Cau 2024 abstract:
“Known since the 19th Century, the compsognathids are among the smallest predatory dinosaurs, and include the first feathered non-avian species found.”
Which genus is not specified. A name drop would be appropriate here. We can guess that genus is Sinosauropteryx prima. In the LRT Sinosauropteryx is a small theropod nesting with Juravenator basal to Oviraptoridae + Theizinosauria.
Defining the Compsognathidae is key. In the large reptile tree (LRT, 2318 taxa, subset figure 2) the holotype of Compsognathus (Fig 1) nests as a basal member of the Ornithomimosauria. Outgroup taxa include the larger CNJ79 specimen also referred to Compsognathus (Fig 1). In the LRT phylogenetically miniaturized taxa often nest at the base of new clades as these two do. In the LRT the Compsognathidae include these two species, their last common ancsestor, Aorun, and all their descendants. That’s a long list that includes Ornitholestes, Tyrannosaurus and penguins.
The Cau 2024 cladogram (his figure 4-a) nests “Compsognathus specimens” deep within “Megalosauroidea” far from his “Ornithomimosauria + Therizinosauria”. 543 taxa and 1944 characters were tested. Cau also nests Ornithischia and Sauropodomorpha outside Herrerasauria. Outgroup taxa were not labeled, but were based on Cau 2018.
Earlier we looked at Cau 2018 in a four-part series. In that paper outgroup taxa included the LRT theropod, Marasuchus, two proterochampsids known from scraps (Dromomeron and Lagerpeton) and Teleocrater, a long-necked quadruped. By contrast, according to the LRT Cau’s dinosaur outgroup should have been the basal bipedal members of the Crocoylomorpha, then Poposauria.
“Traditionally, compsognathids have been considered small and unspecialized coelurosaurs, closer to birds than large-bodied forms like allosauroids and megalosaurids.”
Sounds like Cau is about to blast ‘tradition’. Cau is also keeping things vague here. A definition (see above) would have been appropriate for readers, but theropod experts may know this definition/list already.
“Yet, all known compsognathids are based on skeletally-immature specimens, and this challenges the accuracy of their traditional phyletic placement.”
This is the first time I’ve heard this asserted. Am I the last to know?
(For an answer, see below.)
At this time perhaps a list of compsognathids is warranted. Perhaps theropod experts know this list, so Cau assumes they know this list.
“Despite the role of heterochrony in dinosaur evolution is widely recognized, the impact of ontogenetic-biased miscodings in shaping theropod phylogenetics is mostly underestimated.”
Is it underestimated? Fewer assertions and more citations would be helpful.
“Herein, I show that the standard framework of theropod macroevolution is biased by a series of coding artifacts which violate semaphoront equality prescribed by phylogenetic systematics.”
An unfamiliar term ‘semaphoront’ pushed me to the dictionary.
semaphoront: biology, rare) An organism as seen in a specific time, as the object of identification or basis for systematics.
From Sharma et al 2017: “A new practice in systematics, “semaphoront” coding, treats developmental stages as terminals. We submit that this approach is antithetical to cladistic practice and constitutes a gross misunderstanding of Hennig’s original idea. We contend that treating ontogenetic stages as terminals both violates tenets of phylogenetic systematics and oversimplifies the complexity of developmental processes. We contend that the application of semaphoront coding to any palaeontological question requires invoking multiple, unjustified assumptions, and ultimately will not yield a possible phylogenetic solution.”
Figure 1. The large (from Peyer 2006) and small Compsognathus specimens to scale. Several different traits nest these next to one another, but at the bases of two sister clades. Note the differences in the forelimb and skull reconstructions here. There may be an external mandibular fenestra. Hard to tell with the medial view and shifting bones.
Cau 2024 abstract continues:
“I introduce “Ontogenetic State Partitioning” (OSP), a novel coding protocol which integrates ontogenetic and morphological variation under a total evidence approach, and apply it to a densely sampled data set focusing on Mesozoic theropods.”
Good! Hope this works as well as standard trait-based analysis.
“The phylogenetic analysis dismissed “Compsognathidae” from being a natural group: its members are identified as juvenile morphs nested among several non-maniraptoriform tetanuran lineages.”
Cau 2018, 2024, provides a competing analysis to the LRT. This deep into the abstract Cau still has not listed any compsognathids, not even the two Compsognathus species, which the LRT found to constitute (along with the LCA, Aorun), a natural group.
We also remember the warnings of Sharma et al 2017 (see above).
“Conservatism in the immature body plan and greater disparity among large-sized adults differentiate the predatory communities dominated by non-coelurosaurian species (e.g., the so called “triumvirates”) from the maniraptoriform-tyrannosaurid faunas (herein named “tyrannies”).
