Avimimus enters the LRT basal to therizinosaurs — not quite oviraptorosaurs

Tsuihiji et al 2017 wrote:
“The oviraptorosaurian Avimimus portentosus is known for possessing ‘avian’-like features.”

By contrast, when added to the large reptile tree (LRT, 2202 taxa, subset Fig 1) Avimimus (Figs 2, 3) nests at the base of the therizinosaurs, a sister clade to the oviraptorosaurs.

Figure 1. Subset of the LRT with the addition of Avimimus at the base of the therizinosaurs.

It’s the smallest phylogenetic move from one clade to another
and feathers were probably present in Avimimus according to phylogenetic bracketing.

Figure 2. Skull of Avimimus in several views from the Whitmer lab, authors of Tsuiiji et al 2017. Colors and minor restoration added here. Note the large medial fontanelle between the nasals and frontals. Note the extremely gracile post-dentary restoration and the rotate naris due to rostral shortening.

Neotony and phylogenetic miniaturization
seem to be reasons for the several odd traits in the Avimimus skull (e.g. short rostrum, large eyes) and metatarsus (mt3 does not reach the tarsus). At 1.5 m Avimimus is not small, but it is primitive and lived during the Late Cretaceous, so one can predict larger ancestors from the Early Cretaceous to someday be discovered.

Figure 3. Avimimus skeletal elements plus the pedal phalanges enlarged. Note the lack of a metatarsal 3 that extends to the tarsus.

Avimimus portentosus
(Kurzanov 1981, Dyke 1998, Tsuihiji et al 2017, MPC-D 100/125, Late Cretaceous, 1.5m long, Figs 2, 3) is widely considered an aberrant oviraptorosaur, but here nests basal to therizinosaurs, as a late-surviving sister to oviraptorosaurs. This toothless taxon had a very short metatarsal 3 that did not reach halfway up the rest of the metatarsus. The rostrum was short, which rotated the naris vertically. The eyes were large. The unpreserved posterior mandible must have been strut-like and fragile because the large mandibular fenestra starts far forward behind the very short dentary.

Figure 4. The large (from Peyer 2006) and small Compsognathus specimens to scale. Several different traits nest these two close to one another, but at the bases of 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. The big one needs a new generic name.

Avimimus and the therizinosauroids have given paleontologists
many sleepless nights since their first discoveries in the fossil record. Now, with something approaching a ‘critical mass’ of taxa, the therizinosaurids nest with oviraptorosaurs, not far from tyrannosaurs, ornithomimids and their last common ancestor, the little holotype of Compsognathus, which should be getting more attention. The big Compsognathus corallestris (Fig 4) needs a new name, like ‘Megalocompsognathus corallestris ‘.

References
Dyke T 1998. Reduced cladistic consensus methods and the avian affinities of Protoavis and Avimimus. Archaeopteryx. 16: 123–129.
Forster CA, Sampson SD, Chiappe LM, Krause DW 1998. The Theropod Ancestry of Birds: New Evidence from the Late Cretaceous of Madagascar. Science 279 (5358): 1915–1919.
Forster CA, O’Connor PM, Chiappe LM and Turner AH 2020. The osteology of the Late Cretaceous paravian Rahonavis ostromi from Madagascar. Palaeontologia Electronica, 23(2):a31. https://palaeo-electronica.org/content/pdfs/793.pdf
Kirkland JI, Zanno LE, Sampson, SD, Clark J M and DeBlieux DD 2005. A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah. Nature 435: 84-87
Kurzanov SM 1981. An unusual theropod from the Upper Cretaceous of Mongolia Iskopayemyye pozvonochnyye Mongolii (Fossil Vertebrates of Mongolia). Trudy Sovmestnay Sovetsko-Mongolskay Paleontologiyeskay Ekspeditsiy (Joint Soviet-Mongolian Paleontological Expedition), 15: 39-49. Nauka Moscow.
Pu H-Y, Kobayashi Y, Lu J-C, Xu L, Wu Y-H, Chang HL Zhang J-M and Jia S-H 2013. An Unusual Basal Therizinosaur Dinosaur with an Ornithischian Dental Arrangement from Northeastern China. PLoS ONE 8(5): e63423. doi:10.1371/journal.pone.0063423
Tsuihiji T, Witmer L et al (5 co-authors) 2017. New information on the cranial morphology of Avimimus (Theropoda: Oviraptorosauria). Journal of Vertebrate Paleontology 37: e1347177.
Xu X, Tang Z-L, Wang X-L 1999. A therizinosauroid dinosaur with integumentary structures from China. Nature. 399 (6734): 350–354. doi:10.1038/20670

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