Sciurumimus: a feathered sister to Ornitholestes and Microraptor

This specimen is old news for many.
It came to my attention in a YouTube video lecture you can see here.

Figure 1. Sciurumimus in situ. Most of the skeleton as buried below the bedding plane.

Figure 1. Sciurumimus in situ. Only some off the tail filaments are colored here. Most of the skeleton as buried below the bedding plane. It would be interesting to dig out the remaining matrix behind the hands and legs with the understanding that sister taxon Microraptor has long feathers there. Total length ~ 60cm.

Sciurumimus albersdoerferi (Rauhut et al. 2012; Late Jurassic, 150 mya; Bürgermeister Müller Museum Solnhofen (BMMS) BK 11) is traditionally considered a basal coelurosaurian theropod, but was originally considered a basal megalosaur. Here it nests with Ornitholestes, close to Microraptor, also a Late Jurassic taxon. Microraptor is an Early Cretaceous taxon.

This specimen
(Fig. 1) has filament-like tail feathers growing in a pattern like squirrel tail hair over the proximal tail as in several ornithischians. There is a small egg-like shape between the ischia. Not sure what it is, but if you’re smiling right now, you’re guessing that this specimen is not a post-hatchling juvenile.

Figure 2. Skull of Sciurumimus with bones colorized.

Figure 2. Skull of Sciurumimus with bones colorized.

It would be interesting
to dig out the remaining matrix behind the hands and legs with the understanding that sister taxon Microraptor has long feathers there.

Figure 3. Sciurumimus manus and pes using DGS to find the best elements from both extremities and reassemble them. Note the shift of pedal digit 2 to metatarsal 3 in situ, repaired in vivo.

Figure 3. Sciurumimus manus and pes using DGS to find the best elements from both extremities and reassemble them. Note the shift of pedal digit 2 to metatarsal 3 in situ, repaired in vivo.

Surprisingly
pedal digit 2 has taphonomically drifted to metatarsal 3. Pedal digits 1 and 5, if present, are string-like and spindly vestiges.

Size vs. Juvenile status
Sciurumimus is correctly sized as a basal coelurosaurian, but tiny compared to megalosaurids. Rauhut et al. considered their specimen a megalosaur “probably early-post-hatchling” with teeth that are “markedly similar to that of basal coelurosaurian theropods.”. They nested Sciurumimus between spinosaurs and Megalosaurus and kin.

So why did Rauhut et al. consider their find a megalosaur? And a juvenile?

From the Rauhut et al. diagnosis: 
Megalosauroid theropod with the following apomorphic characters:

  1. axial neural spine symmetrically “hatchet-shaped” in lateral view;
  2. posterior dorsal neural spines with rectangular edge anteriorly and lobe-shaped dorsal expansion posteriorly;
  3. anterior margin of ilium with semioval anterior process in its dorsal half.

Unfortunately these traits also describe
Compsognathus and Ornitholestes, among other coelurosaurs. So, in theitr discussion Rauhut et al. list other synapomorphies of megalosaurids present in Sciurumimus.

  1. An elongate anterior process of the maxillary body
  2. a medially closed maxillary fenestra
  3. a very slender anterior process of the lacrimal
  4. a lateral blade of the lacrimal that does not overhang antorbital fenestra’
  5. the presence of a deep fossa ventral to the basioccipital condyle
  6. a splenial foramen that opens anteroventrally
  7. a slightly dorsally expanded anterior end of the dentary
  8. a pronounced ventral keel in the anterior dorsal vertebrae
  9. the absence of a posteroventral process of the coracoid,
  10.  and an enlarged manual ungual I.

Note:
The LRT recovers a different theropod tree topology and nests Sciurumimus apart from megalosaurs by using different character traits and by interpreting Sciurumimus differently. This was done without referring to more recent papers, like Godefroit et al. 2013, or having the specimen to study firsthand. According to Wikpedia, Godefroit et al. nested Sciurumimus with the much larger Sinraptor, among taxa tested here, and far from Ornitholestes. So now we have three competing theropod tree topologies. At least now Sciurumimus actually looks like a transition between Ornitholestes and/or Compsoognathus and micorraptors.

Juvenile traits present in Sciurumimus, according to Rauhut et al. include:

  1. the body proportions, with a very large skull and rather short hindlimbs
  2. lack of fusion in the skeleton (unfused neurocentral sutures in all of the vertebral column
  3. unfused sacral vertebrae
  4. lack of fusion between elements of the braincase
  5. a coarsely striated bone-surface texture in all skeletal elements
  6. and a very regular pattern of tooth development in the maxilla, possibly indicating that no teeth had been replaced.
FIgure 6. Ornitholestes nests as a sister to Sciurumimus, between Compsognathus and Microraptor.

FIgure 4. Ornitholestes nests as a sister to Sciurumimus, between Compsognathus and Microraptor. Sciurumimus is about the size of Microraptor at 60 cm in length.

Figure 6. Subset of the LRT that includes Sciurumimus and kin.

Figure 5 Subset of the LRT that includes Sciurumimus and kin.

Note:
The Solnhofen formation, from which Sciurumimus was recovered, has no large megalosaurid theropods. But it does have small coelurosaurian theropods, like Compsognathus, which nests as a more primitive relative one node away in the LRT (Fig. 5).

As readers may recall
the LRT finds several clades of birds, near-birds and bird mimics, including a basal radiation of several clades of post-Archaeopteryx birds that became extinct by the end of the Cretaceous. Furthermore, as readers may recall, small theropods appear at the base of many theropod clades, including those that create giants. So, there may not have been a gradual size decrease leading to birds, throughout the Theropoda, just including the Troodontidae (which includes extant birds).

References
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.

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. Nature498 (7454): 359–362.

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