A new, tiny choristodere, Mengshanosaurus, enters the LRT

It’s tiny and probably a hatchling because sister taxa are much larger.
Mengshanosaurus minimus (Yuan et al. 2021; Early Cretaceous, China; skull length 3.5cm) nests between Ikechosaurus and Champsosaurus in the LRT (subset Fig. 3). Note the indented remnants of the antorbital fenestra in the hatchling model (Fig. 1). Apparently the post-frontal fontanelle is not a pineal opening. Sister taxa do not have a pineal opening.

Figure 1. Full scale model of CT-scanned Mengshanosaurus skull.

Traditional skull misinterpretations continue in Yuan et al.
Yuan et al did not properly label several fused bones (corrected Fig. 2 right) because they don’t know which taxa are choristodere outgroups and last common ancestors. That remains a traditional academic enigma that no one else seems to want to resolve, confirm or refute (Fig. 3) by simply adding taxa to find out.

Figure 2. Holotype of Mengshanosaurus.

Similarly,
the authors had no idea where to nest choristoderes as reptiles. In the large reptile tree (LRT, 1879+ taxa; subset from 2013 in Fig. 3) choristoderes nest as derived proterosuchids. Tiny transitional taxa, like the BPI 2871 specimen, lose the antorbital fenestra. Sister clades within the Pararchosauriformes include the Parasuchia and Proterochampsia. Euarchosauriformes derived from Euparkeria evolve to Archosauria, Rauisuchia, Erythrosuchia, etc.

Figure 3. Subset of the large reptile tree focusing on the pararchosauriformes and the Choristodera.
Figure 3. Subset of the large reptile tree from 2013 focusing on the pararchosauriformes and the Choristodera. This has not changed much, but for the addition of taxa, like Mengshanosaurus between Ikechosaurus and Champsosaurus.

If you don’t know where your clade resides,
keep adding taxa until it becomes apparent and all candidate sister taxa are considered. Or just sneak a peek at the LRT. Don’t overlook tiny taxa. Often tiny taxa bridge gaps, forming transitions at the genesis of major clades in a process known as phylogenetic miniaturization. This time a tiny taxon just turned out to be a hatchling.

Figure 4. The choristodere, Champsosaurus laramiensis (USNM PL 544147) has a vestige antorbital fenestra in the usual place, anterior to the orbit. Here the frontal fontanelle is also present, as in Mengshanosaurus.

PS
Sometimes adult choristoderes also retain a vestige of the antorbital fenestra (Fig. 4).


References
Yuan M, Li D-Q, Ksepka DT and Yi H-Y 2021.
A juvenile skull of the longirostrine choristodere (Diapsida: Choristodera), Mengshanosaurus minimus gen. et sp. nov., with comments on neochoristodere ontogeny. Vertebrata PalAsiatic in press DOI: 10.19615/ j.cnki.2096-9899.210607

wiki/Mengshanosaurus – not posted yet

When whale experts get into trouble, and don’t seem to care

Taxon exclusion is once again today’s topic.
The cladogram in Geisler et al. 2014 (Fig. 1`) passed the review process even though it nested Hippopotamidae with Georgiacetaus, here provided with graphics of the two skulls to illustrate the absurdity. The odontocete portion of the same cladogram (the lower 2/3s) does not mismatch sister taxa, but the outgroup portion is not so carefully constructed. It is cherry-picked. The article authors, editors and referees let this happen. Until now, no one has raised a hand to say ‘this is wrong and this is why.’

Figure 1. From Geisler et al. 2014, rotated 90º, then the top portion enlarged with graphics added. ‘Bos’ = cattle. ‘Sus’ = pigs. Both are also inappropriate outgroup taxa for a study on echolocation in odontocetes. Where were the detractors and suppressors when this was published?

Whale experts Geisler et al. published two years prior
to the recovery of odontocetes arising from tenrecs (Fig. 2) in the large reptile tree (LRT, 1878+ taxa), apart from mysticetes arisng from desmostylians (Fig. 3) anthracobunids, hippos and mesonychids. Even so, professionals with PhDs should not be waiting for amateurs to show them a list of taxa they should employ in analysis. Nor should they be cherry-picking inappropriate taxa just because they know they can get away with it in the paleo whale expert community.

