McCabe and Nesbitt 2021
assigned a disarticulated Late Triassic scapulocoracoid and humerus (MCZ 101542) to Lagerpeton(Fig. 1) in the absence of any pervious similar bones for the Lagerpeton holotype.
Workers have been trying to rebuild a chimaera of Lagerpeton from disassociated parts for several years now, hoping it will somehow shed some insight into dinosaur and pterosaur origins.
This is all for naught because Lagerpeton is a bipedal chanaresuchid that ran on two toes, not an archosaur (dinosaurs + crocs) or fenestrasaur (pterosaurs and their ancestors).
Figure 1. Tropidosuchus and Lagerpeton compared to the new material (MCZ 101542).
How can McCabe and Nesbitt assign that pectoral girdle? The holotype of Lagerpeton lacks any pectoral girdle material. So we can only imagine missing elements based on phylogenetic bracketing and comparative anatomy.
Figure 2. MCZ 101542 scapulocoracoid and humerus compared to Dromomeron humerus.
Given that, does the MCZ 101542 material closely resemble comparable bones in closely related taxa? In the large reptile tree (LRT, 1810+ taxa) Lagosuchus nests with Tropidosuchus (Fig. 1), not with dinosaurs or pterosaurs.
A problem arises. Tropidosuchus (Fig.1) has a larger, hourglass-shaped scapula with a short ‘waist’. By contrast the MCZ 101542 scapula (Fig. 1) has a smaller, straighter, narrower, more rectangular shape. So, maybe we should look for a better match… if there is one.
Figure 2. MCZ 101542 compared to Marasuchus and Lagosuchus.
Is material from another taxon a little more similar? Marasuchus (Fig. 2; PVL 3871) has a more robust, but otherwise similarly straight scapulocoracoid with a dinosaurian deltopectoral crest located about a third the way down the slender humerus, and more similar in scale. Lagosuchus (Fig. 2; UPLR 090) has a similarly gracile scapulocoracoid (at least what’s left of it). It’s all iffy.
McCabe and Nesbitt also make comparisons when they note, “Compared to Lagosuchus talampayensis (PVL 3871), the scapular blade of MCZ 101542 is much more strap-like (near parallel anterior and posterior side) and the distal end expands more in Lagosuchus talampayensis.”
Their table 2 lists ‘species’ Marasuchus‘ with specimen number PVL 3871. So their Marasuchus (PVL 3871) is not Lagosuchus (UPLR 090; Fig. 2).
McCabe and Nesbitt also write “The glenoid of MCZ 101542 is directed posteroventrally like that of other avemetatarsalians (e.g., lagerpetids, Lagosuchus talampayensis, silesaurids, dinosaurs).”
In the LRTAvemetatarsalia is a junior synonym for Reptilia because it also include pterosaurs. Lagerpetids are proterochampsids, not dinosaur relatives. And, once again the authors’ Table 2 does not match their text with regard to nomenclature and specimen numbers.
Figure 3. Ixalerpeton compared to MCZ 101542.
The protorosaur, Ixalerpeton,
(Fig. 3) is similar in size to MCZ 101542, but the shapes are slightly different.
The authors note, “Within Lagerpetidae, the humerus of Ixalerpeton polesinensis (ULBRA-PVT059) is more robust than MCZ 1010541 (Fig. 4), with proportionally much larger proximal and distal expansions. The proportions of the humerus of Lagosuchus talampayensis (PVL 3871) matches that of MCZ 101541, with overall weakly expanded articular ends.”
Would you like to see a ‘Hail Mary’ pass based on taxon exclusion? The authors report, “Overall, the gracile proportions of MCZ 101541 (= MCZ 101542 = the humerus) are unlike early archosaurs and their close relatives.”
When workers give up like this,
it’s usually due to taxon exclusion, whether intentional or not.
Figure 4. Procompsognathus has proportions that precisely fit the MCZ 101542 material.
