Here's a pretty arachnid—it's phylogenetically rather distant from the more familiar spiders, belonging to its own order, the Pycnogonida, and it's a marine organism, and this particular one has been dead for 425 million years, so I don't think I'm treading on Mrs Tilton's turf. Pycnogonids are primitive chelicerates related to ticks and mites, and they make their living as predators and scavengers. This one, Haliestes dasos, is the oldest sea spider known.

Haliestes dasos and the extant pycnogonid Nymphon gracile. a–j, OUM C.29571, holotype of Haliestes dasos. a, b, d–j, 'Virtual' reconstructions. a, Left anterior oblique stereo pair; b, right lateral view; c, horizontal section number 67 from serial grinding, with inset of proboscis; d, dorsal stereo pair; e, cephalosoma and anterior part of trunk, ventral stereo pair, walking legs largely removed; f, ovigers, antero-ventral view (scale as in i); g, palps, antero-dorsal view (scale as in i); h, chelicerae, dorsal view (scale as in i); i, proboscis, anterior view (ventral up); j, left posterior oblique stereo pair; k, dorsal view, Nymphon gracile, Recent, Galway Bay, Ireland (Oxford University Museum of Natural History, Zoological Collections 2003-13-0015). All scale bars 500 µm. ce, cephalosoma; ch, chelicera; e1–e3, elements of trunk end; et, eye tubercle; lp, lateral process; mo, mouth; ov, oviger; pa, palp; pb, proboscis; s1–s9, segments of appendages; se, setae; te, trunk end; tr, trunk; w1–w4, first to fourth walking legs. Note: w4 distally, on both sides, is not preserved, and the incompleteness, distally, of w2 and w3, right side, and w2, left side, represents data lost in processing. The point of colour change shown here between different structures, such as cephalosoma and chelicerae, is somewhat arbitrary.

920K Quicktime movie

This little guy was discovered in Silurian deposits in England, and one interesting feature is that it was preserved in three-dimensions. That means the fossil itself isn't particularly pretty, unfortunately—it's a stack of thin slices through a block of stone, as can be seen in the frame from a movie at right, illustrating a scan through the whole series. Click on the movie if you want to see the actual fossil itself: it's a shifting pattern of dots and blobs.

In these days of wondrous technology, though, the individual sections can be scanned and stacked and manipulated by computer to produced 3-D images of the animal, as shown in the picture above. The pictures on the left side are all stereo pairs, and if your eyes have the knack of being able to fuse pairs, you'll bee able to see Haliestes dasos pop up out of the screen at you. (Hint: click on the image to get a slightly larger picture. Stick your face up close to the screen and defocus your eyes while looking between two members of a pair, and move your head back slowly. It helped me to cover the other images with a piece of paper to reduce visual distractions.)

Siveter DJ, Sutton MD, Briggs DE, Siveter DJ (2004) A Silurian sea spider. Nature 431:978-980.