PZ Myers. 2004 Nov 15. Parapandorina and Megasphaera. <http://pharyngula.org/index/weblog/parapandorina_and_megasphaera/>. Accessed 2008 Dec 01.

Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Monday, November 15, 2004

Parapandorina and Megasphaera

I was just browsing through some journals and bumped into this well-known paper describing the phosphatized microfossils discovered in China a few years ago. These are thought to be embryos because they are of the appropriate size, exhibit uniformity of size of individual cells, lack any differentiation, and possess enveloping membranes. In the case of Parapandorina, they also observe 2ⁿ cells in each, and the overall size, no matter how many individual cells within, is constant. The authors can't say much more than that, though—there just aren't enough traits present at these early stages to do more than classify them as developing metazoans. It's not even clear that they are necessarily bilaterians or triploblasts. But heck, they are 570 million year old embryos, and they are pretty, so I thought I'd share some of the lovely images.

Here's the abstract:

Phosphorites of the uppermost Neoproterozoic Doushantuo Formation (ca. 570 ± 20 Ma) exposed at Weng'an, South China, contain globular microfossils characterized by distinctively sculpted outer coverings and precise patterns of cell division. Forms assigned to the genera Megasphaera and Parapandorina were originally interpreted as problematica and volvocacean green algae, respectively, but newly discovered populations support their reinterpretation as the eggs and embryos of early animals. The fossils' large size (about half a millimeter in diameter), persistent enveloping membrane, precise cleavage pattern, and inferred physiology are fundamentally different from those of volvocaceans or other algae, but are expected features of animal embryos preserved in early stages of cleavage. In this light, Megasphaera is interpreted as an egg enveloped within its egg case, and Parapandorina represents equally and totally cleaving stereoblastulas. Despite their exquisite preservation, the phylogenetic placement of Megasphaera and Parapandorina cannot be resolved with confidence, due largely to the absence of recognizable adult morphologies to which they might be linked. Individual characters of Megasphaera and Parapandorina can be found in eggs and embryos of extant sponges, cnidarians, and bilaterians.

Here's Parapandorina and Megaclonophycus:

Parapandorina raphospissa (1–6) and Megaclonophycus onustus (7–12). 1, Note the diagenetic apatite crystal (arrow). SRA-1, 370; 2, SRA-1, 377; 3, SRA-1, 356; 4, SRA-1, 342; 5, SRA-1, 264; 6, photomicrograph. SRA-1, SRA-1-5, 20.4X109.4, 48–12. 7–12, Megaclonophycus onustus; 7, WJY-19E, 415; 8, K94-21, 271; 9, SRA-1, 95; 10, magnified view of the central part of 9; 11, photomicrograph. WJY-19E, WJY-19E-5, 15.3X92.8, 51–3; 12, photomicrograph. WJY-19E, WJY-19E-7, 5.6X103, 50-7. The scale bar in 1 represents 120 μm for 3, 11, 12; 75 μm for 6; 20 μm for 10;100 μm all other pictures.

And here's Megasphaera. In this case, you aren't looking at the cells, but at the surfaces of external enveloping membranes.

Megasphaera ornata n. sp. 1–2, polygonal envelope ornamentation. Note the secondary spherulitic overgrowth (arrowed) on internal body; 3–4, polygons with fractal branching; 5–6, tubercular envelope ornamentation; 7–10, tubercles with dimples at top (arrows in 10). Note the phosphatized filaments on internal body surface in 8; 11–12, paratype (HUHPC-62991), notice anastomosing ridges on envelope. 1, SRA-1, 60; 2, SRA-1, 77; 3, SRA-1, 28; 4, SRA-1, 19; 5, SRA-1, 53; 6, WJY-19E, 228; 7, SRA-1, 80; 8, SRA-1, 4; 9, WJY-19E, 218; 10. magnified view of the arrowed area in 9; 11, SRA-1, 56; 12, magnified view of the central part in 11. The scale bar in 1 represents 100 μm for 1, 2, 4, 6–9, 11; 125 μm for 3; 85 μm for 5; 5 μm for 10; 20 μm for 12.

Xiao S, Knoll AH (2000) Phosphatized animal embryos from the neoproterozoic Doushantuo formation at Weng'an, Guizhou, South China. J Paleonotology 74(5):767-788.

1. A buddy of mine who has looked those guys over along with some better images he says are now available, although not online, yet, says there's a decent chance they're cnidaria.
   
   I quote " the embryos are invaginated at both poles...looks like gastrulation at one end and probably an anemone type suction holdfast at the other".
   #: Posted by DarkSyde  on  11/15  at  11:22 PM
2. I'm blown away by the fact that I can sit here in my modest office and have at my fingertips, macro photography of the finest scientific standards, of the the most recent discoveries available at my finger tips, and that the same info is available in Ouwagadougou or on the Ross Ice Shelf. We be blessed. Thanks Dr. Myers.
   #: Posted by  on  11/16  at  12:05 PM