Today’s New York Times describes one man’s obsession with an enigmatic, living deep-sea creature that looks morphologically identical to one that lived hundreds of millions of years ago. The “creature” is described in a new paper in the journal Deep-Sea Research II (reference at bottom).
In 1976, a deep-sea camera being towed over the mid-Atlantic Ridge showed images of a strange creature (or the burrow of a strange creature): sets of tiny holes forming a hexagonal pattern. Each set was 2.5 – 7.5 cm (1 – 3 in.) across. Here’s one of those early photos:
Fig. 1. One of original 1976 towed camera photos of patterns in sediment on the east wall of the axial valley of the Mid-Atlantic Ridge (all photos from original paper)
The patterns appeared virtually identical to a 500-million-year-old fossil (almost certainly of an invertebrate) from the Cambrian that was named Paleodictyon nodosum. This species represented by this fossil was thought to have gone extinct 50 million years ago:
Fig. 2. Cast of fossil P. nodosum on the sole of an Eocene turbidite near Vienna Austria. Note partial erosion on right side, showing nodose pattern of vertical shafts.
The similar “appearance” of the ancient and living form prompted the authors to also classify the modern one as P. nodosum.
But what is this thing? It could be either the body parts of a creature itself or simply remants of an animal’s burrow. Whatever it is, it doesn’t look like anything else we know.
Rutgers biologist Peter Rona has spent the past 30 years trying to find out what P. nodosum is, and attempting to capture a specimen in deep-sea dives in the submersible vehicle ALVIN. Here’s a more recent image, with laser dots for scale; notice how the tubes form a dome that is raised above the substrate:
Fig. 3. High-definition TV image of P. nodosum at the discovery site on the Mid-Atlantic Ridge with laser beams for scale (10 cm separation). Note the shield-shaped elevation, marginal elevated rim and mote, and color (pale pink) of the area of the pattern compared with the surrounding veneer of gray calcareous lutite (image courtesy The Stephen Low Company).
So far the creature has eluded capture. On-site “erosion” studies (blowing away surface layers) have confirmed that the vertical holes are connected to each other below the surface by horizontal tunnels. Below is a reconstruction of what the creature (or its burrow) looks like in three dimensions:
Fig. 4. 3-D plasticine reconstruction of P. nodosum (or its burrow)
The New York Times has a nice graphic showing how these tubes, when raised above the substrate in a dome-like shape (as they are in P. nodosum) trap water more readily, allowing it to circulate through the system of tubes and perhaps allowing the creature to harvest edible microorganisms.
To make a long paper short, Rona et al. think that this pattern (and that of its eponymous ancestor) could represent either a “burrow consistent with interpretation of the ancient form as a trace fossil,” or “a compressed form of a hexactinellid sponge adapted to a sedimentary substrate, which means that the ancient form is a body fossil with possible affinity to the Ediacara fauna.” Paleontologist Dolph Seilacher thinks the traces are a “kind of farm where an unknown type of worm or other organism raises miro-organisms to eat.”
Is this really a “living fossil”? Well, the ancient fossils are morphologically identical to what is produced by the present-day creature. That’s why ancient and modern forms are given the same Latin name. And there is certainly a phylogenetic connection between the two, though because the forms are separated by millions of years we can’t test their reproductive compatibility to see if they’re the same biological species. But it hardly matters. “Living fossils” are recognized by morphological similarity, not genetic identity. What is exciting is the finding of a truly unique animal of unknown affinity, and its connection with a similar creature from the Cambrian. We’re a long way from knowing all the strange creatures with whom share our planet.
Paleodictyon nodosum: A living fossil on the deep-sea floor. 2009. Peter A. Rona, Adolf Seilacher, Colomban de Vargas, Andrew J. Gooday, Joan M. Bernhard, Sam Bowser, Costantino Vetriani, Carl O. Wirsen, Lauren Mullineaux, Robert Sherrell, J. Frederick Grassle, Stephen Low, Richard A. Lutz . Deep-Sea Research II. In press, available online 28 May 2009