Figure 8. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.
Figure 2. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

The mysticetes in Geisler et al. 2014
(Fig. 1) likewise arose from taxa omitted by Geisler et al. 2014 (Fig. 3).

Figure 1. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.
Figure 3 Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Geisler et al. 2014 reported,
“All extant odontocetes seem to echolocate; however, exactly when and how this complex behaviour—and its underlying anatomy—evolved is largely unknown.”
Geisler et al 2914 made no mention of Gould 1965, who found evidence for echolocation in tenrecs (Fig. 2), extant rat-sized denizens of Madagascar that nest basal to echolocating odoncetes in the LRT. The phylogenetic analysis that nested tenrecs with odontocetes was first presented here in 2016.

Bottom line:
If the ‘closely related’ taxa in your cladogram do not look like taxa that could have evolved from the outgroup taxa, then go back in and add taxa until they do resemble one another, no matter if you can get away with it or not. Do not cherry-pick taxa. Let your wide gamut and comprehensive cladogram tell you which taxa are outgroups and ingroups. If you don’t have one of those, start one today.

This is not an isolated incident,
as longtime readers know.

References
Geisler JH, Colbert MW and Carew JL 2014. A new fossil species supports an early origin for toothed whale echolocation. Nature 508:383–386.
Gould E 1965. Evidence for Echolocation in the Tenrecidae of Madagascar
Proceedings of the American Philosophical Society 109 (6): 352-360. online here.

An online manuscript on the triple origin of whales
is found on ResearchGate.org here.

Is Borealodon a stem mysticete? No.

Due to taxon exclusion
whale experts Shipps, Peredeo and Pyenson 2019 described Borealodon (Fig. 1) as a ‘new stem mysticete’. They falsely reported “The earliest mysticetes had teeth” under the myth of a monophyletic clade ‘Cetacea’. They omitted the actual stem mysticetes, the desmostylians (Fig. 2), from their cladogram (Fig. 3).

Figure 1. Elements of Borealodon from Shipps, Peredo and Pyenson 2019, to scale and colored here.

Borealodon has archaeocete teeth
(Fig. 1) because it is an archaeocete, basal to odontocetes. Some Borealodon teeth have a premolar-like shape with twin roots. Others have an incisor-like shape with a single root.

Figure 2. Two subsets of the LRT showing how various ‘whales’ are related to one another whenever more taxa are added. Prior whale cladograms omitted many of these pertinent taxa.

When more taxa are added,
as in the large reptile tree (LRT, 1876+ taxa; subset Fig. 2) Borealodon nests with Aetiocetus within the clade that includes tenrecs and odontocetes. Mysticetes (baleen whales) nest elsewhere in the LRT (Fig. 2), with hippos, mesonychids, desmostylians and anthracobunids (including Mammalodon and Janjucetus, which both have teeth).

Figure 3. Cladogram from Shipps, Peredo and Pyenson 2019. Colors added based on the LRT. The traditional clade ‘Cetacea’ has been invalid (= polyphyletic) since 2016.

The convergence between odontocetes and mysticetes
was documented in the LRT in 2016, but this has been completely overlooked by whale experts world-wide ever since, who continue to consider the clade Cetacea monophyletic several years after the LRT found otherwise (Fig. 2).

Figure 4. Subset of the LRT with only whale or candidate outgroups included. Note the apparent reappearance of legs after the appearance of baleen whales when pertinent taxa are omitted as in Shipps Peredo and Pyenson (Fig. 3).

What is a stem mysticete?
A desmostylian is a stem mysticete in the LRT. Desmostylians have been traditionally omitted from mysticete studies by whale experts, including Biscont 2006, Boessenecker and Fordyce 2017 and Shipps, Peredo and Pyenson 2019.

References
Shipps BK, Peredo CM and Pyenson ND 2019. Borealodon osedax, a new stem mysticete (Mammalia, Cetacea) from the Oligocene of Washington State and its implications for fossil whale-fall communities. R. Soc. open sci. 6: 182168. http://dx.doi.org/10.1098/rsos.182168References
Bisconti M 2006. Titanocetus, a New Baleen Whale from the Middle Miocene of Northern Italy (Mammalia, Cetacea, Mysticeti) Journal of Vertebrate Paleontology Vol. 26, No. 2 (Jun. 12, 2006), pp. 344-354 (11 pages) Published By: Taylor & Francis, Ltd.
Boessenecker RW and Fordyce RE 2017. Cosmopolitanism and Miocene survival of Eomysticetidae (Cetacea: Mysticeti) revealed by new fossils from New Zealand. New Zealand Journal of Geology and Geophysics. 60(2):

