Shuvuuia: Night digger or Cretaceous tick-bird with Sharovipteryx proportions?

Choiniere et al. 2021
bring us new inner ear data on Shuvuuia (Figs. 1, 4), a tiny alvarezsaurid theropod dinosaur we earlier considered as a Cretaceous tickbird (Fig. 2). Instead of perching on rhinos, Shuvuuia would have picked insects off larger, sometimes feathered dinosaurs (Fig. 1). Other workers, including Choiniere et al. follow tradition in considering Shuvuuia a digger. Presumably that would be difficult given the tiny forelimb proportions (Fig. 4).

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 1. Giant Deinocheirus, a contemporary of Mononykus and Shuvuuia, might have served as the host and dining room for a series of ever smaller and more specialized parasite eaters.

From the Choiniere et al. abstract:
“Owls and nightbirds are nocturnal hunters of active prey that combine visual and hearing adaptations to overcome limits on sensory performance in low light. Such sensory innovations are unknown in nonavialan theropod dinosaurs and are poorly characterized on the line that leads to birds. We investigate morphofunctional proxies of vision and hearing in living and extinct theropods and demonstrate deep evolutionary divergences of sensory modalities. Nocturnal predation evolved early in the nonavialan lineage Alvarezsauroidea, signaled by extreme low-light vision and increases in hearing sensitivity. The Late Cretaceous alvarezsauroid Shuvuuia deserti had even further specialized hearing acuity, rivaling that of today’s barn owl. This combination of sensory adaptations evolved independently in dinosaurs long before the modern bird radiation and provides a notable example of convergence between dinosaurs and mammals”.

Not sure why the authors are being so coy in their abstract.
They could have gotten right to the point, as they did for the publicity in the science website Phys.org. The authors reported Shuvuuia had “a fragile, bird-like skull, brawny, weightlifter arms with a single claw on each hand, and long, roadrunner-like legs. This odd combination of features has baffled scientists since its discovery in the 1990s. The eyes of Shuvuuia were also of note, as they had some of the proportionally largest pupils yet measured in birds or dinosaurs, suggesting that they could likely see very well at night. The extremely large lagena of this species is almost identical in relative size to today’s barn owl, suggesting that Shuvuuia could have hunted in complete darkness.”

Learn more about the lagena below.

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.

These traits could have had a different explanation.
Other than owls, most birds bed down for the night. The authors reported to Phys.org that “Many carnivorous theropods such as Tyrannosaurus and Dromaeosaurus had vision optimized for the daytime,” so these theropods also slept at night on stable ground or tree limbs. On the other hand, if you are a Cretaceous tickbird, the substrate on which you live and sleep is itself alive and moving about, like a rocking ship in a storm. In this scenario Shuvuuia needed a better-than-average balancing organ and clinging forelimbs, which is what they say it had!

When is the best time to jump on a giant dinosaur?
Probably at night, when it is sleeping. Perhaps that is why larger, but still relatively tiny, ancestral Haplocheirus (Fig. 1) had larger eyes… all the better to seek out dinosaur hosts by moonlight.

Figure 3. The ear of a bird with the cochlea and lagena highlighted. The cochlea is the hearing organ. The lagena has other duties in birds.

So, what is the lagena?
The lagena in birds can be related to their navigation abilities (birds are supposed to be capable of orienting within the magnetic field of the Earth due to the magnetic properties of the lagenar otoconia; this structure can also provide detection of movements along the vertical axis.”

According to Wildlife-sound.org,
“The function of the lagena is uncertain, but it is considered that it is more likely to be concerned with balance rather than hearing.”

“There is a conspicuous difference between the hearing organ or cochlea of the mammal and the bird. That of the mammal is a thin, coiled tube while the bird’s cochlea is relatively short, broad and has only a slight curve; both organs are filled with fluid. With the bird, as with the mammal, a basilar membrane traverses the cochlea; it carries sensitive hair cells with nerve fibres running to the auditory nerve and hence to the brain. The hair cells are covered by a tectorial membrane and have a far greater concentration per unit area of membrane than those of the mammalian cochlea.”

Funny thing…
in the Choiniere et al. text, there is no mention of the lagena, which they emphasize in the publicity. Rather the authors discuss the elongation of the cochlear duct, the part devoted to sound reception and conversion to nerve impulses.

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 4. 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.

Choiniere was quoted in Phys.org
“Nocturnal activity, digging ability, and long hind limbs are all features of animals that live in deserts today, but it’s surprising to see them all combined in a single dinosaur species that lived more than 65 million years ago.”

Figure 3. Sharovipteryx reconstructed. Note the flattened torso.
Figure 5. Sharovipteryx reconstructed. Compare proportions to Shuvuuia in figure 4.

The digging ability of Shuvuuia has always been suspect,
given its Sharovipteryx-like proportions (Fig. 6), with arms barely deeper than the chest and long, gracile legs better suited to leaping and running.

With regard to the ‘large’ eyes of Shuvuuia — size matters.
I’s a tiny taxon with much larger ancestors (Fig. 1). In archosaurs juveniles have proportionately larger eyes than adults. Shuvuuia was phylogenetically miniaturized as an adult, retaining juvenile traits. There’s nothing more to it than that. Surprised this wasn’t brought up among the 13 co-authors. Apparenlty no one was given the assignment to be the Tenth Man.

