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).
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.
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.
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.”
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.
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.”
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.
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.
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.
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