The bird manus ‘frameshift’ hypothesis returns in 2021

de Bakker et al. 2021 write:
“The influential ‘frameshift’ hypothesis postulates an evolutionary change in the phenotype of one or more digits in the lineage leading to birds (Wagner and Gauthier 1999; Stewart, et al. 2019). The hypothesis aimed to reconcile conflicting data from developmental biology (Welten, et al. 2005; Richardson 2012; de Bakker, et al. 2013) and palaeontology (Wagner and Gauthier 1999) about the homologies of the wing digits. Developmental data suggest an evolutionary loss of digits I and V from the ancestral pentadactyl forelimb (Kundrát, et al. 2002). For example, putative pre‐cartilage domains for these digits are seen transiently in the chicken and ostrich embryo wing bud (Kundrát, et al. 2002; Welten, et al. 2005; de Bakker, et al. 2013).”

Figure 1. Tawa is the baslmost theropod in the LRT. Like its predecessors, this taxon has a tiny digit 4 and a smaller digit 5. These taxa give rise ultimately to birds with three digits, 1, 2 and 3 without a phase-shift or frame-shift as discussed by de Bakker et al. 2021.

de Bakker et al. 2021 write:
“Palaeontological data, by contrast, point to a pattern of reduction and loss affecting digits IV and V in archosaurs, the clade which includes birds.”

Readers, students, professors,
I encourage you to follow the fossils, not the genes or transient embryo details, which too often deliver false positives. As an example: Bakker et al. nest ducks with chickens and nest swifts with hummingbirds in their figure 8 based on genes. The LRT does not recover those results using traits.

When genetic and embryo results start matching adult trait results,
then we can start listening to gene and embryo studies. In June 2021, gene studies = alchemy. Bogus results occur far too often using gene studies. They only confuse, distract and misdirect. They waste the time of tuition-paying students. Genes never shed light on deep time studies. Embryos also confuse, distract and misdirect while wasting the time and money of students.

,Use traits to learn how every taxon came to be.
You can do this alone, for free if you have Internet access.

References
de Bakker MAG et al. (15 co-authors) 2021. Selection on phalanx development in the evolution of the bird wing. Molecular Biology and Evolution, msab150 doi: https://doi.org/10.1093/molbev/msab150

https://academic.oup.com/mbe/advance-article/doi/10.1093/molbev/msab150/6308563

A skunk and two otters enter the LRT

Three taxa
enter the large reptile tree (LRT, 1875+ taxa) today. Mephitis, the skunk (Fig. 1), Enhydra, the sea otter, and Lontra, the river otter, are all extant members of the clade Carnivora.

All three are traditional musteloids (= weasels).
In the LRT (subset Fig. 2) the otters do nest with Mustela, the weasel, but Mephitis, the skunk, nests between Procyon, the raccoon, and the mongoose (Herpestes).

Figure 1. Skull of Mephistis, the striped skunk from Digimorph.org and used here with permission. Colors added here. Note the concave surface of the upper molar (light green).
Figure 1. Skull of Mephistis, the striped skunk from Digimorph.org and used here with permission. Colors added here. Note the concave surface of the upper molar (light green).

Convergence
In dorsal view (Fig. 1) the squamosal of the skunk skull has a lateral process. Where else in Tetrapoda do we find such a squamosal process or shape? 1) Coelurosauravus and 2) Triceratops.

Figure 2. Subset of the LRT focusing on the clade Carnivora with the addition of Miphitis, Lontra and Enhydra.

A folded, wingless enantiornithine hatchling, IVPP V12707, enters the LRT

Wang et al. 2021 described a tiny Early Cretaceous
enantiornithine hatchling presently known only by its museum number, IVPP V12707 (Fig. 1). Uniquely, the back half of the specimen is folded over the front half, and the wings are missing. How does this happen? I have no idea. Phylogenetic bracketing indicates the wings were large.


Figure 1. IVPP V12707, an enantiornine hatchling close to Chiappeavis and Pengornis. Images from Wang et al. 2021. The specimen is unfolded below. The wings are not present. Shown almost twice life size on a 72 dpi monitor. The ectopterygoids (purple comma-shaped) are shown in two possible configurations here.