Headline grabbing?
Choinierre et al. wrote: “sensory evolution in birds and their theropod stem lineage is poorly understood [but see, e.g., (6–9)]. This is a substantial shortcoming in our understanding of dinosaurian biology and of the structure of Mesozoic ecosystems.” Did Choinerre shed light on this problem, if it is indeed a problem? Or did they just add to the myth? The words ‘balance’, ‘neotony’ and ‘paedomorphism’ are not mentioned in the text. Sometimes workers get lost in the details and lose the overall perspective, blinding themselves to alternate possibilities.

Artists!
Start showing alvarezsaurids riding bareback, clinging to Late Cretaceous Mongolian dinosaur quills and feathers.

PS added 48 hours later:
Birds with a similar wide diameter maximum iris diameter, like Apus, the swift, and Struthio, the ostrich, are apparently active day and night. In these taxa the iris itself must be highly variable to adapt to changes in lighting. That puts a whole new spin on dinosaurs. These facts were not highlighted on the authors’ chart, nor were their stories told. Like them, maybe Shuvuuia stayed awake night and day (not scratch digging!) clinging to mothership dinosaurs loaded with insect prey.

References
Choiniere JN et al. (+12 co-authors) 2021.
Evolution of vision and hearing modalities in theropod dinosaurs Science 372 (6542): 610-613 DOI: 10.1126/science.abe7941

wiki/Lagena
springer.com/article/lagena

Publicity:
phys.org/news/2021-05-shuvuuia-dinosaur-dark.html

10 thoughts on “Shuvuuia: Night digger or Cretaceous tick-bird with Sharovipteryx proportions?

  1. A point you seem to have missed: it’s not the size of the eye that is used as a proxy for nocturnality, it’s the size of the scleral ring relative to the orbit (which in turn is a proxy for the size of the pupils, not the eye itself). This has been shown by comparison within modern taxa to be an extremely robust proxy. Also pupil size is not a paedomorphic trait to the best of my knowledge.

      • yeh i saw that you were arguing that. My comment was in reponse to your rather snide paragraph where you ask why none of the 13 authors had considered large eye size being due to paedomorphosis. the reason obviously being that they weren’t talking about large eye size

      • Neil, thank you for bringing this up for clarification. We are all learning as we go.

        Their Fig. 1 showed a nocturnal owlet with a wide maximum iris aperture and an unrelated diurnal pygmy parrot with a slightly smaller maximum iris aperture. This was meant to set up the analogy in dinosaurs when they showed a presumably nocturnal Hapolocheirus with a wide diameter maximum iris aperture and a presumably diurnal Erlikosaurus with a narrow maximum iris aperture. To your point nocturnal species do need a faster F-stop (= optic ratio), but I see this as cherry-picking unrelated taxa to ‘prove’ a point.

        Instead the authors should have showed a related velociraptor-type with a small sclerotic ring next to Haplocheirus to demonstrate the differences, and then showed the related nocturnal Shuvuuia to scale, noting the slower F-stop iris (documented in their Fig. 1 chart), despite the relatively larger eyeball-to-skull ratio.

        Other birds with a similar wide diameter maximum iris diameter, like Apus, the swift, and Struthio, the ostrich, are apparently active day and night. In these taxa the iris itself must be highly variable to adapt to changes in lighting. That puts a whole new spin on dinosaurs. These facts were not highlighted on the authors’ chart, nor were their stories told. Like them, maybe Shuvuuia stayed awake night and day (not scratch digging!) clinging to mothership dinosaurs loaded with insect prey.

      • The paper analysed sclerotic proportions in more than 400 taxa, including extant for which diel preference was known and Mesozoic to provide the phylogenetic context. The data was analysed using a machine learning algorithm that accounts for phylogenetic non-independence of the taxa. This isn’t cherry picking taxa, this is you being too lazy to check what they actually did:

      • Thanks for sending me to the SuppData, Neil. It provides info we both need to consider.

        To your point, Struthio and Apus are indeed included in their optic ratio analysis. My fault not to look at the SuppData.

        The authors’ algorithm determined that Struthio and Apus both had a probable diurnal optic ratio. We know both keep active day and night. The authors note, “the scleral ring and orbit morphology, extended from previous analyses (9), has a mean accuracy of 92.0% for the classification of extant species as nocturnal or non-nocturnal.” The authors do not indicate whether scores for Apus and Struthio were ‘hits’ or ‘misses’. The authors don’t seem to realize there is a third possibility: ‘sleepless’.

        And finally, I traced the skull, created a reconstruction and ran an analysis on the referred specimen of ‘Shuvuuia,’ IGM 100/0977. That’s the one with the sclerotic ring the authors used to measure its optic ratio. As you know, the Shuvuuia holotype lacks a scleral ring. Turns out IGM 100/0977 is not a Shuvuuia, but very close to Gallimimus, a coeval, typically larger, ostrich-like dinosaur, also toothless with large eyes. This new taxon in the LRT will be the subject of a future blog.

        Thanks again for pointing out my oversight.

    • Thanks, Nick. A word search for ‘lagena’ in the text produces no results. Having never heard of ‘the lagena’ before, I looked it up. “an extension of the saccule of the ear in some vertebrates, corresponding to the cochlear duct in mammals.” The illustration (above) indicates the bird cochlea includes the lagena at it tip, but what is it? A saccule extension sounds like a smaller sac coming off a larger sac. A duct is a passageway, a hole that leads elsewhere. Confusing. Sometimes scientists clarify what they say and mean. Sometimes they obfuscate.

      • The cochlear duct is the bony feature. The lagena is within. It is an attempt to be clear they are describing bony features, not that soft tissues are preserved. We can reasonably assume the lagena is present at the end of the CD and some past authors use the terms interchangeably because in some taxa there isn’t an elongate bit before the lagena… There’s no conspiracy here, just you need to read more.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.