In this case there is a close match for the gracile proportions
of MCZ 101542 and it comes from a taxon that happens to be missing the scapulocoracoid and humerus, the Late Triassic theropod from Germany, Procompsognathus(Fig. 4), a taller relative of Marasuchus in the LRT. Like a lock and a key, a Yin and a Yang, the MCZ material is a perfect fit including the narrow, but deep anterior torso required to fit the narrow but deep scapula and coracoid. The authors did not mention Procompsognathus. So taxon exclusion continues to be a problem here. If inappropriate, at least it should have been considered and eliminated.
Still, this is only a gutsy guess.
See how reconstructions can help?
The LRT uses more complete taxa
whenever possible. To assign two bones to a specific genus is getting close to “Pulling a Larry Martin.” Be careful when you go there. It’s worth a shot (Fig. 4), but it’s easy to be wrong.
References McCabe MB and Nesbitt SJ 2021. The first pectoral and forelimb material assigned to the lagerpetid Lagerpeton chanarensis (Archosauria: Dinosauromorpha) from the upper portion of the Chañares Formation, Late Triassic. Palaeodiversity, 14(1) : 121-131.
Summary for those in a hurry: A unique morphology + a unique niche + a unique prey assemblage = a unique hunting technique.
From the Hone and Holtz 2021 abstract: “We conclude that …the pursuit predation hypothesis for Spinosaurus as a “highly specialized aquatic predator” is not supported. In contrast, a ‘wading’ model for an animal that predominantly fished from shorelines or within shallow waters is not contradicted by any line of evidence and is well supported. Spinosaurus almost certainly fed primarily from the water and may have swum, but there is no evidence that it was a specialised aquatic pursuit predator.”
Hone and Holtz pay little attention to the fact that Spinosaurus was the only large theropod that had such short hind limbs and had a dorsal fin much deeper than its ribcage. The authors cherry-picked less obvious traits to support their hypothesis, giving only passing notice to what makes Spinosaurus unique.
From the Hone and Holtz introduction: “In short, these animals [spinosaurs] acted like large herons or storks, taking fish and other aquatic prey from the edges of water or in shallow water, but also foraging for terrestrial prey and scavenging on occasion.”
In paleontology, if Spinosaurus is going to be compared to large herons or storks, it should look overall like a giant heron or stork. It does not.
Figure 1. Aquatic Spinosaurus to scale with contemporary Early Cretaceous giant fish.
Hone and Holtz keep hammering away at a single point: “The hind limbs of Spinosaurus do potentially provide evidence for aquatic locomotion and even striking at prey underwater, but specifically not in the sense of pursuit predation.”
“Surface swimming is considerably less efficient than submerged swimming and incurs considerable extra wave drag for animals moving at, or just below, the surface.”
The problem is, Hone and Holtz want Spinosaurus to be built for speed, like a sailfish, if they are to grant it a submerged aquatic existence. Unfortunately, the authors are caught in a logic rut based on some sort of straw man. They end up cherry-picking traits less important traits while trying to weave their story away from the larger, unique traits.
Spinosaurus was not built for speed.
It didn’t need to be built for speed. Look at the prey taxa available (Fig. 1). Lungfish, giant bichirs and coelacanths are big, fat and slow-moving fish. Sawfish are lethargic bottom-dwellers. Drag is not a factor when moving slowly, like Spinosaurus.
As the only aquatic dinosaur,
Spinosaurus may have developed a sail to help regulate body temperature while staying submerged (except to lay eggs). It may have never needed to stand bipedally, like its theropod sisters. Hence the small legs and quadrupedal center-of-balance.
The tiny backset naris of Spinosaurus
was on its way to complete closure. That’s not a problem as many extant birds without nares demonstrate. They can all breathe throughout their mouth and throat.
Figure 2. Diagram from Dal Sasso et al. 2005, colors and overlay added to show dorsal expansion of the maxilla to cover an elongate naris.
Hone and Holtz summarize their study: “If swimming to engage prey, based on the drag, performance and body shape it would be limited to lunging attack in shallow waters, not pursuit predation at speed in open water.”
No. Spinosaurus was a slow swimmer, unaffected by drag. It would not be limited to lunging attacks in shallow water, contra Hone and Holtz. Rather, slow, steady underwater predation with its sail exposed to maintain a 99º body temperature in an 80º river is still the best explanation for this unique theropod.
