The clade Troodontidae: Who’s in and who’s out?

Summary for those in a hurry:
Compared to traditional taxon lists (Figs. 3, 4), the LRT taxon list for the Troondontidae is greatly reduced (Fig. 2). That means many traditional troodontids nest elsewhere.

We start today with a new taxon.
Zanabazar junior (Norell et al. 2009, originally Sauronithoides junior Barsbold 1975; IGM 100/1; est. 2.3m long, 27cm skull length; Fig. 1), is a late-surviving (Late Cretaceous) basal troodontid in the large reptile tree (LRT, 1274+ taxa; subset Fig. 2). The specimen includes a nearly complete skull and braincase, part of the pelvis, some tail vertebrae, and parts of the right hindlimb. The teeth are relatively small.

Figure 1. Skull of Zanazabar from Digimorph.org and used with permission. Bones are colored here.

Figure 1. Skull of Zanabazar from Digimorph.org and used with permission. Bones are colored here.

Figure 2. Subset of the LRT focusing on theropods leading to birds, including the two newest additions, Bambiraptor and Zanabazar.

Figure 2. Subset of the LRT focusing on theropods leading to birds, including the Troodontidae and the two newest additions, Bambiraptor and Zanabazar.

Prior to the LRT
authors nested Zanabazar as a highly derived troodontid (Figs. 3, 4).

Figure 2. Current cladograms of the Troodontidae currently found in Wikipedia pages.

Figure 3. Current cladograms of the Troodontidae currently found in Wikipedia pages.

Those other authors
also nested LRT pre-bird anchiornithids (Sinovenator, Almas, Daliansaurus, Sinusonasus, Jinfengopteryx) and one scansoriopterygid bird (Mei) in the Troodontidae (Fig. 4). Prior authors include several taxa known from scrappy data that will not be tested in the LRT. These include Talos, Byronosaurus, Troodon, IGM 100/44, Linhevenator, and Philovenator.

Figure 2. Wikipedia cladogram from Shen et al. 2017. Overlay limits LRT Troodontidae to the taxa in the white box.

Figure 4. Wikipedia cladogram from Shen et al. 2017. Overlay limits LRT Troodontidae to the taxa in the white box. Others are in the bird lineage.

On the other hand,
the LRT (Fig. 2) includes troodontid taxa not included or nested elsewhere (e.g. Rhamphocephalus, Haplocheirus, Shuvuuia, Halszkaraptor). Readers will note that several of these taxa are alvarezsaurids that now nest within the Troodontidae in the LRT. This is a novel hypothesis of interrelationships. If there is another prior citation, please let me know so that can be promoted.

According to Wikipedia
“During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages.” By contrast, most taxa included in the LRT are largely complete.

Wikipedia continues:
“More recent fossil discoveries of complete and articulated specimens (including specimens which preserve feathers, eggs, embryos, and complete juveniles), have helped to increase understanding about this group.” None of these sorts of taxa currently in the Troodontidae in the LRT (Fig. 2).

The first question is:
What is the definition of Troodontidae?

Looking for a definition online @ yourdictionary.com
“Any member of a family (Troodontidae) of small, bird-like theropod dinosaurs with large brains, large eyes, and a retractable claw on the second toe of each hind foot, similar to a farmer’s sickle, used for slashing at prey.”

This is a trait-based definition, subject to convergence. We call this “Pulling a Larry Martin.” Only a phylogenetic nesting in a wide gamut cladogram can determine what is and what is not a troodontid and that starts with a definition of the clade.

According to Wikipedia (Troodon) 
“the entire genus is based only on a single tooth.” and this tooth has been considered to belong to a wide variety of Reptilia. “Phil Currie, reviewing the pertinent specimens in 1987, showed that supposed differences in tooth and jaw structure among troodontids and saurornithoidids were based on age and position of the tooth in the jaw, rather than a difference in species.”

So, there’s a definite problem
in defining both Troodon and the Troodontidae. Even so, the theropoddatabase.com has compiled a few that may prove useful.