From the Wang et al. abstract:
“The transformation of the bird skull from an ancestral akinetic, heavy, and toothed dinosaurian morphology to a highly derived, lightweight, edentulous, and kinetic skull is an innovation as significant as powered flight and feathers. Our understanding of evolutionary assembly of the modern form and function of avian cranium has been impeded by the rarity of early bird fossils with well-preserved skulls.”

Not in the large reptile tree (LRT, 1873+ taxa; subset Fig. 2) where there are plenty of taxa to clearly demonstrate a very gradual transition from dinosaurian morphology to bird morphology.

It is also worth noting that Wang et al. decided the IVPP specimen had an akinetic palate without finding the palatines (Fig. 1 imagined in gray). Otherwise scorable palatal traits are no different than for related taxa in the LRT.

Figure 2. Subset of the LRT focusing on basal birds and the Enantiornithes. Note the IVPP hatchling does not nest with Gretcheniao in the LRT, as it does in Wang et al. 2019.

From the abstract:
“Here, we describe a new enantiornithine bird from the Early Cretaceous of China that preserves a nearly complete skull including the palatal elements, exposing the components of cranial kinesis. Our three-dimensional reconstruction of the entire enantiornithine skull demonstrates that this bird has an akinetic skull indicated by the unexpected retention of the plesiomorphic dinosaurian palate and diapsid temporal configurations, capped with a derived avialan rostrum and cranial roof, highlighting the highly modular and mosaic evolution of the avialan skull.”

Modular and mosaic evolution are two myths that need to be eliminated from the literature. Adding taxa gets rid of these concepts. Don’t be confused by convergence and reversal. The IVPP specimen does not have a dinosaurian skull, defined by Wang et al. as, akinetic. Perhaps the authors were thinking of ostriches, parrots and finches for their typical birds, rather shoebill storks, toucans, penguins and hummingbirds.

Wang et al. write,
“As in most other enantiornithines, the facial margin is dominated by the maxilla, rather than the premaxilla as in crown birds.”

Not true. The maxilla dominates the rostrum in many crown birds (just a few listed above).

Figure 3. Chiappeavis, Pengornis and STM-34-1 to scale.
Figure 3. Chiappeavis, Pengornis and STM-34-1 to scale.

The IVPP hatchling nests in the LRT
between Chiappeavis and Pengornis as derived enantiornithines. Wang et al. nested the IVPP hatchling with Gretcheniao (Wang et al. 2019; Figs. 4–7), a taxon not in the LRT. So, curious about this, I looked up Gretcheniao and added it to the LRT (Fig. 2).

Figure 4. Gretcheniao in situ from Wang et al. 2019 and isolated from the matrix.

Wang et al. were able to understand the crushed post-crania,
of Gretcheniao, but were stymied by the disarticulated skull (Fig. 5) which included an overlooked finger and some neck bones. Their postdentary (pd) is identified here as a parietal. Their second (crescentric-shape) frontal (f) is identified here as a posterior portion of the nasal.

Figure 5. From Wang et al. 2019. The authors were not able to figure out the bones of the skull and did not realize a finger and a few cervicals were in the middle. Compare to figure 6.

A new reconstruction of the skull of Gretcheniao
(Fig. 6) along with the scoring of the post-crania (Fig. 7) nests this taxon within Enantiornithes. In the LRT (Fig. 2) Gretcheniao nests closer to Pengornis (Fig. 3) than to the IVPP hatchling (Fig. 1).

Figure 6. Tracing and reconstruction of the skull of Gretcheniao using DGS. Cervicals are numbered.

Fingers 2 and 3 of Gretcheniao
were quite robust. Finger 1 was tiny. The premaxillary and maxillary teeth were tiny, but the dentary teeth were robust. The tail vertebrae were quite robust and so were their transverse processes.

Figure 7. Elements of Gretcheniao separated more or less into their in vivo positions using Photoshop. Compare to its sister, Pengornis, in figure 3.

Sometimes it is important to have firsthand access to a fossil.
However, in this case firsthand access did not deliver the data presented here (Fig. 6) using Photoshop to colorize and reconstruct published details. Unfortunately, if you follow this method, you too can be shunned from paleontology. Here, as always, academia gets the final vote.