“The information provided through recent discoveries may suggest an increase in aquatic affinities for Spinosaurus, and it may have been able to swim with its tail, and even swim well compared to other theropods, but nothing presented to date contradicts the fundamentals of the ‘wading model’ and does not support active pursuit predation.”
Hone and Holtz failed to consider a semi-active, semi-submerged method of predation. “Nothing presented to date”= failure to consider all options. Spinosaurus is indeed a “highly specialized aquatic predator”, just not a fast one.
Earlier we looked at Spinosaurus in its environment here, its ability to swim deep here and its tiny naris here.
Smyth et al. 2020
brings us a new, articulated, partial, crushed skeleton of a small Aptian (Early Cretaceouse) compsognathid theropod with interesting soft tissue. The authors compared the integumentary structures of Ubirajara jubatus to those of the standard wing bird-of-paradise. A reconstruction (Fig. 1) shows four “stiff rod-like structures projecting from its shoulders,” according to Karina Shah, writing for NewScieintist.com.
We’ve never seen anything like this,
which makes it newsy. But is it real?
This taxon will not go into the LRT
because too little is known of the skeleton (Figs, 2, 3).
Figure 1. Ubirajara illustration showing proposed four “stiff rod-like structures projecting from its shoulders.”
The specimen reconstruction (above) was restored from a plate and counter plate (Fig. 2) with bones at the periphery and a big glob in the middle.
Figure 2. Plate and counter plate image and tracing from Smyth et al. 2020. The tracings were combined by Smyth et al. here in figure 3. Sorry for the low resolution. This is just for display.
Fortunately, Smyth et al. provided a combined tracing (Fig. 3). Note both legs are missing.
Or are they?
Instead Smyth et al. identify two pairs of straight 15 cm rods, which you can see in their illustration above (Fig. 1). Their diagram shows BMFIs directed outside the blob, aiming toward the top of the scapula.
Occam’s Razor suggests
those paired rods emanating from the shoulders may instead be long, straight legs, knees flexing near the shoulders, splitting posteriorly as shown on the overlays (Fig. 3) toward an absent pelvis for the femur and an absent foot for the tibia. This alternate restoration is a guess based on the scant evidence shown here and an aversion to completely new structures. But somebody has to say it, just to open this discussion. If I’m wrong, I’m wrong.
Figure 3. From Smyth et al. 2020 with overlays suggesting the possibility that the paired rods growing from the shoulders may instead just be legs with knees near the shoulders. Just a hypothesis awaiting confirmation or refutation. Here the vertebrae are also renumbered and the hand is reconstructed.
Anyone can make a mistake. Even if there are four co-authors. We’ve seen this sort of thing before in Yi qiand Ambopteryx where the authors mistook a displaced ulna or radius for a novel bone, their styliform. The important thing is to not perpetuate the myth of an entirely new structure, if it is a myth. This Ubirajara example is not so clear (based on indistinct impressions) so I could be wrong. Let’s figure this out. This is the loyal opposition talking, building on the tenth man rule (from World War Z).
Figure 4. Ubirajara rough reconstruction from diagram in Smyth et al. 2020 (Fig. 3).
Has anyone else
come up with this novel hypothesis? Let me know if this leg idea can be readily refuted.
References Smyth RSH, Martill DM, Frey E Rivera-Silva HE and Lenz N 2020. A maned theropod dinosaur from Brazil with elaborate integumentary structures. Cretaceous Research. doi:10.1016/j.cretres.2020.104686
Updated November 11, 2021 with another look at candidate sister taxa.
O’Connor et al. 2020 present a late surviving, Late Cretaceous basal theropod dinosaur lacking maxillary teeth, Falcatakely forsterae (Figs. 1–4). The authors reconstructed their crushed and slightly disarticulated fossil using µCT scans.
Figure 1. Falcatakely from O’Connor et al. 2020 µCT scans, then moved slightly on frame 2.
the authors mistakenly considered Falcatakely an enantiornithine bird with a “unique development of beak”. This was due to taxon exclusion. They assumed they had a bird when a more inclusive analysis of omitted taxa indicates they did not.