“Triumvirates” usually refer to three of something. What are the three somethings?
Googling this term takes you first to three giant theropod skeletal models.
‘Tyrannies”? Is this a new trend in renaming clades?
Again, am I the last to know this trend?
Added not quite a day later:
Cau’s text reports, “The term “tyranny” used here (and inspired by the analogous “triumvirate” sensu Sereno & Brusatte, 2008) equates to “tyrant-dominated fauna” of Holtz (2021).
Hopefully that helps readers familiar with theropods. It still seems vague = poorly defined for Early Cretaceous taxa.
“This clade-specific differentiation among the communities is confirmed by an analysis of the predatory guild structures including all growth stages: triumvirates and tyrannies result as particular cases along a continuum of communities regulated mainly by alternative contributions of the small-and medium-sized classes.”
An unfamiliar term ‘guild structures’ pushed me to the dictionary, again. A guild structure is tripartite: 1. apprentice, 2. journeyman and 3. master.
Triumvirate is different, referring to three ruling men sharing power.
“The oldest tyrannies (early Late Cretaceous in age) cluster among non-tyranny communities, supporting the hypothesis that tyrannosaurid-dominated faunas acquired their peculiar structure only after the extinction of the non-coelurosaurian components.”
In the LRT the basalmost members of the tyrnannosaurus clade are Fukuiraptor, Tianyuraptor and Huaxiagnathus. All three are indeed Early Cretaceous. The latter is considered a traditional compsognathid. I don’t know if Cau intends his ‘oldest tyrannies’ to mean these three taxa.
It’s always better to be specific, not suprageneric, whenever possible.
“The macroevolutionary trajectory that led the maniraptoriforms to realize the avian-like biology may have precluded them from occupying hypercarnivorous large-bodied niches:”
Aren’t all theropods more or less ‘avian-like’? Among maniraptoriforms, isn’t Utahraptor among the ‘hypercarnivorous large-bodied’? I would add the terror bird, Phorusrhacos, to that list. If not, where is the large-bodied lower limit in size? Cau assumes readers know this. Some readers (= I) don’t know this.
“this bauplan constraint would have favored the tyrannosauroids in opportunistically assuming the apex predatory roles in Late Cretaceous Asiamerica but not elsewhere.”
Cau wrote about compsognathids as juveniles in this abstract. That’s a paper in itself. Now Cau is on the opposite end of the theropod continuum: apex predators. Hopefully Cau will tie this all in together with a discussion on hatchling tyrannosaurs, which were likely the size and shape of the larger Compsognathus species (CNJ79, Fig 1), which Cau appears to be considering a juvenile, if I’m reading this abstract correctly.
“The large-scale structure of the Cenozoic radiation of birds is coherent with the framework introduced herein.”
The “radiation of birds”? Evidently Cau is going to tie this topic in with immature compsognathids and apex predatior tyrannosaurs. We’ll see…
Figure 2. From 2023, the theropod subset of the LRT. Note how many small theropods are located at the bases of theropod clades that produced giant members. That’s phylogenetic miniaturization.
Wikipedia reports,
“Some authors have proposed that Compsognathidae is not a monophyletic group, and at least some compsognathids represent juvenile specimens of larger tetanuran theropods, such as carnosaurs and tyrannosaurs.”
Two of those three authors were Andrea Cau (2021, 2024). The multi-author third paper included, “the juvenile theropod dinosaurs Sciurumimus and Juravenator’ in the headline. The LRT nested these two taxa with relatively small taxa (Ornitholestes and Sinosauropteryx respectively), not with much larger = ontogenetically older taxa.
Small does not automatically = young.
If there is something of value in the text of Cau 2024,
I will be happy to shed some light on that in future posts when I have time to read it. Let me know if I missed anything pertinent. Otherwise, in the abstract to this paper Cau is following a method Sharma et al warned others not to follow (see above).
References
Cau A 2018. The assembly of the avian body plan: a 160-milliony ear long process. Bollettino della Società Paleontologica Italiana, 57: 1-25.
Cau A 2024. A Unified Framework for Predatory Dinosaur Macroevolution. Bollettino della Società Paleontologica Italiana, 63(1): 1-19.
Sharma PP, Clouse RM and Wheeler WC 2017. Henning’s semaphront concept and the use of ontogenetic stages in phylogenetic reconstruction. Cladistics 33(1):93–108.
wiki/Compsognathidae
Cau 2018: Evolution of the avian body plan – part 4
The Pterosaur Heresies 