Natovenator: a ?swimming theropod buried in eolian Gobi sands

Sunday, December 4, 2022 13:25

% of readers think this story is Fact. Add your two cents.

Found in wind-blown sand dune deposits,
like so many other Late Cretaceous Gobi desert dinos, Natovenator (Lee et al 2022, Fig 1) was given a duck-like or penguin-like (Fig 1) niche ONLY because the authors reported the swept-back dorsal ribs made the torso streamlined and deeper than wide, unlike most theropods.

All that is true. But Moloch (Fig 1) has similar swept-back ribs and no one has ever called this extant Australian desert lizard ‘streamlined’ or ‘aquatic’.

Cherry-picking traits, like swept-back ribs, is called “Pulling a Larry Martin.” Instead, always start and end with your own wide gamut cladogram, like the large reptile tree (LRT, 2173 taxa, subset Fig 6).

Figure 1. Natovenator compared to other taxa with streamlined ribs, including Uria, the flying penguin ancestor, Jurapteryx, a Solnhofen bird, and Moloch, an Australian horned lizard of the desert.

Nearest tested relative,
desert sprinter, Halszkarapator (Figs 2, 3), lacks dorsal ribs on an otherwise articulated fossil. So synapomorphy cannot be tested here. This loss was likely due to taphonomy because tetrapods never lack dorsal ribs. Shuvuia (Fig 2) seems to have swept-back ribs. Only the anteriror ribs of ancestral Sinornithoides (Fig 2) were swept back.

Figure 2. Halszkaraptor in situ. Note the lack of articulated dorsal ribs, probably due to taphonomy. Suddenly their loss takes on phylogeenetic importance!

From the Lee et al 2022 abstract:
“It is primarily because most known non-avian theropods are terrestrial, barring a few exceptions. However, clear evidence of streamlined bodies is absent even in the purported semiaquatic groups.”

A decidedly non-streamlined lizard, Moloch (Fig 1), also has has swept-back, streamlined dorsal ribs (Fig 1). This paleo-problem would have been delightful to Kansas professor, Larry Martin, who used to tease students with just this sort of mental exercise.

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.

Earlier
a tick-bird hypothesis was proposed for the phylogenetically-miniaturized descendants of Haplocheirus (Fig 6). First imagine dromaeosaurids flock-jumping onto larger prey dinosaurs to kill it (Fig 3). Next imagine smaller, less lethal, transitional taxa flock-jumping onto larger host dinosaurs, not to kill it, but to eat the insect parasites found among the feathers. This continues until a well-adapted Natovenator decided to more or less live permanently on its huge feathery host (Fig 3).

Figure 4. Natovenator resting on a large host dinosaur, like Deinocheirus in figure 3. Note how it clings in a different, but convergent pattern, to its alvarezsaurid cousins, Shuvuuia and Mononykus. This explains the atypical wider-than-deep ribcage of Natoveenator better than an aquatic niche hypothesis. The humerus is 1.5x too large in this graphic modiifed from Lee et al, when compared to the actual humerus shown in figure 5.

From the Lee et al 2022 abstract:
“Here we report a new theropod, Natovenator polydontus gen. et sp. nov., from the Upper Cretaceous of Mongolia. The new specimen includes a well-preserved skeleton with several articulated dorsal ribs that are posterolaterally oriented to streamline the body as in diving birds. Additionally, the widely arched proximal rib shafts reflect a dorsoventrally compressed ribcage like aquatic reptiles. Its body shape suggests that Natovenator was a potentially capable swimming predator, and the streamlined body evolved independently in separate lineages of theropod dinosaurs.”

Other aquatic types (penguins and ducks) do not have extremely gracile, almost needle-like limbs, like those found in Natovenator. This bony morphology marks Natovenator has a hitchhiker, like a land remora.

Figure 5. Natovenator skull from Lee et al. 2022. Colors added here. Note the dorsolateral nares. The humerus (h) is no longe than the orbit-occiput length.

Other niche options:
Given a close relationship to Shuvuuia (Fig 1) and other phylogentically miniaturized alvaresaurids derived from Haoplocheirus (Fig 3) and Sinornithoides (Fig 1), Natovenator was likely an insect eater, plucking parasites from a giant host dinosaur hide.

Note the similar, but different pectoral hook apparatus,
on Natovenator (Fig 4) using three needle-like unguals, rather than one robust pinching ungual anchored by a deep sternum, as in Shuvuuia (Fig 2). During cold desert nights, Natovenator could have been warmed by close association with its host. Hence the wider-than-deep ventrally flat torso, distinct from most theropods. During hot desert days Natovenator could have loosened its grip to get up and walk around its host, seeking cooler areas, perhaps in the shadows.

Figure 6. Subset of the LRT focusing on theropods close to Natovenator.

Depositional setting
According to Eberth et al 2009, “In particular, the Baruungoyot Formation comprises a stacked succession of tabular redbeds consisting of fine grained sediments deposited in alluvial (channel, floodplain, and floodbasin), lacustrine, and eolian environments. Dinosaur
tracks and turbation are locally abundant, especially in overbank mudstones and channel abandonment mudstones that are overlain by sand and silty channel fill deposits.”

If Natovenator was indeed lacustrine, these mudstones would have made a great burial site. Instead the gracile sprinter was buried in eolian (wind-blown) sand deposits.

Natovenator was exceptionally gracile,
distinct from robust submerging water birds. Natovenator had no clear aquatic locomotory adaptations, other than those swept-back ribs, which closely resemble those of an extant desert lizard that does not demonstrate streamlining elsewhere (Fig 1).

References
Eberth DA, Badamgarav D and Currie PJ 2009. The Baruungoyot-Nemegt transition (Upper Cretaceous) at the Nemegt type area, Nemegt Basin, South Central Mongolia. Journal of the Paleontology Society of Korea 25(1):1–15.
Lee S et al (7 co-authors) 2022. A non-avian dinosaur with a streamlined body exhibits potential adaptations for swimming. Nature communications biology https://doi.org/10.1038/s42003-022-04119-9

wiki/Natovenator

Source: https://pterosaurheresies.wordpress.com/2022/12/05/natovenator-a-swimming-theropod-buried-in-eolian-gobi-sands/