Figure 2. Falcatakely from O’Connor et al. 2020 µCT scans, then missing elements added in frame 2. Note the lacrimal (in red) is partly the jugal (cyan, frame 2).
From the O’Connor et al. abstract: “Mesozoic birds display considerable diversity in size, flight adaptations and feather organization1,2,3,4, but exhibit relatively conserved patterns of beak shape and development Here we describe a crow-sized stem bird, Falcatakely forsterae gen. et sp. nov., from the Late Cretaceous epoch of Madagascar that possesses a long and deep rostrum, an expression of beak morphology that was previously unknown among Mesozoic birds and is superficially similar to that of a variety of crown-group birds (for example, toucans).”
Note the first two words. They assumed they had a ‘unique’ Mesozoic bird without first testing in a phylogenetic analysis. Evidently they got excited by the possibility of getting published in Nature. That part came true.
Colleagues: The time to get excited is AFTER a wide gamut analysis documents and firmly nests your taxon. Otherwise you’ll end up like the authors of Oculudentavis, which was also mistakenly considered a bird. (I wonder if Nature will demand a similar retraction?)
Figure 3. Falcatakely from O’Connor et al. 2020 µCT scans, then non-palate elements darkened in frame 2. Compare to figure 1 for a more realistic narrowing of the snout.
Whenever you think you have a ‘unique’ anything add taxa. Expand your taxon list.
Uniqueness in evolution is an oxymoron.
Something should resemble your ‘unique’ taxon. The O’Connor team’s mistake was due entirely to taxon exclusion, not looking far enough with a wide enough taxon list.
Figure 4. Cladogram from O’Connor et al. 2020 where they exclude basal theropods and nest Falcatakely with dissimilar enantiornithine birds by default.
Late-surving Late Cretaceous Falcatakely
nests among the basalmost theropods with Tawa, in the large reptile tree (LRT, 1768+ taxa).
Figure 5. Late Cretaceous Falcatakely compared to scale with Late Triassic Tawa and Early Cretaceous Pengornis. Note the loss and lack of an antorbital fossa in Tawa and Falcatakely. The premaxillary teeth are tiny, distinct from related theropods and distinct from Pengornis. 160 million years separates these new sisters.
The lack of maxillary teeth in Falcatakely
is notable in this bird-mimic. Tiny teeth line the down-tipped premaxilla. Note the loss and lack of an antorbital fossa. Now we can look for transitional taxa.
With so many theropod fans out there,
let’s see how many confirm the basal theropod affinities of Falcatakely. In either case, it’s still a wonderful fossil find and the authors did a wonderful job of documenting the material. (Next time, just add more taxa).
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 juniorBarsbold 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 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 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 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 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 theropoddinosaurs 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.
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 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 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.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 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.
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.
Short one today, both in text length and taxon height.
Figure 1. Bambiraptor figures from Burnham et al. 2000. Colors added.
Bambiraptor feinbergi (Burnham et al. 2000; Late Cretaceous, AMNH FR 30556) was originally considered a juvenile Sauronitholestes. The brain size is the largest among Mesozoic dinosaurs. Here it nests basal toVelociraptor + Balaur in the large reptile tree (LRT, 1724+ taxa, subset Fig. x). Proportions are closer to Arachaeopteryx according to the authors.
Figure 2. Bambiraptor to scale compared to Velociraptor, Balaur, Hapolcheirus, Archaeopteryx and Gallus.
Figure 3. Subset of the LRT focusing on theropods leading to birds, including the two newest additions, Bambiraptor and Zanazabar.
at Zanazabar soon.
References Burnham DA, Derstler KL, Currie PJ, Bakker RT, Zhou Z and Ostrom J H 2000. Remarkable new birdlike dinosaur (Theropoda: Maniraptora) from the Upper Cretaceous of Montana. University of Kansas Paleontological Contributions 13: 1-14.
Traditionally the small, but extremely robust hand claws of Mononykus and Shuvuuia (Figs. 1, 2) were considered digging tools. If so, their forelimbs would have been distinctly different from the digging forelimbs of all other fossorial tetrapods based on size alone, not to mention the rest of the bird-like morphology that does nothing to support a digging hypothesis.