This blogpost was a presentation, a contribution, a demonstration, an experiment.
You are free to accept it or not, but it would be best if you tested it, like a good scientist. Use every taxon and tool available. Don’t leave the description and tracing of a fossil half-finished after skilled preparators have spent months laboriously exposing every sliver.

References
Wang M, Stidham TA, Li Z, Xu X and Zhou Z 2021. Cretaceous bird with dinosaur skull sheds light on avian cranial evolution. Nature Communications. https://doi.org/10.1038/s41467-021-24147-z

Operculum: origin, disappearance, reappearance and disappearance

The fish operculum first appeared
in the sturgeon, Acipenser, among tested taxa in the large reptile tree (LRT, 1870+ taxa; subset Fig. 2). More primitive taxa, from the hagfish (Myxine) to Thelodus, had several gill openings lined up without an operculum covering the serial openings.

Figure 1. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.
Figure 1. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to sharks. The operculum is the lavendar plate between the jaws and pectoral fins.

The operculum first disappeared
and multiple gill openings reappeared in sharks and rays, like Manta and Loganellia.

The operculum first reappeared
in ratfish (Chimaera), then reappeared again in paddlefish (Polyodon) and again in bony fish (Gregorius).

The operculum finally disappeared
in placoderms, and again in moray eels (Gymnothorax) + gulper eels (Eurypharynx) and again in tetrapods (Greererpeton, Fig. 1).

Figure 2. Subset of the LRT focusing on taxa with an operculum.

Frogs and salamanders also have an operculum,
but it is not the same (= homologous) structure. It just has the same name and acts like an eardrum (Fig. 3).

Figure 2. Skull of the frog, Rana
Figure 3. Skull of the frog, Rana. The operculum is the gray series of concentric circles below the squamosal and above the ptergyoid.

At sharks.org, they asked,
“A classical evolutionary question has been how and when these differences arose during Chondrichthyian evolution, and what the common ancestor of Elasmobranchs and Holocephalans looked like. Did sharks gain additional branchial rays after they diverged from a common ancestor who had a chimaera-like pattern, or did chimaeras lose most of their branchial rays after diverging from a common ancestor who had a shark-like pattern?”

Then concluded,
“So elephant sharks have lost the ability to make posterior branchial rays, but still carry the embryonic pattern for where they should form. It seems the common ancestor of Elasmobranchs and Holocephalans had the gills of a shark, rather than the gills of a chimaera,” according to Gillis et al. 2011 in Neil Shubin’s lab. This is confirmed by the LRT (Fig. 2).

The takeaway message is this:
Don’t get caught Pulling a Larry Martin! (= defining a clade based on a single trait or a dozen traits). You’ve just seen how one trait can appear and disappear and reappear and disappear. This is but one example of several hundred I could have shown. Only the last common ancestor method can define a clade. A last common ancestor can be hypothetically recovered in phylogenetic analysis using 150 to 250 multi-state character traits and as many taxa as you have time to do. Convergence and reversal are out there, so beware!

References
Gillis JA, Rawlinson KA, Bell J, Lyon WS, Baker CVH and Shubin NH 2011. Holocephalan embryos provide evidence for gill arch appendage reduction and opercular evolution in cartilaginous fishes. Proceedings of the National Academy of Sciences USA 108:1507–1512.

https://www.sharks.org/blog/blogs

Reconstructing the skull of Changyuraptor

Han et al. 2014
had a tough time tracing the skull of the bird-mimic Changyuraptor (Fig. 1).

Figure 1. DGS tracing of the skull of Changyuraptor. Gray portions added to repair broken or buried bones.

Earlier
the post-cranial traits of Changyuraptor were employed to nest this bird mimic. Today a little extra effort revealed enough of the skull to make a reconstruction (Fig. 1) with gray areas added to fill out the broken, crushed and buried pieces.

Figure 1. Changyuraptor to scale with Ornitholestes, Scriurumimus and Microraptor.
Figure 2. Changyuraptor to scale with Ornitholestes, Scriurumimus and Microraptor.

Changyuraptor yangi (Han et al. 2014; Early Cretaceous) nests here between Ornitholestes and Microraptor (Fig 2) and is midway in size.

Figure 3. This cladogram for Saurornithoides also recovers the nesting of Chanyuraptor between Microlestes and the microraptors.

