Phylogenetic miniaturization among alvarezsauroid dinosaurs

Qin et al. 2021 bring us their views
on the phylogenetic miniaturization that started with Haplocheirus (Fig. 1) and ended with tiny Shuvuuia (Fig. 1) in the clade of Alvarezsauridae. This has been known for several years.

Actually there are other examples of phylogenetic miniaturization.
Several in birds (e.g. hummingbirds, kingfishers, swifts, any bird with short legs and long-legged ancestors). Microraptor is a bird-mimic evolving from a larger Ornitholestes. Rahonavis is smaller than its therzinosaurid relatives. Marasuchus and Compsognathus each had larger ancestors. like Tawa. The authors also mention scanoriopterygids, but this is a clade of often flightless taxa evolving from small Solnhofen birds. Their sizes vary considerably.

The authors attribute this miniaturization to a diet of ants and termites.
The alternate idea, that of Cretaceous tickbirds seeking insects among the feathers of much larger dinosaurs (Fig. 1) was not considered.

The authors include Aorun zhaoi within the Alvarezsauroidea.
The large reptile tree (LRT, 1885 taxa) does not support that hypothesis of interrelationships. In the LRT Aorun nests basal to the second round of theropods, including two species of Compsognathus.

The authors state,
“The holotype of Haplocheirus sollers and a specimen of Shuvuuia deserti are juveniles, without any evidence supporting an advanced ontogenetic stage.” This is based on histological slices, measuring growth rings.

The authors are forgetting,
during phylogenetic miniaturization juveniles become sexually active at an earlier age. Smaller adults produce smaller eggs, which produce smaller adults, etc. We also see this in pterosaur phylogeny. This need not be continuous throughout dozens of generations, but may come and go and reverse as the niche permits.

The authors say they pruned the taxa they considered to be juveniles from their studies. Maybe that was a good idea. Maybe not.

The authors report,
“Alvarezsauroids had no adaptations for flight or gliding, so characters shared by tiny alvarezsauroids and birds, like the keeled sternum and atypical vertebrae,
might be related to size reduction or ecological niche shift rather than adaptation to powered flight.”

Indeed. Ask what was Haplocheirus (Fig. 1) doing? Then ask what would it be doing better if it were considerably smaller? (See figure 1.)

The authors report,
“This sharp decrease in size may be related to some of the bizarre alvarezsauroid modifications
seen only in the Late Cretaceous late-branching species.”

No. Basal taxa always provide some clues as to what made their clade different. Look at Haplocheirus again. I’ll bet no one can claim that it was eating ants and pulling up termite mounds. Rather, it was leaping on larger prey and hanging on while feeding. Same with tiny Shuvuuia.

The authors report,
“What could have been the specific ecological drivers of the sharp size reduction and speciation of alvarezsauroids? Early studies of alvarezsaurs compared the forelimbs to fossorial
mammals, even inferring a burrowing habit, and according to relevant research on living burrowing mammals, a fossorial habit might limit the basal metabolic rate and further lead to a
smaller body size.“

No. You have to start your observations with Haplocheirus. It does not have forelimbs comparable to fossorial (= burrowing) mammals. Neither do the tiny taxa. This is yet another myth that really needs to die.

The authors report,
“Recent functional analyses suggest that the body forms of alvarezsauroids cannot be explained by adaptations for fossorial habits.”

True. They would have been light and agile leapers, not diggers. Look at the whole animal, not just the forelimbs.

By the way,
specialized forelimbs are not necessary for bird-sized theropods to eat ants and termites. Doves, certain owls and chickens will pursue termites on the ground. Birds that eat ants include antbirds, flickers, sparrows, wrens, grouse and starlings.

References
Qin Z, Zhao Q, Choiniere JN, Clark JM, Benton MJ and Xu X 2021. Growth and miniaturization among alvarezsaurroid dinosaurs. Current Biology. https://doi.org/10.1016/j.cub.2021.06.013

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