Figure 1. Forelimb of Mononykus. Large deltopectoral crest pulls humerus toward the sternum like a clasp.
Maybe there’s another answer.
For a moment, let’s not focus on Mononykus and Shuvuuia. Let’s broaden our view to see what related taxa are doing with their forelimbs. Let’s see if phylogenetic bracketing and environment can provide clues to the Mononykus forelimb mystery.
Figure 2. Shuvuuia and Mononykus to scale in various poses. The odd digit 1 forelimb claws appear to be retained for clasping medial cylinders, like tree trunks and dinosaur backs. The forelimb is very strong. Click to enlarge.
Outgroup taxa include Haplocheirus (Fig. 3) and, more distantly, Velociraptor (Fig. 3). These two have forelimbs more typical of theropods with three digits and digit 2 longer than 1. Both come with a reputation and ability to jump on large dinosaurs (Fig. 4).
That’s similar to
what extant tickbirds (oxpeckers) do to large African mammals (Fig. 4), though not with the intention of ripping into their flesh with a wicked pedal digit 2.
Figure 3. Haplocheirus sollers traced from several photos. This specimen is 15 million years older than Archaeopteryx and tens of million years older than dromaeosaurs and alvarezsarids. Click to enlarge. Note the robust pedal digit 2 and manual digit 1.Haplocheirus sollers (Choiniere et al. 2010 Late Jurassic, 150 mya, 2m long) is a a theropod dinosaur from the Jurassic that nests at the base of the alvarezsaurids (including Mononykus and Shuvuuia) and also basal to the Cretaceous dromaeosaurids (including Velociraptor), ~and~ basal to Jurassic proto-birds (including Aurornis, Fig. 2).
In modern day Africa tickbirds are often seen happily perching atop rhinos and other larger mammals (Fig. 5), cleaning them of parasites and riding them like passengers on a bus… yet always able to fly away or jump off and run away.
To scale with other dinosaurs of their time and place
(Fig. 3) it becomes clear that alvarezsaurids and Mononykus were relatively about the size of tickbirds and able to do the same job (plucking off parasitic insects) for their mutual benefit.
Figure 4. Giant Deinocheirus, a contemporary of Mononykus, might have served as the host and dining room for a series of ever smaller and more specialized parasite eaters.
Clearly Mononykus and Shuvuuia are highly specialized taxa leaving no descendants. In the large reptile tree (LRT, 1692+ taxa) these alvarezsaurids evolve from larger theropods like Hapolocheirus. As the ancestors of Mononoykus and Shuvuuia grew smaller, so did their forelimbs, pelvis, killer toe and teeth. These tiny theropods became more and more specialized for their insect-plucking, hitchhiking niche. As they became phylogenetically-miniaturized, smaller alvarezsaurids were able to hitch rides on smaller and smaller dinosaurs.
Figure 5. Tickbirds (oxpeckers) sitting atop a pair of rhinos, perhaps a modern analog for mononykids.
So the little adducting forelimbs of Mononykus and Shuvuuia acted like little hair clips, keeping these little dinosaurs attached to the skin and feathers of their hosts. That’s really all they were good for. Not flying. Not flapping. Not digging. Not display. Just mighty adduction. Those tiny forelimbs with big thumbs were perfect for clipping to giant host dinosaurs. The long legs of Mononykus would have been just long enough to walk through high feathers, like a human walks through tall grass. Or to run and hop on one new dinosaur after another. Active and highly coordinated, alvarezsaurids would have had the same agility as modern birds when they cavort on tree branches, tree trunks and rhino backs, all without using their ‘hands.’
This may be a novel hypothesis.
If not, please provide a citation so I can promote it.
Added a day later in response to the above promise: Thank you, Tyler. From the abstract: “I propose that bizarre structures may have served to defend against parasitic dorsal attacks from riding dromaeosaurs. Frequent dismounts from large living dinosaurs may explain the origin of feathers, gliding and avian flight.”
