Rahonavis returns! (still without resolution due to taxon exclusion)

Forster et al. 2020
bring us up to date on Rahonavis (Fig. 1), a tiny theropod with long forelimbs that has been traditionally hard to nest. Forster et al. adds Rahonavis to two previously published analyses of bird-like theropods …still without resolution (due to taxon exclusion).

Figure 2. Rahonavis nests in the LRT as a tiny derived therizinosaur based on the few bones currently known.

Figure 1. Rahonavis nests in the LRT as a tiny derived therizinosaur based on the few bones currently known.

From the abstract:
“Recent phylogenetic analyses place Rahonavis either within the non-avialan Unenlagi- inae, an early-diverging clade within Dromaeosauridae, or at the base of Avialae. Rahonavis is one of the best represented and preserved Gondwanan paravians, and remains a pivotal taxon for understanding the evolution and biogeography of paravians.”

So the most recent analyses by Forster et al.
have not nested Rahonavis with confidence. In one study (based on the phylogenetic analyses of Lefèvre et al. 2017) Rahonavis nests between Archaeopteryx and Balaur. In the other study (based on Brusatte et al. 2014) Rahonavis nests at the base of three troodontids, Buitreraptor, Austroraptor and Unenlagia. In that study Balaur nests with Velociraptor and kin and all are derived from a sister to Mahakala.

So Rahonavis is not the only theropod
to shift places between the two published studies.

By contrast
the large reptile tree (LRT, 1698+ taxa; subset Fig. 3) nests Rahonavis outside the taxon lists employed by Forster 2020. In the LRT Rahonavis nests with Jianchangosaurus, Falcarius, and Beiapiosaurus, three therizinosaurs not tested by Forster et al. We looked at this still heretical nesting of Rahonavis a few years earlier here. Prior to the addition of therizinosaurs (and a thousand other taxa), Rahonavis nested with Velociraptor in the LRT. So back then, with fewer taxa, the LRT also suffered from taxon exclusion.

Following Forster et al. 2020,
to move Rahonavis to the Berlin specimen of Archaeopteryx adds 18 steps. To move Rahonavis to Buitreraptor adds 14 steps in the LRT. Those smnall numbers are based on the relatively few bones, all post-cranial, preserved by Rahonavis.

Figure 1. Jianchangosaurus nests at the base of the Maniraptora in Cau 2018, but with therizinosaurs in the LRT.

Figure 2. Jianchangosaurus nests at the base of the Maniraptora in Cau 2018, but with therizinosaurs in the LRT. Note the large pedal ungual 2, as in Rahonavis.

Therizinosaurus
are typically larger than Rahonavis, and feathered. Thus phylogenetic bracketing indicates Rahonavis was likely feathered, too.

Rahonavis (orignally Rahona ostromi – Forster et al. 1998, Late Cretaceous, 70mya, UA 8656, 70 cm) was originally considered a bird-like theropod. The partial skeleton is much smaller than sister taxa among the Therizionsauria. The pubis was ventrally oriented and the radius + ulna were very long. The tail was comparatively short with few vertebrae.

Figure 3. Subset of the LRT focusing on theropods and basal birds. Colors added for large (greater than a meter), medium (about a meter), and small (less than a meter) in length. Compare to figure 2 from Rezende et al. Note the depth of small taxa, some of which give rise to large taxa.

Figure 3. Subset of the LRT focusing on theropods and basal birds. Colors added for large (greater than a meter), medium (about a meter), and small (less than a meter) in length.

By minimizing taxon exclusion,
the LRT manages to nest all included taxa with high confidence and high Bootstrap numbers. As demonstrated over and over again, it doesn’t matter how many characters you use (at least 200 multi-state characters). You just have to barely nest taxa with complete resolution. You can only do this by adding taxa. That minimizes taxon exclusion, the biggest single problem in paleontology right now, whether workers know it or not.

Solve your problems by adding taxa.
It works here all the time. It will work for you, too.


References
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

wiki/Rahonavis

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