Ozimek in water: Two new hypotheses

The hyper-slender limbs
of the protorosaur, Ozimek (Fig 1), are unique within the clade Tetrapoda. They are so slender that one wonders how Ozimek was able to move about, with or without the additional burden of that large skull on the end of that long slender neck. We looked at this taxon earlier here and here.

FIgure 1. Ozimek skeleton in vivo. Water and grainy lake bedding are indicated. Neutral buoyancy is one answer to the riddle of those hyper-slender limbs.

FIgure 1. Ozimek skeleton in vivo. Water and grainy lake bedding are indicated. Neutral buoyancy is one answer to the riddle of those hyper-slender limbs.

Originally
(Dzik and Sulej 2016) Ozimek was described as a glider related to Sharoviptyerx, but it is much larger, not related, no lateral membranes are present either in Sharovipteryx or Ozimek, the ribs of Ozimek enclose a cylindrical torso, unlike the flat torso of Sharovipteryx,  and most importantly the limbs in Ozimek are much more slender, relative to those in Sharovipteryx despite being many times larger.

Phylogenetically
the large reptile tree (LRT) nests Ozimek with Prolacerta. There are no gliders in that clade. Jaxtasuchus is another long-necked protorosaur, but it was armored with bony scutes, and was not directly related to Ozimek, yet similar in size. While the metapodials are not compact in any protorosaurs, neither do they spread widely, so the manus and pes of Ozimek were likely narrower (Fig. 1) than originally restored by Dzik and Sulej.

Let’s remember
that gliders may be slender, but their ‘wing struts’ must berobust enough to support their entire weight on extended unsupported limbs, multiple ribs, as in Draco, or extended dermal rods, as in Coelurosauravus. Gliders may also have strong pectoral and pelvic girdles to anchor those gliding limbs. Ozimek will never be described as anything but weak and slender. If it was the size of a fly it might have glided, but at the size of a praying mantis or Sharoviipteryx, that possibility is less likely, let alone at its current size, 3x longer and 9x heavier than Sharovipteryx.

Tanystropheus underwater among tall crinoids and small squids.

Figure 2. Tanystropheus in a vertical strike elevating the neck and raising its blood pressure in order to keep circulation around its brain and another system to keep blood from pooling in its hind limb and tail.

Converging on the long-necked tritosaurs,
Tanystropheus (Fig. 2) and Langobardisaurus (Fig. 3), Ozimek had similar overall proportions,  but with more slender limbs. Tanystropheus is best considered an underwater biped (Fig. 2) based on many geologic clues. Langobardisaurus may have been amphibious, but facultatively bipedal either wet or dry.

Figure 2. Langobardisaurus compared to Ozimek and its sister, Prolacerta.

Figure 3. Langobardisaurus compared to Ozimek and its sister, Prolacerta.

Geological setting
Dzik and Sulej report, “the [Ozimek] fossils under study occur in the one-meter thick lacustrine horizon where the dominantspecies are aquatic or semi-aquatic animals. Shallow freshwater conditions existed at deposition. This taken together with conchostracans occurring within the fauna suggest an abundance of periodic ponding at a lake shore.” Ponds are generally placid bodies of water. Dzik and Sulej report, periodic flooding events transported terrestrial taxa to the ponds during redeposition events. The strata also contain large parasuchians (phytosaurs), large metoposaurs, lungfish and other fish. “Most of the articulated Ozimek gen. nov. specimens were found at the boundary between the red upper and grey lower units of the lacustrine horizon, but not in the grainstone lenses.”

Neutral buoyancy
Ozimek does not have the robust solid ribs that would indicate it was a bottom dweller. The slender limbs were hollow, but not pneumatic, so they were likely filled with bone marrow, according to the authors. In any case, their very slenderness minimizes the amount of air or fat they can contain.

After wrestling with various niche scenarios for Ozimek
the morphological and analog evidence indicates that IF Ozimek was a slow-moving reptile supported by a placid aquatic medium, slender limbs could evolve. The long neck raised and lowered the skull, both for breathing and prey capture. Lateral motions were limited. The long cervical ribs would have kept the neck fairly straight, like a flexible fishing rod. Ozimek was likely a sit and wait predator in shallow ponds. It is difficult to envision it as a giant glider or a terrestrial predator. The limbs were too slender.

Figure 4. Ozimek hitching a ride on top of Metoposaurus.

Figure 4. Ozimek hitching a ride on top of Metoposaurus as a possible parasite, cleaner, or egg predator.

A second hypothesis:
Sometimes when taxa have unusual traits, those develop due to a relationship with other individuals, or even other genera. If Ozimek entertained a remora-like lifestyle, hitched to the top of a giant, slow-moving Metoposaurus (Fig. 4), it would have been protected by the  bulk of its patron and able to feed on leftovers from the giant predator’s meals. Or maybe it fed on parasites attached to Metoposaurus. Or on worms stirred up when Metoposaurus was settling in. Or on Metoposaurus eggs when they were laid. In that role, the limbs of Ozimek would not have needed to remain ambulatory because Metoposaurus would have done the walking. The long fingers and sharp claws (unguals) of Ozimek might have helped anchor it to the back or the flat skull of the giant amphibian.

References
Dzik J and Sulej T 2016. An early Late Triassic long-necked reptile with a bony pectoral shield and gracile appendages. Acta Palaeontologica Polonica 61 (4): 805–823.

 

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