Fraser G 2014. “Bizarre Structures” Point to Dromaeosaurs as Parasites and a New Theory for the Origin of Avian Flight. The Journal of Paleontological Sciences: JPS.C.2014.01 PDF
In counterpoint, Fraser was postulating the origin of larger wings and feathers for dismounting dromaeosaurs. He also discussed the origin of frills, plates and spikes on large host herbivores to dissuade dromaesaurs from mounting in the first place. Unfortunately, nowhere does he discuss the alvarerzsaurids or Mononykus and the development of its bizarre tiny forelimbs. Evidently they were not on his ‘radar’. Even so, thank you for bringing this paper to my attention. A good read!
A few more data points and citations:
Velociraptor mongoliensis(Osborn 1924; Late Cretaceous, 75 mya; 6.8m long) The tail was long and stiffened with elongate chevrons and zygapophyses. The deep pubis was oriented posteriorly with a large pubic ‘boot’.
Haplocheirus sollers (Choiniere et al. 2010 Late Jurassic, 150 mya, 2m long) The tail was not stiffened with elongate accessory processes.
Mononykus olecranus (Perle et al, 1993; Late Cretaceous ~70 mya, 1 m in length) Only digit I remained full size on the stunted hand. The proximal ulna (the elbow) was enlarged. The pubis was shorter and lacked a pubic boot.
Shuvuuia deserti (Chiappe, Norell and Clark 1998, Late Cretaceous) was smaller and retained digits 2 and 3 as vestiges.
Halszkaraptor escuilliei (Cau et al. 2017; Late Cretaceous) was originally considered an aquatic dromaeosaur related to Mahakala, but here nests with Shuvuuia. A distinctly different manual digit 3 was the longest, but the gracile thumb retained the largest claw. The hands did not act like hair clips.
References Cau A, et al. 2017. Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs. Nature. doi:10.1038/nature24679 Chiappe LM, Norell MA and Clark JM 1998. The skull of a relative of the stem-group bird Mononykus. Nature, 392(6673): 275-278. Choiniere JN, Xu X, Clark JM, Forster CA, Guo Y, Han F 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang, China. Science 327 (5965): 571–574. Perle A, Norell MA, Chiappe LM and Clark JM 1993. Flightless bird from the Cretaceous of Mongolia. Nature 362:623-626. Perle A, Chiappe LM, Rinchen B, Clark JM and Norell 1994. Skeletal Morphology of Mononykus olecranus (Theropoda: Avialae) from the Late Cretaceous of Mongolia. American Museum Novitates 3105:1-29.
A Smithsonianmag.com writer is trying to make the ordinary extraordinary by claiming, “The Rise of Meat-Eating Dinosaurs Is More Complicated Than We Thought. “
Figure 1. Herrerasaurus from Black 2020. This is a basal dinosaurs. This is not an omnivore.
Writer Riley Black (formerly Brian Switek) writing in smithsonianmag.com declares: “The earliest dinosaurs arose about 235 million years ago during the Middle Triassic. They didn’t look much like modern favorites Triceratops or Spinosaurus. Instead, these lanky creatures didn’t get much bigger than a German shepherd. The current spate of evidence suggests they were omnivorous.”
Black also provides this image (Fig. 1) of middle Triassic Herrerasaurus, the basalmost dinosaur in the large reptile tree (LRT, 1688+ taxa) and this is no omnivore. This taxon is 3 meters or 16 feet long, not the size of a German shepherd.
Black continues: “Up until now, paleontologists thought theropods remained generally small and on the ecological sidelines from about 235 through 201 million years ago. It was only after a mass extinction at the end of the Triassic, at the 201 million-year mark, that carnivorous dinosaurs started to get big. But that view is starting to change thanks to a new reading of the bone trail by scientists who think large meat-eaters may have appeared much earlier. Virginia Tech paleontologist Christopher Griffin says a key player in this story is Herrerasaurus.”
Confused? So am I. This brings us back to where the LRT starts, regarding dinosaurs. Everyone in paleo knows Herrerasaurus is a Middle Triassic carnivorous dinosaur. The little German shepherd-sized dinos are either made up or never existed. In either case, Black doesn’t list or illustrate them.
