SVP abstracts 23: Alvarezsaurids identified as termite eaters again

Qin et al. 2020 suggest
alvarezsaurids, tiny theropods with odd hook-like hands, were termite eaters. According to tradition and Qin et al. alvarezsaurids would have used their strong, hook-like fore claws to rip open termite nests.

By contrast,
earlier a tick bird analog (Figs. 1, 2) was proposed here.

Figure 3. 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.

Figure 3. 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.

From the Qin et al. abstract:
“Alvarezsauroidea is a group of bizarre theropods with highly specialized anatomy. Some of the late-branching members of this clade evolved extremely small body size—as small as early birds.”

“Alvarezsauroids until now have generally been overlooked in studies of theropod body mass evolution because they lacked sufficient lineage sampling and because the evidence for skeletal maturity of tiny specimens was lacking. Moreover, unlike the evolution of flight in paravians, the phylogenetically independent reduction in alvarezsauroid body mass lacks an obvious functional correlate.”

Unless they became parasite pluckers, like tick birds.

“The recent discoveries of early-branching alvarezsauroid fossils from China make a more thorough investigation of body mass evolution in this clade possible.”

Haplocheirus (Fig. 1) nests at the base of the alvarezaurids in the large reptile tree (LRT, 1751+ taxa) and in all other studies. These close relatives of velociraptors are traditionally portrayed as capable of leaping on the backs of larger dinosaurs. That’s where the ticks are, hiding among the feathers. That explains the phylogenetic miniaturization of alvarezsaurids, widely recognized as convergent with birds.

“We conducted detailed osteohistological analysis and bone co-ossification comparisons for Chinese alvarezsauroids, including the Late Jurassic Haplocheirus sollers, Aorun zhaoi, and Shishugounykus inexpectus, the Early Cretaceous relatively large-sized alvarezsauroids Xiyunykus pengi and Bannykus wulatensis, and the Late Cretaceous alvarezsauroid Xixianykus zhangi.”

In the LRT Aorun does not nest with alvarezsaurids, but at a much more basal node. Several of the others have not yet been tested due to their partial and often post-cranial only remains. Often these authors consider the large fore claws ideal for ripping open termite mounds.

“Together with previously published histological data and observations of anatomical characters such as bone ossification, we present a general age and ontogeny estimation for alvarezsauroid specimens. We use our results to estimate adult body mass of all alvarezsauroids and to critically assess the hypothesis of lineage-specific size decrease in Alvarezsauroidea.”

Figure 3. Tickbirds sitting atop a pair of rhinos, perhaps a modern analog for mononykids.

Figure 2. Tickbirds sitting atop a pair of rhinos, perhaps a modern analog for mononykids.

“Our results reveal that size evolution within alvarezsauroids had an initially divergent start followed by a single body size miniaturization event. This miniaturization process started at around 90 million years ago, had a significantly high rate, and culminated in parvicursorines that attained the smallest non-paravian dinosaur body masses in its final stage. Alvarezsauroid lineage richness increased after the miniaturization began, and potentially involved a secondary radiation of small-sized taxa at the end of the Cretaceous. Our results also support the idea that these late-branching small-sized alvarezsauroids occupied an obligate myrmecophagous (termite-eating) ecological niche. This hypothesis is also supported by their unusual low growth rates strategies revealed by our osteohistological studies, and their highly specialized anatomical features indicated by previous research.”

Analog extant termite eaters (anteaters, pangolins) are not tiny, bird-shaped taxa. Analog extant tick pluckers (Fig. 2) are bird-shaped taxa. Authors Michael Benton and David Hone, are part of the contingent promoting the unlikely termite-eater hypothesis.


References
Qin Z, Zhao Q, Choiniere J, Benton M and Xu X 2020. Comparative osteohistology of alvarezsaurs informs hypotheses for their body size evolution. SVP abstracts 2020.

https://pterosaurheresies.wordpress.com/2020/05/21/mononykus-and-shuvuuia-cretaceous-tickbirds/

https://www.smithsonianmag.com/science-nature/a-new-ant-eating-dinosaur-xixianykus-67587048/

Mononykus and Shuvuuia: Cretaceous tickbirds

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.

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 1. 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. The forelimb is very strong. Perhaps these taxa rest vertically and run horizontally. Click to enlarge.

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 1. 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).

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 3. 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.

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 3. Tickbirds sitting atop a pair of rhinos, perhaps a modern analog for mononykids.

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.

wiki/Mononykus
wiki/Halszkaraptor
wiki/Shuvuuia

Here’s the blogpost that inspired this one.

Haplocheirus: a basal alvarezsaroid and more…

Earlier we looked at two Cretaceous alvarezsauroid theropod dinosaurs that likely clung to trees while resting. Today we’ll look at a basal alvarezsauroid theropod dinosaur from the Jurassic with bigger claws and more primitive traits…

Figure 1. Theropods in the large reptile tree. Haplocheirus is highlighted in yellow.

Figure 1. Theropods in the large reptile tree. Haplocheirus is highlighted in yellow.

Figure 1. 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).

Figure 1. 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).

Despite the obvious similarities
to Velociraptor, Haplocheirus is treated only as a basal alvarezsauroid in Wikipedia. Certainly manual digit 1 is more robust in Haplocheirus, as in alvarezsaurids. But just as certainly, pedal digit 2 is more robust with a ginglymoid (pulley-shaped) joint (Fig. 1), as in dromaeosaurids (deinonychosaurs).

Figure 5. Balaur (in vertical and horizontal configurations) compared to Haplocheirus and Velociraptor, Aurornis, Archaeopteryx and Gallus. Balaur nests with Velociraptor in the large reptile tree.

Figure 5. Balaur (in vertical and horizontal configurations) compared to Haplocheirus and Velociraptor, Aurornis, Archaeopteryx and Gallus. Balaur nests with Velociraptor in the large reptile tree.

Phylogenetic bracketing indicates that Haplocheirus was covered with primitive feathers. The present tree (Fig. 2) is similar to that of other prior dinosaur cladograms.

The large reptile tree now includes 550 taxa.

References
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.