A very early chordate fossil
In Nature 417, 841-844, Dominguez, Jacobson and Jefferies (1) describe an important early transitional. It is Jaekelocarpus oklahomensis, a fossil that has been known for some time, but that is now re-analysed by Dominguez et al. But before we point out why it is a transitional, let us review the basic zoology
Within animals there are two major groups: protostomes (which include insects, snails and worms) where the nervous system is organised down the belly (ventrally) and deuterostomes (which includes people) where the nervous system is organised down the back (dorsally). Now , living deuterostomes are organised into three major phyla, the echinoderms (starfish and similar), the hemichordates (a small and rather obscure phylum which is actually irrelevant to this tale) and the chordates (a phylum that includes the vertebrates: that is, all the fish, amphibians, reptiles, birds and mammals on the planet.) (2)
Living echinoderms have several diagnostic features (things which are very characteristic about them, and which can be used to distinguish them from other animals) (3).
1) They have calcite skeletons which are made of plates of calcite. Each plate is, crystallographically, a single crystal.
2) They have radial symmetry (usually 5-fold symmetry in
most star-fish) but other degrees of radial symmetry are known. Radial
symmetry means that they are circular or star shaped. Living echinoderms do
not have bi-lateral symmetry (that is with a left and right) or a
differentiated 'head' and 'tail' (unlike people and all other chordates)
Living chordates also have some diagnostic features (4).
1) They all have a notochord (a non-bony structure that runs the length of the back and provides them with some rigidity and an attachment for muscles) at some stage in their lives. In vertebrates, the notochord appears only in embryos and is replaced in the adult organism by the articulated backbone.
2) They have bi-lateral symmetry.
3) They have a differentiated 'head' and 'tail'
4) They all have pharyngeal slits at some stage in their life cycle. These slits are openings from the pharynx or throat to the outside. They are used in primitive chordates to filter food, are modified by addition of gills in fish and used for breathing, and in most land animals (like you and me) appear only in the embryo and are converted into other structures (such as the lower jaw and parts of the cardiovascular system) before birth. Living chordates do not have radial symmetry or a calcite skeleton.
OK, now let's return to Jaekelocarpus. The fossil of this creature came from Upper Carboniferous rock dated at 320 Million years to 340 Million years. It was examined using a technique called computer X-ray microtomography. X-ray tomography is now a routine technique for creating 3-D images of organic structures including the human body. The same technique is used here, but with the added feature of high magnification, as Jaekelocarpus is a tiny little thing. So we see all the details in three dimensions of the animal:
|Microtomographic reconstructions of J. oklahomensis Burke Museum, University of Washington, UWBM74305. Plates shown in life position. After ref (1)|
Jaekelocarpus is actually a creature called a mitrate, all of which are now extinct - or at least as far as we can tell. However, there are several different species of mitrate known in the fossil record. Like all mitrates, it consists of a head and a tail (a fore-, mid- and hind-tail). The head is made up of a set of ten calcite plates. The mouth is at the front of the head and there are two gaps on the head (the antero-dorsal gaps) which open into the pharyngeal area. The tail flexes downward.
The key new information in the paper is the investigation through the computer x-ray tomography of a pair of structures on the inside of the cranial skeleton which are the same in structure as the ciliated gill slits of larval tunicates (tunicates are existing marine animals that live their adult life anchored to a substrate, but are free swimming in their larval form. They are chordates). The low height of the slits and tubes (about 40µm along their length independent of the length of the structure) suggests that in life they were lined with fine cilia (hairs) which acted to pump water through the tubes.
Dominguez et al point out that : 'X-ray microtomography strengthens the comparison with gill slits by revealing the anterior boundaries of the slits for the first time and yielding dimensions consistent with the presence of a ciliary pump'
This detailed analysis showed:1) the gill slits were elongated in a front to back direction;
These are diagnostic of Jaekelocarpus being a tunicate
So what do we have here?:
So Dominguez et al conclude that this creature is a transitional between an ancestral chordate, retaining primitive features like a calcite skeleton and downwardly flexing tail, and is in the stem group of living tunicates.
For further information, go here (5), which includes animated slices through the fossil
are other amazing 3-D movies of the fossil, on Nature's website (6)
There we have it: a plain transitional carrying in itself diagnostic features of what are, today, two exclusive groups (echinoderms and chordates), positioned in the stem group of the clade of tunicates.
Postscript - 3rd December 2003
I was recently contacted by Dr Bertrand Lefebvre of UMR Biogéosciences, Université de Bourgogne who pointed out that the diagnosis of Jaekelocarpus as a stem tunicate (or calcichordate) is controversial. Other workers consider mitrates to be derived echinoderms and challenge the interpretation of the grooved structures on the internal wall of the calcite skeleton as gill slits (7). Dr Lefebvre interprets the grooves as gonoducts connecting paired gonads and not gill slits (which is as we have seen, a diagnostic for chordates).
We should also note that there is a third (and actually most long standing) hypothesis which systematically places mitrates in the stem group of echinoderms.
1. Dominguez, Jacobson and Jefferies, Paired gill slits in a fossil with a calcite skeleton, Nature 417, 841 - 844 Return to text
2. http://tolweb.org/tree?group=Deuterostomia&contgroup=Bilateria Return to text
3. http://tolweb.org/tree?group=Echinodermata&contgroup=Deuterostomia Return to text
4. http://tolweb.org/tree?group=Chordata&contgroup=Deuterostomia Return to text
5. http://digimorph.org/specimens/Jaeckelocarpus_oklahomensis/ Return to text
6. http://www.nature.com/nature/journal/v417/n6891/extref/nature00805_s1.htm#1 Return to text
7. David, Lefebvre, Mooi and Parsley, Are homalozoans echinoderms? An answer from the extraxial-axial theory, Palaeobiology 26(4), 529 - 555 (2000)
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