“The known carnivorous dinosaurs during the later part of the Triassic appeared to be smaller and less imposing than the crocodile relatives they lived alongside (such as Postosuchus from the southwestern United States). Thanks to a better understanding of dinosaur growth, however, paleontologists have found that some of those little theropods were hiding a secret.”
Postosuchus is not a crocodile relative in the LRT. Herrerasaurus is closer to crocodiles in the LRT because only crocodylomorphs and dinosaurs make up the clade Archosauria.
Black continues: “The few remains we’ve found of larger Triassic theropods come exclusively from immature animals that are still growing rapidly,” Griffin says. These young carnivores would have grown to lengths exceeding 18 feet in adulthood. That’s a little less than half a full-grown T. rex, but enough to make you want to avoid meeting such a carnivore face-to-face.”
A few Late Triassic theropod lengths: Tawa is 2m long. Coelophysis is 3m long. Both are represented by adult skeletons. Again, where are these imaginary few remains?
Black finally raises the curtain on her main attraction: “Late last year, Ludwig-Maximilian University of Munich paleontologist Oliver Rauhut and colleague Diego Pol named an exceptional skeleton of a Middle Jurassic carnivore they called Asfaltovenator. This was a large animal, more than 25 feet long, that approached the average size of the later Allosaurus and bears more a passing resemblance to the later dinosaur.”
This is no big deal. Between the Late Triassic and the Late Jurassic we expect to find theropod dinosaurs bigger than their ancestors and smaller than their descendants with transitional morphologies.
Black concludes with a quote: “There is much more to be learned about theropod evolution during this time,” Rauhut says, with finds like Asfaltovenator hinting at what remains to be uncovered.”
Again, no big deal. There is always ‘much more to be learned’ about all taxa ‘during this time.’ Some people complain because I was a journalism major. Sometimes that degree comes in handy.
References Black R 2020. The Rise of Meat-Eating Dinosaurs Is More Complicated Than We Thought. online here.
We looked at tiny, feathered Sciurumimus albersdoerferi (Germany, Rauhut et al. 2012; BMMS BK 11) and larger bones-only Ornitholestes (North America) earlier as Late Jurassic sisters in the large reptile tree (LRT, 1659+ taxa). After a recent review, these two continue to nest as sisters at the base of the Microraptor (Fig. 3) +Sinornithosaurus clade. So no news here… except now let’s combine the extraordinary size difference between the two and the widely accepted observation that Sciurumimus is a juvenile.
That brings to mind: a juvenile of what?
The LRT indicates a juvenile ornitholestid (Fig. 1). The overall morphologies are strikingly similar and the size difference is appropriate. Other published studies recover other nestings.
Rauhut, et al. 2012
(Suppdata) nested Ornitholestes between ornithomimosaurs and deinonychosaurs, far from Sciurumimus, which Rauhut et al. nested Sciurumimus between an unresolved clade of giant spinosaurs + megalosaurs and giant Monolophosaurus. Like Rauhut et al., the LRT nests also nests Ornitholestes between ornithomimosaurs (+ tyrannosaurs + oviraptors + therizinosaurs) and deinonychosaurs.
Key differences in the LRT include
the use of two Compsognathus specimens. The each nest at the base of their own clade, a hypothesis of interrelationships overlooked by Rauhut et al.
the inclusion of three Microraptor specimens and two Sinornithosaurus specimens, adults of which are closer in size and morphology to Sciurumimus. This brings to mind the possibility that phylogenetic miniaturization and neotony played a part in the evolution of these bird-mimics. These closely related taxa were omitted by the Rauhut et al. selection process.
Figure 1. Sciurumimus compared to Ornitholestes and Microraptor to scale.
In their study of the wonderfully preserved
anchiornithid, Aurornis, Godefroit et al. nested Sciurumimus between Monolophosaurus + Sinraptor and Zuolong, all more primitive taxa in the LRT. In Godefroit et al. these taxa are far from Ornitholestes, which nested with another small compsognathid, Juravenator. Juravenator nests with equally small, but shorter limbed Sinosauropteryx in the LRT. Evidently few theropod studies agree with one another in the details.
