I got my start in biology as a paleontologist. Back in graduate school, I was fascinated by the origin and early evolution of animals, and Ediacaran and early Cambrian fossils more than 500 million years old were the focus of my study. Little by little, though, despite the best efforts of my wonderful advisor Jim Valentine, I learned that I wasn't really that good at (or perhaps suited for) paleontology. I often got hung up on the uncertainties involved in understanding marks left on rocks by animals separated from today by over 500 million years in time and evolution. Nevertheless, as I shifted my research interests toward living animals, those in lineages that diverged early in the history of animals (comb jellies, sponges, placozoans, and especially jellyfish and kin) I have maintained a paleobiological perspective.
So, last year when a paper came out describing a 560 million year old fossil as a "muscular cnidarian", the oldest record for muscle in the fossil record, I was of course intrigued. But what really caught my eye, and those of two my close collaborators, was that the authors specifically compared the musculature of this fascinating animal, which they named Haootia quadriformis, to living stalked medusae (class Staurozoa). [Note: they carefully did not assign the fossil to Staurozoa.]
As it turns out, thanks to the wonderful research being conducted by a Phd student from Brazil, Lucília Souza Miranda, I have been learning quite a bit about the musculature, and other anatomical aspects, of staurozoan bodies. So Lucília, her major professor Antonio Carlos Marques, and I all avidly devoured the details presented in Liu et al.'s paper.
Accepting that the 560 million year old impressions were indeed those of muscles (something I will come back to below), we concluded that Liu and colleagues description of the muscular system in Haootia quadriformis didn't quite fit that of extant staurozoans. In particular, you can see that the muscles are largely circular (which is something seen in large swimming medusae), whereas the bottom-dwelling staurozoans have more prominent longitudinal, radially organized muscles. Liu et al. did observe, however, the presence of longitudinal muscles in the stalk connecting the animal to the substrate and the presence of longitudinal muscles running up into the branching arms of the animal, both of which are consistent with what is seen in living staurozoans.
Thus, the major difference between Liu and colleagues' reconstruction of the musculature of Haootia quadriformis and that of living staurozoans is the prominent circular muscles all throughout the calyx (cup-like part of the body). Circular muscles are present in staurozoans, but they are always restricted to the calyx margin, as seen above in Manania uchidae. This musculature fits with the behavior of the bottom-dwelling stalked medusae, which do not open and close their bodies. Of course, free swimming medusae do open and close to propel themselves through the water, and it is these animals, relatively closely related to stalked medusae, that do have more prominent circular musculature.
After comparing the musculature of Liu et al's reconstruction to that of living staurozoans, we did something a bit odd: we invoked how hard it is to interpret fossils, which nearly always involve a three dimensional organism being squashed completely or nearly flat with parts overlaying one another. Maybe Liu et al. just misinterpreted all of those impressions of muscles? Given their expertise and experience, probably not, but we went on to offer up a hypothetical reinterpretation, one that would be consistent the musculature of staurozans, and basically through that out there for the experts to consider. A somewhat weak finish in a way, but one very much in keeping with my personal experience in paleontology.
Liu and colleagues did reconsider our re-imagination of Haootia quadriformis in a very good natured response to our paper, and urged further investigation of the remarkable holotype fossil specimen that was only recently collected from the field. Our paper had pointed out that such strong circular musculature would be consistent with an animal capable of vigorously contracting its body and Liu et al. rightly replied that body plans and feeding strategies separated from the present by so many millions of years might not have analogs in modern seas. But I still wonder, what would a bottom-dwelling animal attached by a stalk be doing with such prominent muscles? How were these putative muscles truly organized in Haootia quadriformis? How do we even know the impressions really represent muscles? All these questions recall those persistent doubts I had in graduate school when I studied to be a paleontologist. Having published a scientific paper on a fossil I guess I am still a paleontologist, or at least sort of one.
Liu AG, Matthews JJ, Menon LR, McIlroy D, Brasier MD. 2014 Haootia quadriformis n. gen., n. sp., interpreted as a muscular cnidarian impression from the Late Ediacaran period (approx. 560 Ma). Proc. R. Soc. B 281, 20141202. (doi:10.1098/rspb.2014.1202)
Miranda LS, Collins AG, Marques AC. 2015 Is Haootia quadriformis related to extant Staurozoa (Cnidaria)? Evidence from the muscular system reconsidered. Proc. R. Soc. B. 282, 20142396. (doi:10.1098/rspb. 2014.2396)
Liu AG, Matthews JJ, Menon LR, McIlroy D, Brasier M D 2015 The arrangement of possible muscle fibres in the Ediacaran taxon Haootia quadriformis. Proc. R. Soc. B 282, 20142949.
The Cambrian Animal. 2015 Wherein scientists DON’T spill blood over a Precambrian animal.
Liam Herringshaw. 2014 Muscling in on a new world.