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Phosphatized pre-Cambrian embryos are cool. It’s amazing that they’ve been preserved at all, and they are spectacularly gorgeous. We can learn about the evolution of development from their superficial appearance, but what we really want to do is poke around their interiors and analyze them cell by cell, something that has been hard to do without destroying them in the process. Until now.
A report in Nature (and a too short mention on a researcher’s web page) describes the application of synchrotron X-ray tomographic microscopy (SRXTM) to these fossilized embryos to resolve their internal structure. It’s a powerful tool, and it’s generating some beautiful images.
The technique basically involves blasting the sample with X-rays and reconstructing the three-dimensional structure on a computer. It’s something I wish we could do with live embryos, but I suspect they have to be mineralized before they can survive the procedure…but that means that these images are often better than anything I’ve seen in embryos that haven’t been dead for over a half billion years.
Here are some reconstructions of blastula stage embryos. You can trace the outlines of individual blastomeres (the cells that make up the embryo) and follow the stacks of cells from the center to the periphery.
(click for larger image)
Divisions between adjacent blastomeres variably preserved on the surface and within. a, b, Museum of Earth Science, Institute of Geology, Chinese Academy of Geological Sciences (MESIG) 20061. Divisions between some, but not all, blastomeres are preserved internally. c, d, MESIG 20062. Divisions between all, or nearly all, blastomeres are preserved to their full extent; the orange and yellow structures are renderings of the morphology of a column of blastomeres. e?g, Geological Museum of Peking University (GMPKU) 2204. Divisions between blastomeres are generally not preserved, and instead the core of the embryo is characterized by the centrifugal addition of diagenetic crust layers (easily distinguished from edge artefacts through their absence from some of the objects seen in the slices); orange structure represents a rendering of one of the cavities within the diagenetic infilling.
Greater detail can also be used to resolve phylogenetic disputes. This is an example of Markuelia. What is Markuelia? It’s a small wormlike creature that has been proposed to be a member of a group called the Scalidorpha, which includes some fairly obscure marine worms, like the priapulids. It’s also been suggested that it might properly belong to the Nematoidea, which includes the nematode worms. One feature that distinguishes the two groups is the arrangement of spines in the mouth and pharynx, both in number and their orientation. Scalidophorans can invert their mouth parts to capture prey—if you remember those wormlike monsters on the recent remake of King Kong, you’ll have an idea of what that’s like.
With the SRXTM, they can see all the spikes and spines and fangy bits of the fossil’s mouthparts, as you can see in this series of scans, where the colored parts are the oral and pharyngeal spines.
(click for larger image)
a?f, GMPKU 2205. Whole-mount tomographic reconstruction of Markuelia hunanensis from the Upper Cambrian of Wangcun, Hunan, Southern China. b?f are rotated 90°; from a. a and b show the whole reconstruction, with all scalids. c?f, 25 circumpharyngeal scalids arranged in rings surrounding the mouth cone (f). 25 scalids are shown in the first three rings (c), 16 scalids are shown in the first two rings (d), and 8 scalids are shown in the inner first circumoral ring (e). g, h, GMPKU2011. Tomographic reconstruction of posterior spines and a section revealing the terminal end of the digestive tract of M. hunanensis. i, j, Swedish Museum of Natural History (SMNH) X2240. Markuelia secunda from the basal part of the Pestrotsvet Formation at Dvortsy, Aldan River, Siberia. Scanning electron micrograph (i) shows four terminal spines arranged in two pairs, while the tomographic reconstruction (j) shows a third, obscured, pair.
It’s toothy like a scalidiophoran. That’s an amazing level of detail.
I think we can look forward to many more images like these—this is going to be a very popular tool for analyzing microfossils.
More can be found at Philip Donoghue’s website.
Donoghue PCJ, Bengtson S, Dong X-p, Gostling NJ, Huldtgren T, Cunningham JA, Yin C, Yue Z, Peng F, Stampanoni M (2006) Synchtrotron X-ray tomographic microscopy of fossil embryos. Nature 442:680-683.
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