Monday, January 31, 2011

The Lost World: The biology of echinoderm "living fossils" (stalked crinoids) on an Antarctic Seamount!!

Dinosaurs Living in Underground Caves! Pre-historic Monsters Living in the Deep Sea!

This is what we envision whenever the term "living fossil" gets brought up!
At this exact moment, a whole bunch of paleontologists' heads are exploding BECAUSE I've used the term "living fossil". (BABOOM!) Why?

The term basically refers to any organism that physically RESEMBLES an organism that we know primarily as a fossil. So, examples would be..the coelocanth, lungfish, nautilus, ginko trees, and tuataras.
These types of organisms (and not just animals) are often abundant or at least, prominent in the fossil record but are really not too common today..

Why does this cause such ire in people who study fossils? Well, I would say that its inaccurate and hard to define. Technically EVERYTHING alive is a "living fossil". Its an ambiguous term at best. And the "good examples" are largely biased by public perception of the most frequently encountered fossil types. So, really the term doesn't mean anything! And yet the term persists...

THAT said.. people (including scientists) continue to have a fascination with "archaic" forms, which continue to live in the "recent" and the ecology that said forms exhibit.

It is through observation of how these modern analogs of extinct fossil animals interact that we hope to gain insight into ecosystems into the past. A romantic might almost say that these provide us with kind of a "window to the past"....

This is the subject of this week's blog.. A NEW paper by David Bowden and associated researchers at New Zealand's foremost oceanographic agency-NIWA (New Zealand Institute of Water and Atmosphere) focusing on "Archaic crinoid-dominated assemblages on an Antarctic Seamount) in the new issue of Deep-Sea Research II.

Stalked crinoids of course, are one of the most prominent of echinoderm "living fossils" and are the ancestors of the smaller feather stars seen today in tropical habitats. The relationship between the two can be seen in this past blog...
The paper features video captured by NIWA's towed video array operated by the R/V Tangaroa
From this area... (in the box). This includes Admiralty Seamount and several other seamounts and islands just north of the Ross Sea.
And it is on Admiralty Seamount that they see a substantial population of a NEW species of a large (~50 cm tall!) stalked crinoid !! (Family Hyocrinidae)

Note that hyocrinids, unlike other stalked crinoids are permanently attached to the bottom.
Out of a total seabed area of approximately 11,300 square meters..they observed 1,348 crinoids!
(images courtesy of David Bowden, NIWA)

Admiralty Seamount is apparently QUITE the hangout for the filter-feeding set. Also seen on the seamount in large densities is the large suspension feeding ophiuuroid Ophiocamax gigas...
(image courtesy of David Bowden, NIWA)

Among some of the more interesting interactions... one of this crinoid being devoured by Porania antarctica
(image courtesy of David Bowden, NIWA)

This is kind of interesting to me..because poraniids have one of the oldest fossil records of modern asteroids. A Triassic fossil poraniid (click here) was described here.

and here we see the sea urchin Sterechinus antarcticus also apparently munching on a crinoid.
(image courtesy of David Bowden, NIWA)

Sea urchins attacking stalked crinoids?? We've discussed that relationship here and here with regards to cidaroid sea urchins versus stalked crinoids in the deep-sea..

So, among the first observations is that with the crinoids and their echinoderm predators, we see a conspicuous LACK of big crab and fish predators. This suggests that it is a "Paleozoic Style" Ecosystem as proposed by Rich Aronson, Dan Blake and others which is present in the Antarctic.

This is an ecosystem dominated by suspension/filter feeders, particularly invertebrates. Certain types of predators, such as crabs and fishes, which are able to crush bony parts were absent-and so many MORE types of invertebrates flourished without being devoured.

This type of ecosystem occurs throughout geologic history wherever and whenever certain types of predators are absent.

One such ecosystem that included suspension feeding ophiuroids and crinoids similar to the ones on Admiralty Seamount was thought to have been present about 65 to 56 million years ago (the Palaeocene) but which again, ended due to a radiation of fishes and crustaceans around that time. Dang fishes and crabs!
How did this particular assemblage of crinoids and associated suspension feeders (without predators) originate?

Physical oceanography may be the key, as several factors (including current and food) influence how the different taxa are transported onto the seamount. As it turns out, Admiralty Seamount is close to the center of a nearby ocean gyre,

(Figure 7B from Bowden et al., 2011)

The seamount's location may be influenced by a confluence of the gyre, plus cold currents from the Ross Sea region, which likely reduces the chances of larvae (small baby forms of marine animals) arriving at the seamount from the "source populations" near shore.

And both of the species above have floating larvae that disperse via ocean currents..
(photo by Allison Gong. Not from Porania nor from an Antarctic species..its just here to help demonstrate a point.)

It may be the location of the seamount under the center of this gyre and the interaction of currents from the Ross Sea shelf that prevent the larvae of predators, such as crabs, from settling and growing out as well as ensuring the preferential settlement of suspension feeding critters like crinoids and ophiuroids.

Thus, it may be the seamount's location (which effectively isolates the crinoids and ophiuroids from predators like crabs and fish) acts as a sort of "refuge" where this "Paleozoic style" ecosystem can continue to persist!

So, how long have these crinoids been around Admiralty Seamount? Some intriguing clues present themselves.

Remember how I said that hyocrinids were anchored? Their holdfasts and attachment points are found all over the sea bottom and are the remants of past animals which have died. (these are the tiny white circles on the rock).
The authors speculate that based on the accumulation of sediments formed from crinoid body parts (ossicles) in conjunction with a conservative age per generation of about 20 years, this alone suggests that the crinoids here may have "persisted for centuries...if not millennia."

Sadly, all good things must come to an end...

The authors also note that the area of seabed at Admiralty Seamount covered by sediments formed by crinoids and these basal disks is much greater than that covered by living animals.

This, in conjunction with the observed predation-the authors speculate that the crinoid population here may already be on its way out... Talk about a gentrified neighborhood!


Jennifer Frazer said...

Way cool crinoid community in a place that is seriously in the sticks! Thanks for sharing. : )

I know "living fossils" isn't PC,
but as a colloquial term for living organisms (e.g. sharks) that still resemble their far-off ancestors, I think it's still valuable for the public.

I mean, we still use little-r reptiles in spite of the fact that term's completely invalid taxonomically as the group doesn't include birds. Why? Because it's a vivid term people really grasp and love.

Similarly, primitive in its original sense just meant "more like the first", without any of the inferiority overtones with which it's now associated. And in my opinion, that's *much* clearer to the public than the soulless "basal" or "less derived". You can watch their eyes glaze over when you use those terms; better to say primitive then qualify it connotes no inferiority than lose your audience, I say.

ChrisM said...

ha! You should hear some of my scientific colleagues complain about my occasional "semantic lapse" every so often...