References
Han G, Chiappe LM, Ji S-A, Habib M, Turner AH, Chinsamy A, Liu X and Han L 2014. A new raptorial dinosaur with exceptionally long feathering provides insights into dromaeosaurid flight performance. Nature Communications DOI: 10.1038/ncomms5382

wiki/Changyuraptor

Mei long 2 in Gao et al. 2012: an Early Cretaceous flightless bird, not a troodontid.

Taxon exclusion mars this otherwise wonderful study
from Gao et al. 2012 of a second Mei long (Fig. 1) found earlier (note the curled in situ specimen in gray, museum number DNHM D2154; in Fig. 1) from the Early Cretaceous of China.

Figure 1. Two Mei long specimens, one in vivo, one in situ.  Click to enlarge.
Figure 1. Two Mei long specimens, one in vivo, one in situ.  

Gao et al. report,
“A second nearly complete, articulated specimen of the basal troodontid Mei long (DNHM D2154) is reported from the Early Cretaceous (Hauterivian-Valanginian) lower Yixian Formation, Liaoning Province, China.”

Unfortunately
the large reptile tree (LRT, 1870+ taxa; subset Fig. 3) nests Mei long far from any troodontids.

If only Gao et al. would have added appropriate taxa.
Here (Fig. 2) are the closest relatives of Mei long within the bird clade Scansoriopterygidae (Fig. 3). Note that some still have long, folding forelimbs replete with feathers. Mei long was more like a small ostrich in the Early Cretaceous. Click HERE to enlarge the image on another web page.

Figure 1. Click to enlarge. Scansoriopterygids to scale.
in the LRT to scale.

Mei long
(IVPP V12733, Xu and Norell 2004, Early Cretaceous) is famous for its 3D preservation in a curled up sleeping posture. Originally considered a young juvenile, bird-like troodontid, Mei long nests in the large reptile tree between the xx specimen of Archaeopteryx and Scansoriopteryx amid the scansoriopterygid basal birds. A second specimen, DNHM D2154 (Gao et al. 2012), was also preserved in a sleeping posture. Mei is derived from a sister to the Munich specimen of Archaeopteryx and was basal to other scansoriopterygids.


Figure 3. Subset of the LRT focusing on basal theropods, including Mei, nesting here in orange within the Scasoriopterygidae following several Solnhofen birds (=Archaeopteryx).
Figure 4. From Gao et al. 2012. Colors added to match figure 3.

References
Gao C, Morschhauser EM, Varricchio DJ, Liu J and Zhao B 2012. A Second Soundly Sleeping Dragon: New Anatomical Details of the Chinese Troodontid Mei long with Implications for Phylogeny and Taphonomy. PLoS ONE 7(9): e45203. doi:10.1371/journal.pone.0045203

Saurornitholestes enters the LRT

Today
Saurornitholestes langstoni (Sues 1978, Evans, et al. 2014; Currie and Evans 2019; Campanian; Figs. 1, 2) enters the large reptile tree (LRT, 1870+ taxa; subset Fig. 3) basal to the bird mimics, Wulong, Sinusonasus, Hesperornithoides, Caihong, Changyuraptor, Microraptor and Sinornithosaurus.

Figure 1. Museum mount of Sinornitholestes.
Figure 2. Skull of Sinornitholestes langstonit. Colorized here.

All these taxa
are derived from Ornitholestes in the LRT (subset Fig. 3).

Figure 3. Subset of the LRT focusing on derived theropods leading to birds.

Not surprisingly, considering that large ‘killer claw’,
Saurornitholestes is also basal to the clades that give rise to Velociraptor and the clade of bird precursors.

References
Currie PJ and Evans DC 2019.
Cranial Anatomy of New Specimens of Saurornitholestes langstoni (Dinosauria, Theropoda, Dromaeosauridae) from the Dinosaur Park Formation (Campanian) of Alberta. The Anatomical Record Advances in Integrative Anatomy and Evolutionary Biology 303(3) DOI:10.1002/ar.24241
Evans DC, Larson DW Cullen,TM and Sullivan RM 2014. “Saurornitholestes” robustus is a troodontid (Dinosauria: Theropoda): Canadian Journal of Earth Sciences, v. 51, p. 730–734.
Sues H-D 1978. A new small theropod dinosaur from the Judith River Formation (Campanian) of Alberta Canada. Zoological Journal of the Linnean Society 62: 381-400.

 

 

reptileevolution.com/saurornitholestes.htm