  1. Troodontidae = Troodon formosus (Gilmore 1924)
  2. Troodontidae = Troodon formosus, Saurornithoides mongoliensis, Borogovia gracilicrus, Sinornithoides youngi but not Ornithomimus velox, Oviraptor philoceratops) (Varricchio 1997)
  3. Troodontoidea Troodon + Saurornithoides (Livezey and Zusi 2007)

What these definitions have in common
are the more completely known taxa, Sinornithoides, Sauronithoides and Zanabazar. Let’s make these, plus their last common ancestor. our working definition. Let’s assume, until proven wrong, that Troodon is similar in most respects. Given these parameters many taxa leave the clade Troodontidae and nest within the bird-line of anchiornithids or within birds (Fig. 2).

FIgure 5. Gobivenator is the most completely known troodontid. It nests with Zanabazar in the LRT.

FIgure 5. Gobivenator is the most completely known troodontid. It nests with Zanabazar in the LRT.

Figure 6. Gobivenator skull, colors added here.

Figure 6. Gobivenator skull, colors added here.

Gobivenator, the most completely known troodontid,
(Fig. 5, 6) was added to the LRT just an hour ago, nesting alongside Zanabazar with very few scoring differences. So, Gobivenator was not forgotten.

Figure 2. Mei long compared to the BSP 1999 I 50, Munich specimen of Archaeopteryx and Scansoriopteryx to scale. Click to enlarge.

Figure 7. Mei long is a scansoriopterygid bird in the LRT. Yes it has small hands and could not fly, but the rest of its traits nest Mei within the bird clade.

Mei
(Fig. 7) nests not with troodontids in the LRT, but with the bird clade scansoriopterygids, between the private #12 Archaeopteryx specimen and Yi qi. Yes, it has small hands and could not fly (like Struthio the ostrich). Moving Mei to the troodontids adds 23 steps. Reversals do happen. A few traits compete against a larger suite. Let your software determine where a taxon nests and make sure you include enough taxa to let convergence happen.

Wikipedia – Mei long reports:
“It is most closely related to the troodontid Sinovenator, which places it near the base of the troodontid family.” In the LRT, Sinovenator (Fig. 8) is not in the Troodontidae, but nests in proximal bird outgroup clades. Moving Mei long to Sinovenator adds 19 steps to the LRT. Taxon exclusion has so far kept Mei long apart from other scansoriopteryids everywhere but here.

Figure 8. Sinovenator nests with anchiornithid birds in the LRT.

Figure 8. Sinovenator nests with anchiornithid birds in the LRT.Likewise,
Sinovenator nests not with troodontids in the LRT, but with the pre-bird anchiornithids, between Almas (Fig. 9) and the BMNHC PH804 specimen of Anchiornis.

The LRT documents a fast track for the origin of birds
from the last common ancestor of Bambiraptor + Zanabazar that leads to the following series of taxa: Anchiornis, Daliansaurus (Fig. 9), Almas, the Daiting specimen of Archaeopteryx, Xiaotingia and the Thermopolis specimen of Archaeopteryx, the last known common ancestor of all birds in the LRT.

Figure 1. Daliansaurus and the origin of birds through Almas and Xiaotingia.

Figure 9. Daliansaurus and the origin of birds through Almas and Xiaotingia.

Daliansaurus liaoningensis 
(Shen et al. 2017; Early Cretaceous, Barremian, 128 mya; 1 m long) nests in the LRT as a basal anchiornithid, not a troodontid.

Almas ukhaa
(Pei et al. 2017; Campanian, Late Cretaceous, IGM 100/1323) nests in the LRT as a basal anchiornithid, not a troodontid.

Several lineages approached and experimented with the bird grade
(e.g. Rahonavis, Microraptor, the Daiting specimen of Archaeopteryx), but only one lineage starting with the Thermopolis specimen of Archaeopteryx created robustly volant and extant birds.

In the LRT,
the reduced clade memberships of Troodontids indicate they are a splinter group,
closer to Bambiraptor + Velociraptor. That combined clade (Fig. 2) is a splinter group to the smaller compsognathids and anchiornithids lineage that led more directly to birds (Fig. 9).


References
Barsbold R 1974. Saurornithoididae, a new family of small theropod dinosaurs from Central Asia and North America. Palaeontologica Polonica. 30: 5−22.
Norell MA et al. 2009. A review of the Mongolian Cretaceous dinosaur Saurornithoides (Troodontidae, Theropoda). American Museum Novitates (3654): 1−63.

wiki/Zanabazar_junior
wiki/Gobivenator
wiki/Troodontidae
wiki/Alvarezsauridae
wiki/Daliansaurus
wiki/Almas_ukhaa

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