Dangerous babies: re-evaluation of "compsognathids"

[ITdactyl]

Scipionyx by Davide Bonadonna

Andrea Cau recently blogged about the possibility that some "compsognathids" may actually be the infant stages of Tetanurans.  He goes on detail about how Scipionyx appears to be a baby Carcharodontosaurid.
https://theropoda.blogspot.com/2021/09/carcharodontosauri-ditalia.html

The blog is in Italian, but I've read through the Google translation and it's okay.

Here's the link to the paper:
https://www.researchgate.net/publication/354418322_Comments_on_the_Mesozoic_theropod_dinosaurs_from_Italy

I recommend reading through both the blog and the paper; the arguments he presented are compelling and the speculations he opened are intriguing (to say the least).

[Faelrin]

With all the extensive research I've been doing on dinosaurs lately I did find it rather peculiar most of the "Compsognathidae" genera were based on young individuals (and many with decent remains luckily).

On one hand it could just be preservation bias (although some of this is certainly true in the case of the Sinosauropteryx specimens from the Jehol Biota for example), but on the other hand if this holds true, that "Compsognathidae" is in fact a polyphyletic group (in whole or part), that's going to shake up the placement of many of these going forward (as if it wasn't already a mess at times such as the case with Sciurumimus). Though well it wouldn't be the first time something like this has happened (see "Fabrosauridae").

I will be curious to see what others here think, and perhaps from any other experts in the field that might be thinking about this going forward.

[stargatedalek]

Just gonna leave out the implications of Juravenator being a megalosaur? It's been fun seeing people in some circles loose their minds over that part.

[ITdactyl]

Do you have links? I saw hints of that "discussion" on Facebook - 'just curious what points are being raised.

But yeah, I did skip that because Andrea Cau's paper was more about Saltriovenator and Scipionyx (and I felt that Juravenator being a megalosaurid has been mentioned before).

[Sim]

The compsognathid situation is very interesting.  Some thoughts I have:

1. Andrea Cau mentioned that Haplocheirus's growth stage has been changed since his analysis, would this affect the phylogenetic position of Scipionyx/Juravenator/Sciurumimus?  I would very much like to see a new analysis by him with Haplocheirus's growth stage corrected, and also with all compsognathids and similar animals's immature qualities filtered out.  It would be very interesting to see where these animals would fall and if any synonymies would occur.

2. Integument.  Among non-pennaraptoran theropods, the only feathers known are filamentous and this portion of theropods seems to include both scaly and filamentously feathered animals.  Perhaps then if compsognathids are not a natural group, some of them were scaly while others were feathered, even if they were all small at that stage.  Compsognathus does have scales preserved on its tail, maybe it was fully scaly after all?  If Juraventor and Sciurumimus are indeed megalosauroids then a feather covering exists in that group.  I don't think that means megalosauroids were necessarily all feathered since a loss of feathers does appear to have occured possibly more than once among dinosaurs.  I can't imagine spinosaurids being feathered for example, their lifestyles to me suggests they were fully scaly.

3. On Facebook, Andrea said the ontogenetic status of every non-maniraptoriform tetanuran under 3 meters should be checked to see if they could be immature specimens of larger theropods.  I have some thoughts on specific genera:
- Compsognathus: I remember reading that it appears to be the largest land predator in its ecosystem, which was an archipelago.  It would be weird if it was the juvenile of a large theropod.  Additionally the larger Compsognathus specimen has been considered an adult, although maybe it isn't?
- Ornitholestes: Since unusual features it has such as the curved lower jaw and sickle claw is seen in adult dromaeosaurids, I don't think Ornitholestes would grow into a different-looking creature.
- Dilong: I've read that even if it's small, the holotype of Dilong exhibits skeletal fusion indicating it was a subadult.  So I imagine Dilong didn't get much larger and had small adults.  This is a relief to me because I like the idea of it being a small tyrannosauroid and it also means it's one less possibility for Yutyrannus to be a junior synonym.
- Sinocalliopteryx: First I just have to say I find it strange it's never been made into a toy, given its fearsome nature.  I wonder if it represents a juvenile of a larger theropod although it's worth noting that one specimen is over 3m long.
- Sinosauropteryx: Boy, this got a lot of attention.  In his blog post Andrea Cau said Sinosauropteryx displays carnosaur features, but on Facebook he suggested a giant specimen of Sinosauropteryx could be a transitional growth stage between traditional Sinosauropteryx and a large tyrannosauroid.  It seems a bit weird to me that he suggests these two different possibilities which contradict each other.  I am sceptical that Sinosauropteryx would grow into a large theropod though.  The holotype has been identified as a subadult and another specimen, around 1m long, has been identified as an adult.  I've seen contradicting claims on the eggs inside a Sinosauropteryx as to whether they were unlaid or a meal or the intestine and I'm not sure what to think.  The giant Sinosauropteryx could be a different genus too, I think.
- Guanlong: The holotype has been classified as an adult.  I don't see it losing the head crest even if it wasn't an adult.

4. I would very much like to see Mickey Mortimer's thoughts on this.