamentous integument from the Late Jurassic of Germany. Proceedings of the National Academy of Sciences. 109 (29): 11746–11751.