Friday, March 21, 2008

Ancient Impressions of Sex

Fossils can tell us a lot of things about ancient life than just what the organisms looked like. Animals are preserved in a variety of ways in the fossil record. The most common mode is for the hard parts or skeletons of animals to be preserved. Skeletons tell us about the external form of the animal but skeletal associations inform us about paleoecology and paleoenvironments. Soft body parts usually decay away quickly but under certain conditions can leave behind impressions upon a soft substrate or are even mineralized as exemplified by phosphatized animal embryos from the late Proterozoic of China. Such rare preservation may tell us about developmental aspects of ancient animals. Another mode of preservations are trace fossils which as the name suggests as traces left on the substrate by activities of animals. These can be tracks and trails recording the locomotion of animals, groove or scratch marks of animals with appendages or tubular structures which usually indicate burrowing by animals into the sediment. These give us behavioral cues about animals. Animals who are immobile can sometimes be preserved in their living positions. That can tell us a lot about life history and ecologic strategies employed by these organisms. Some recent research has found just such fossils from south Australia in their living positions and it makes an interesting story.

Evidence from molecular phylogeny and the fossil record suggests that multicellular animals (metazoans) evolved around 600 million years ago in the late Proterozoic from a colonial protist ancestor. The Choanoflagellates are taken to be the nearest living equivalent of the ancestral protist. By mid Cambrian (about 520-515 million years ago) animals had evolved complex body plans recognizable in living phyla. There is an interesting temporal control on the mode of preservation of animals during this period which is taken as a reflection of how environments were modified by the evolution of successive grades of biological complexity. One of the earliest record of metazoan body fossils is the Ediacaran biota (570 to 540 million years ago), which are mostly impressions of animals preserved as body casts. In the late Proterozoic bacterial mats covered the shallow sea floor extensively. Animals often left impressions upon these soft mats or on soft mud. The mats trapped fine sediment which filled these impressions. Bacteria also produced a bio-geochemical environment which was conducive for precipitating of calcium carbonate which lithified the fine sediment thus preserving the filled impressions as a cast. Later in the early Cambrian when the first grazers evolved, bacterial mats growing on the sea floor were eaten up. As animals evolved locomotion bioturbation would also have destroyed any delicate body impressions by stirring up sediments. This type of preservational environment disappeared from the shallow marine shelf areas. Tracks, trails and burrows became more common as the sediment interface became increasingly disrupted by animal locomotory and feeding activities. The presence of grazers led to the evolution of predation. Predator- prey evolutionary arms races led to the origin of body armor for protection in the prey and breaking past the protective armor in the predators. Hard body skeletons appear in the fossil record by early-mid Cambrian.

Back to the main theme. Science daily reports on the discovery from the Ediacaran biota of Australia of the earliest instance of preservation of sexual reproductive strategies in animals. The star fossil caught in the act is Funisia dorothea a tubular organism preserved as lithified impressions.

Droser lab, UC Riverside

Judging by the image above the fossils appears to be a lithified cast. The tubular ropy structures have a slight relief against the enclosing material suggesting a slightly higher resistance to weathering. Funisia tubes were rooted to the sea-floor much like corals and sponges. The interesting thing about the fossils is the closely packed tubes which have been interpreted as a pattern of sexual propagation that accompanies animal sexual reproduction. Living in such densely packed communities increases the chances that sperm and eggs released in the water have the best chance of meeting. Living organisms like sponges and coral often reproduce sexually producing huge numbers of offspring. The researchers believe that the dense packing of similar sized fossils is highly suggestive of a similar larval "spatfall" i.e. large number of larvae being born at one time. I wrote above that this may be the first preserved evidence of sexual propagation in animals. This is not the same as saying that this is the first or earliest instance of sex in animals. One of the authors says "In Funisia, we are very likely seeing sexual reproduction in Earth's early ecosystem -- possibly the very first instance of sexual reproduction in animals on our planet." This is highly misleading. Sex is very ancient process, preceding the evolution of animals by hundreds of millions of years. Bacteria too have a form of sex, so do plants and fungi. Single celled Eucaryotes had evolved forms of sexual reproduction much before the advent of multicellular animals like Funisia.

Ediacaran biota are morphologically diverse. Some forms are thought to represent extinct clades which left no descendants. The taxonomic affinity of the fossil Funisia dorothea is not known and may represent one such extinct group. Some fossils have been shown to represent possible stem ancestors of Cambrian clades. These include Cnidarian and possible Arthropod stem forms. Such morphologic diversity implies diverse life strategies. These appear to have evolved very early in the history of animal evolution. I am not sure I agree with the general tone of the report which suggests that this will cause something of a rethink on animal evolution. When evolutionary radiations take place in empty ecologic zones, as late Proterozoic shallow shelf areas undoubtedly were, evolutionary novelty appears relatively quickly in response to major adaptive opportunities. This pattern of early evolution of novelty is seen not just during the Ediacaran radiation but throughout the history of life whenever empty adaptive zones have been filled for example during the early-middle Cambrian and later after the Permian and Cretaceous mass extinctions. That animals show sexual reproductive strategies early in animal evolution should not come as such a huge surprise.

No comments:

Post a Comment