Showing posts with label Great Appendage. Show all posts
Showing posts with label Great Appendage. Show all posts

Tuesday, February 10, 2009

More Devonian Anomalocaris

Thanks to John and Callan I now have a copy of the paper. It’s brief but interesting.

So lets have a look at Schinderhannes bartelsi from the Lower Devonian Hunsrück Slate.

S. bartelsi is a great appendage arthropod with radial jaws. Great appendage arthropods, as the name suggests, are a group of arthropods with big frontal appendages, which are not known after the Middle Cambrian. Some, like Anomalocaris had radial jaws, but most did not. It may be that the great appendage is homologous (or directly comparable) to the frontal appendages in chelicerates. So finding a great appendage arthropod in the Lower Devonian could be something of a missing link, if indeed the great appendage and chelicerate appendage is homologous.

In this instance, it is S. bartelsi's shared characters with Anomalocaris that are really interesting.
Fig. 1. Holotype of Schinderhannes bartelsi. (A) Ventral. (B) Interpretative drawing of ventral side. l, left; r, right; A1, great appendage; A2, flaplike appendage; sp, spine; fm, flap margin; te, tergite; ta, trunk appendage. (C) Partly exposed dorsal side, horizontally mirrored. (D) Interpretative drawing of dorsal side. (E) Interpretative drawing of great appendages, combining information from the dorsal and ventral sides. (F) Radiograph. (G) Reconstruction. Scale bar, 10 mm [for (A) to (G)]. (H) Mouth-part. Scale bar, 5 mm.

Appendages
2, spiny, check.
But the reconstruction has weird cross-linkages between the two appendages. What’s with that? It looks like the appendages have been tied together. This means that the appendages have no independent movement and must work as a locked pair. I’m not convinced of this interpretation. It might be that what is overlapping is the series of up to five or six regularly spaced orthogonal spines along the length of the main spines which are attached to most of the segments of the appendage. In other words the segments of the appendage have a large spine attached to it, which would point downwards if the appendage were stretched out in front of the head. These spines in turn have a smaller set of spines coming off the leading edge, kind of like a comb, with the backbone of the comb representing the large spine coming off the appendage segment, and the set of five or six smaller spines, the teeth of the comb.

They could be used as a net to trap small prey, or, more likely given that they are short, as a means of added grip to ensure the prey doesn’t get away once in the grasp of the appendages. But the bondage reconstruction doesn’t work for me (maybe I need to more adventurous?)

Eyes
2, massive, check
They are enormous! This means S. bartelsi was most probably a predator, possibly adapted for low light conditions, close to the bottom.

Mouth
1, radial, check.
This is the most interesting feature that really links S. bartelsi to Anomalocaris. Radial jaws are so rare, and while the one in S. bartelsi has a lot less plates that the traditional Anomalocaris, it’s hard to argue against S. bartelsi as being distantly related in some way to Anomalocaris. A great appendage on it’s own? Meh. But a great appendage AND a radial jaw? That's fairly convincing.

Highly specialised swimming appendage
2, weird, check
No, wait! Hang on! What? This is just too weird. Highly specialised swimming appendages on the trunk I could accept, but not attached to the head. That just will not work. We rarely have large, functioning, head appendages posterior of the mouth. Maybe if it was some weird rigid structure coming off the posterior margin of the head shield, then yeah, but you just can’t have your highly specialised swimming head appendages AND flap them.

They are most probably highly specialised appendages on the first trunk segment. This would explain the spines along the leading edge of the appendage (exopod flaps maybe?), and would allow them to move as the trunk appendaged are set up for movement. They may be highly modified flukes, such as those on the last but one trunk segment.

There's something wierd going on with the trunk appendages, but they are not well preseved, and the paper doesn't make much of them as it isn't critical to the classification.

100 Million year gap
The authors place S. bartelsi between Anomalocaris and the other arthropods, with some justification. So where did all the round mouthed great appendage arthropods go for 100 million years?

Well, as the paper suggests, preservation plays a role here. These great appendage arthropods tend to be only lightly mineralized, and so do not preserve in normal situations. They are generally found only in areas of exceptional preservation, which would limit the numbers of fossils as there are just too few decent sites with exceptional preservation.

Also competition with the new predators on the block (cephalopods, the new-fangled vertebrates, etc), may well have pushed these arthropods to marginal areas, reducing and restricting their distribution.

It is a great pity that there is only one specimen, and an even greater pity that the quarry where S. bartelsi was found has now closed.

One way to check to see if great appendage arthropods with radial jaws are around, is to find evidence of their attacks – they leave distinctive bite marks. Has anyone surveyed the Hunsrück Slate fossils for bite marks?

Gabriele Kühl, Derek E. G. Briggs, and Jus Rust. A Great-Appendage Arthropod with a Radial Mouth from the Lower Devonian Hunsrück Slate, Germany.
Science 323, 771 (2009). DOI: 10.1126/science.1166586

P.S. I note that Derek resisted the urge to reference Anomalocaris briggsi in the paper, which is a pity 'cos then I would have got a citation in Science!

P.P.S. I've just received word that there will be paper on a new "funky" Anomalocaris coming soon. Details when it's published.