Rauhut et al. 2012 reported, “Our analysis confirms Sciurumimus as the basalmost known theropod with evidence of feather-like integument.” By contrast, in the LRT, Tawa-like, feathered Sincalliopteryx (Fig. 2) is more primitive, despite its late appearance (Early Cretaceous) in the fossil record.
Figure 2. Late surviving Sinocalliopteryx currently nests basal to Late Triassic Coelophysis, derived from Late Triassic Tawa. It has the most primitive presence of feathers despite its late appearance.
currently nests basal to Late Triassic Coelophysis, and was derived from Late Triassic Tawa. In the LRT, Sinocalliopteryx has the most primitive presence of feathers among theropods despite its appearance tens of millions of years later than its phylogenetic genesis.
Figure 3. Microraptor gui (IVPP V 13352) reconstructed from tracings in figure 1. There are no surprises here, except a provisional closer relationship with Compsognathus than with Velociraptor. Microraptor has a large pedal claw two, but it is not quite the killing claw seen in droamaeosaurs.
The Ornitholestes + Sciurumimus + Microraptor + Sinornithosaurus clade were bird-mimics and bird-mimic ancestors not directly related to birds or bird ancestors in the LRT.
References Godefroit P, Cau A, Hu D-Y, Escuillié F, Wu, W and Dyke G 2013. A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds. Nature. 498 (7454): 359–362.
Rauhut OWM, Foth C, Tischlinger H and Norell MA 2012. Exceptionally preserved juvenile megalosauroid theropod dinosaur with filamentous integument from the Late Jurassic of Germany. Proceedings of the National Academy of Sciences. 109 (29): 11746–11751.
Poust et al. 2020 bring us news of a small, subadult theropod with some interesting traits, Wulong bohaiensis (Early Cretaceous; D2933). They considered the specimen a microraptorine dromaeosaurid.
Figure 1. Wulong in situ, plus the original published diagram. The specimen is somewhat surrounded by a few coprolites = cop.
By contrast, the large reptile tree (LRT, 1637+ taxa) nests Wulong among similar, small, long-legged troodontids, between Buitreraptor and Caihong. While this topology differs from that of other workers, the same can be said of nearly every clade in the LRT. That’s why this blog has been self-labeled ‘heretical’.
Figure 2. Wulong skull, original diagram, DGS colors applied to bones and reconstruction based on the DGS tracings.
So, why the different views? That appears to be due to taxon exclusion. There is no indication in the text that Buitreraptor and Caihong were included in analysis. There is no indication that the authors created a reconstruction, which helps identify bones, their ratios and proportion in crushed taxa like Wulong. More importantly…
Figure 4. Wukong manus DGS tracing and reconstruction. Note the 180º rotation of the manus relative to the radius and ulna.
… several taxa converge on birds and small feathered theropods converge with each other in the LRT. The differences between the clades should not be determined by a few traits (= Pulling a Larry Martin), but here are gleaned after phylogenetic analysis of several hundred traits. As mentioned earlier, you can’t nest a specimen within a clade by a small number of cherry-picked traits because there is so much convergence within the Tetrapoda. Rather, run an analysis and find out which taxon is the last common ancestor of a derived clade. Those, then, are the validated clade members.
Figure 3. Wulong pelvis.
Figure 4. Wulong pedes, original tracing and reconstruction based on DGS tracings.
the coracoid is fenestrated in the middle. The ilium includes a prepubis process. Some feathers are preserved.
The authors report, “Wulong is distinguished by several autapomorphic features and additionally, has many characteristics that distinguish it from its closest well-known relatives. Compared with Tianyuraptor and Zhenyuanlong, Wulong is small and its forelimbs are proportionally long.”
By contrast, in the LRT Tianyuraptor and Zhenyuanlong are not related to troodontids, microraptorids or dromaoeosaurids. Tianyuraptor and Zhenyuanlong are basal to tyrannosaurids.
References Poust AW, Gao C-L, Varricchio DJ, Wu J-L and Zhang F-J 2020. A new microraptorine theropod from the Jehol Biota and growth in early dromaeosaurids. The Anatomical Record. American Association for Anatomy. DOI: 10.1002/ar.24343