Friday, January 25, 2019

Whale-Lizards of the Triassic III: Revenge of Eretmorhipis

You might fondly remember the two previous posts in this series: Part I and Part II which describe a particularly bizarre group of basal ichthyosauromorphs called hupehsuchians. Known for their "bony body tubes" and wide, toothless mouths, hupehsuchians are a surprisingly diverse group that includes five monospecific genera: Nanchangosaurus, Hupehsuchus, EohupehsuchusParahupehsuchus, and EretmorhipisEretmorhipis is the most recently-described hupehsuchian (Chen et al. 2015) but also one of the more mysterious, as the holotype does not include a skull or even cervicals. I should mention this is also the case with Parahupehsuchus although it's missing most of the tail, too.

Based on hupehsuchians with skulls (Hupehsuchus, Nanchangosaurus, and Parahupehsuchus), the general feeling has been that these marine reptiles were lunge-feeders in the style of pelicans and baleen whales. Motani et al. (2015) have summarized this quite well and even speculate that Hupehsuchus, at least, may have had soft-tissue structures for filter-feeding along the premaxillae.

But then we come to Eretmorhipis. New complete specimens described this week (Cheng et al. 2019) demonstrate that hupehsuchians, as a group, were far more ecologically diverse than we thought.

Listen, guys, I don't even have words for how strange this is.
Eretmorhipis was clearly not splitting the atom. That is a very tiny head, folks. I do question whether that skeletal accurately portrays the size of the forelimbs. For comparison, here's the holotype:

Note that the holotype does not preserve those dorsal plates, either.

Forelimbs larger than hindlimbs, sure, but Wreck-It Ralph big? I dunno.

Other things you may notice about Eretmorhipis: the small dermal armor ossicles (?) that cap the top of the dorsal vertebrae in every other hupehsuchian has grown to enormous size. The authors compare them to the conspicuous dorsal plates of Stegosaurus, and I can't think of a better analogue. There are ten such structures arranged in a line from the back of the neck to the base of the tail. While this may be a logical extension of the minimal dorsal armor of its relatives, these "plates" are still surprising, especially in a marine animal you'd want to be hydrodynamic.

But let's go back to that almost tragically tiny head. In profile, it looks sort of like a toothless crocodile skull without room for eyes or nostrils. In dorsal view, it looks like somebody put a pair of tweezers where the nose should be (the same can be said for the mandible). In fact, as the authors write, the skull looks a lot like that of Ornithorhynchus anatinus--the duck-billed platypus.

Here are the two side-by-side: platypus on the left and Eretmorhipis on the right. The blue space represents labial cartilage which makes up the soft-tissue "beak" in platypuses and is inferred for Eretmorhipis. The platypus has a pair of foramen, one in front of each eye, for the ethmoidal nerve which provides sensory branches to the nasal cavity. Eretmorhipis appears to have analogous foramen, which are extremely rare in reptiles and unknown in other hupehsuchians. Besides gross differences in form, however, the two are quite different. The external nares of Ornithorhychus are way down at the end of the snout whereas they are about halfway up the skull in Eretmorhipis. The mandible of Eretmorhipis was also seemingly frozen in a mirror image of the snout, un-fused symphysis included, and was probably not capable of bowing to the extent hypothesized for Hupehsuchus.

But it's the size of the eyes that the authors are most impressed by: Eretmorhipis has mere vestiges of eyes, seemingly useless in terms of resolution. Eretmorhipis was about the size of Hupehsuchus but had eyes half as large. This, in addition to the inferred snout structure, implies that Eretmorhipis was not using sight as its primary sense, but touch. Although exceedingly rare in amniotes, it's possible that Eretmorhipis had electroreceptors in addition to its enhanced tactile snout.

The "bony body tube" is also present in Eretmorhipis, and Chen et al. (2019) note that the tail must not have been all that flexible either given that the hemal spines span three caudals apiece and are almost horizontally-oriented. Given the rigidity of its axial skeleton, Eretmorhipis must have relied heavily on its giant paddle-shaped limbs to get around.

Seemingly the entire clade is restricted to an enormous lagoon spanning about 1200 kilometers (745ish miles) east to west and 500 kilometers and 500 kilometers (310ish miles) north to south and they all lived together at the same time. Now that's a lot of lagoon to go around, but one does wonder how multiple species of hupehsuchian (and more are coming) avoided direct competition with each other. Eretmorhipis accomplished this by going blind and grabbing prey that could be sensed by touch alone.

I'm sure this isn't the last entry in this multi-part series and I look forward to whatever curveballs this group throws our way in the future.

1 comment:

  1. So many questions about these guys. Reminds me a bit of a Pipefish. The body must have been very rigid because of the armor? Used those big paddles to maneuver slowly up and down the water column?

    The head is bizarre if it hunted like a platypus it's prey must have been tiny. Maybe a mudsucker like paddlefish, suckerfish, used currents to detect food in the lagoon floor?