Visitor Creator: Dr Romain Sabroux
Marie Curie Fellow in Earth Sciences, College of Bristol
I’ve to make a confession. I’m not a lot of a diver.
As a marine biologist, this in all probability sounds odd. However when you make one thing as demanding as SCUBA diving, particularly if you find yourself on an precise scientific expedition and that you must pattern a number of instances per day for an entire month, you want a great cause. My cause could be the animals I’ve been finding out for eight years now: the pycnogonids, also referred to as sea spiders.
Sea spiders are arthropods – these animals with an exoskeleton and articulated legs, like spiders, bugs, crabs and millipedes. Pycnogonids have 4 pair of legs however they aren’t spiders. They reside within the sea, however they aren’t crustaceans. They’re a gaggle aside, characterised by an extended proboscis on the tip of which stands the mouth; by a tubercle on the highest of their head, which carries two pairs of eyes wanting in all instructions like a periscope; by an typically very tiny and slender physique; and by their lengthy legs, during which the digestive and reproductive organs unfold. They even use their legs to put their eggs! Sea spiders reside on the underside of the seas all world wide, searching for the sponges, anemones, corals, algae, detritus, and all the opposite issues they might feed on.
In our areas, sea spiders are actually, actually, inconspicuous and small. It’s nearly unimaginable to see them if you find yourself diving. That doesn’t imply you can’t acquire them: within the discipline, divers pattern bulks of algae, sand, muds, and so forth. that I can frisk again within the lab below a microscope. Ultimately, I see much more sea spiders staying all of the day in a lab, than any diver on the sphere.
Observe that not all sea spiders are small nevertheless, as some gigantic species (with a leg span of a number of tens of centimetres) dwell within the Antarctic Ocean or in abyssal waters. However I’m very delicate to chilly… not mentioning to 600 bars of water strain.
Jurassic Sea Spiders: The Identical, However Greater!
We all know little or no in regards to the previous range of sea spiders; solely eleven fossil species have been found which are unambiguously recognized as pycnogonids, and their fossil document is gaped with lengthy hiatus of a whole lot of hundreds of thousands of years. Sea spider fossils are solely present in fossiliferous websites of outstanding preservation, with peculiar circumstances that foster even the preservation of probably the most delicate animals.
Two of those websites have been discovered respectively in southern France and southern Germany, within the websites of La Voulte-sur-Rhône and Solnhofen. They offered us with valuable testimonies of the ocean spiders residing in Europe some 150 million years in the past, throughout a interval referred to as the Jurassic. Whereas they clearly belong to sea spider species or households that don’t exist these days, their physique plan was very alike the extant ones. Remarkably, these species have been additionally fairly massive (as much as 12 cm of leg span). A dive to catch would have in all probability been beautiful. However the greatest species discovered within the Jurassic have been doubtless residing in a stage of the ocean referred to as the “aphotic” zone, that’s the sea depth at which the sunshine from the solar fades. We all know this from the morphological variations discovered within the species which lived alongside them, together with massive eyes. Additionally it is coherent with the households of among the sea spiders present in these fossil websites. A lot for a dive within the Jurassic.
Swimming With The Devonian Sea Spiders
However I do know of an excellent higher spot for SCUBA diving. In Western Germany, some 400 million years in the past. The biota of Hunsrück is a remarkably well-preserved witness of a shallow waters surroundings of a interval referred to as Devonian. Sea lilies, starfishes and brittle stars have been overlaying the underside of the ocean, which was additionally navigated by numerous armoured fishes, trilobites, eurypterids, and even marine scorpions (Palaeoscorpius). There have been additionally sea spiders. And what sea spiders!
These have been remarkably totally different from the species we are able to observe at this time. The construction of the legs doesn’t correspond to extant species, and is comparatively variable between species, suggesting they belong to very divergent teams. The bottom of the legs introduced unusual annular buildings, the character of which isn’t but properly understood. Additionally they introduced an extended posterior stomach fabricated from a number of segments, which completely differ from the extant species which have solely a small unsegmented bud on the similar place. One of many fossil species, Flagellopantopus blocki, had a really lengthy flagellum. One other one, Palaeopantopus maucheri, appeared to haven’t any head in any respect.
Although my favorite sea spider fossil ever was additionally probably the most abundantly discovered: Palaeoisopus problematicus. The curious title of this species comes from the difficulties that palaeontologists skilled to determine it. It was initially regarded as an isopod, a gaggle of crustacean that features, amongst many others, the terrestrial woodlice (most isopods are literally marine). This error was quickly “corrected” in a while, however palaeontologists saved mistaking the stomach for the pinnacle for over 30 years! This remarkably massive species had a comparatively broad physique in comparison with extant species, and its stomach was significantly lengthy and divided in 5 segments. However probably the most exceptional function of Palaeoisopus, have been their 4 pairs of enormous, flatten legs. Their paddle-shape articles are prone to have been used to swim; its first pair of legs additionally current lengthy, curved claws, and it’s doubtless they used it to catch their preys. Now I do know that my opinion could also be fairly unpopular right here, however I might have cherished to swim with gigantic swimming sea spiders. Sadly, I can’t.
The Blues Of A Palaeontologist
It’s doubtless that sea spiders like Palaeoisopus, Flagellopantopus or Palaeopantopus belong to totally different teams of sea spiders than the extant ones: they shared many morphological options and an extended frequent evolutionary historical past, however in some unspecified time in the future their phylogenetic tree break up into just a few teams. Because it appears, a lot of the lineages of pycnogonids disappeared in some unspecified time in the future between the Devonian and the Jurassic, leaving just one remaining group: the pantopods. That is the distinctive surviving lineage that latter diversified within the Jurassic species, as within the trendy ones.
So why have been all different lineages immediately worn out? And when? These are among the questions I attempt to reply in my analysis.
The evolution of pycnogonids echoes with an concept that shocked me lots once I learn it for the primary time as an undergraduate pupil. An concept proposed by Stephen J. Gould in his guide Great life, a passionate reflection over the range of Burgess Shale (a fossil fauna of marine invertebrates residing 125 million years earlier than Palaeoisopus). In response to Gould, evolution proceeds by decimation of lineages, adopted by propagation throughout the boundaries of the surviving teams; quite than in a progressive, steady diversification and complexification of life. One thing fairly like this will likely have occurred with sea spiders. Their previous range was wealthy in types, physique plans, and possibly of their biology and their ecology. However most of this range disappeared by some means, and solely pantopods diversified afterward, at all times counting on the identical elementary physique plan. It’s doubtless that we are going to by no means see one thing as lovely as a swimming sea spiders once more.
I typically get melancholic fascinated about all these lovely issues which have existed, and that I’ll by no means see. However there’s some consolation in pondering that some teams, together with pantopods, have withstood the check of time and diversified in a wealth of colors, morphologies, biologies, and habitats, ultimately right here for me to gaze upon, via the lenses of a microscope.
Dr Romain Sabroux is a Marie Curie Analysis Fellow presently working within the Palaeobiology Division on the College of Bristol.
Article edited by Rhys Charles
Sabroux, R. et al. (2019) 150-million-year-old sea spiders (Pycnogonida: Pantopoda) of Solnhofen. Journal of Systematic Palaeontology. 17: 1927 – 1938