A novel fossil seed roils botany theory.

A novel fossil seed roils botany theory

Seeds surfaced less than 100 million years after land plants evolved from their water-borne ancestors. These nutritional nuggets - the reproductive units of the world's most highly evolved plants - arose from spore-producing predecessors that lacked sophisticated structures for collecting pollen. Because all early fossil seeds found so far look almost identical in structure, most plant biologists assume that a single group of spore-spreading plants, or Pteridophytes, led to nonflowering seed-bearing plants, the gymnosperms, says paleobotanist Lawrence C. Matten of Southern Illinois University in Carbondale.

But two european paleobotanists recently put a kink in this theory. Sifting through sediments in southern France, they found an intact fossilized seed that looks like no other from the early Carboniferous-the period about 350 million years ago during which plants apparently acquired the seed-bearing habit. Whereas other fossil seeds feature a funnel designed to trap wind-carried pollen grains, this one sports a stub where the funnel should be, reports Jean L. Galtier of Universite des Sciences et Techniques in Montpellier, France.

The newly found fossil might represent the first evidence of an intermediate stage between pteriodphytes and early seed plants. Alternatively, it may have served a primitive gymnosperm that later gave rise to a subgroup of today's gymnosperms, Galtier says. The latter interpretation, implying that gymnosperms evolved from more than one ancestral group, could cause scientists to reevaluate the "monophyletic" theory for gymnosperm origins, Matten says.

Since the seed lacks a specialized wind-pollination piece and contains some loose, air-filled tissue characteristic of water plants, the plant from which it sprang may have lived in an aquatic or marshy habitant, suggest Galtier and Nicholas P. ROwe in the July 20 NATURE. The primitive plant's seed was probably adapted to receive pollen from water instead of wind, Galtier says.

Although the fossil might reflect a prepollination stage in the plant's development, this explanation is unlikely because "the well-preserved tissue at the seed apex is clearly mature," writes Bill Chanoler at London University's Royal Holloway and Bedford new College in Egham, England, in an article accompanying the report.

Over the past decade, Galtier has found numerous fossilized plant fragments at the same site, all of which he believes stem from the same species. But only last year did he stumble acroiss the singular seed.