Paleontology

I've been in a downhill grind lately on these blogs. I decided to talk about something I'm actually excited about.

There have been a lot of interesting fossil work published in the last year or two. Usually, you hear in the news about this species or that fossil found. But there is rarely any context reported. What we would like to hear about is fossil ecologies: how did the world work a few hundreds of millions of years ago. We don't know that yet but there have been some interesting insights.

First, let's go to Mars.

Poor little Spirit has been dragging a non-functioning wheel for some time now. Eventually, somebody noticed that it was digging up the ground and what was underneath was pretty interesting: silica. Now, silica (the main ingredient of window glass) on earth when found in high concentrations is usually the mark of a hot spring or a fumerole. Both are locations of archaebacteria. Some have speculated that life originated on earth after being knocked over here from Mars.

Once life got started, it eventually evolved into something that could fossilize. Figuring out what life forms looked like prior to the obvious fossiles has been a problem for some time. About a year ago, it was reported that some rocks that pre-date the advent of modern day forms contained some pretty interesting fossils. It looks like these may be embryo fossil molluscs. A year later nearly to the day a different group found some very small fossils that appear to date from the early Cambrian. These are tiny crustaceans. If they were already in existence at the beginning of the Cambrian, then they must have evolved earlier. This pushes back by a fair amount the beginning of a very large and diverse group of animals and, by implication, perhaps some other groups as well. The sudden appearance of many different multicellular forms in the Cambrian is referred to as the Cambrian explosion. The discovered crustacea imply that the roots for the Cambrian may lie not in the Cambrian itself but before it.

Adding on to that, a new find published in October of 2007 of a jellyfish shows that Cnidaria was fully formed within the Cambrian-- again, suggesting the features that describe that phylum were in place earlier than thought.

Crustacea and Cnidaria represent two great flows of animal life: the Bilateria and the Radiata. Bilateria are animals that are bilaterally symmetrical and Radiata are those animals that are radially symmetrical. These are higher order abstractions of animals beyond phyla. They exemplify biological strategies for metazoan life. Since the animals appear in the Cambrian it has always been assumed the strategies predated the Cambrian. Now that the animals appear earlier than expected it could be that these strategies may have existed long before the Cambrian and the Cambrian presented an opportunity for those strategies to get traction. Once school of thought suggests it was the invention of predation that triggered the explosion. If so, given the new evidence, predation may have caused the explosion but the foundation of the new forms was already in place.

Modern phyla have their roots in the Cambrian and before. However, some phyla did not originate before the Cambrian as has been shown in some recent work published in the spring of 2007 with the discover of Orthrozanclus reburrus. O. reburrus shows a simultaneous strong resemblance to molluscs, annelids and brachiopods suggesting it might be a relative of a common ancestor of those three groups and may clarify the family tree. I'm waiting to see how the embryo mollusc find mentioned above relates to O. reburrus. It could be that the embryos found were not mollusc embryos but the fossil relative (such as O. reburrus) of the three groups.

Moving forward in time we reach the point of land occupation by life. For a long time this was thought to be by primitive plants. However, a report published in the spring of 2007 re-examined that idea. It turns out the original colonizers of the land were not plants but the fungi Prototaxites. A tree like organism that went extinct 350 mYears ago was long thought to be a plant but its microstructure now reveals it to be a fungi. Which makes a great deal of sense. May of the structural components of fungi were ideal for land colonization and, presumably, there was plenty of fodder up there for them to eat. Not to mention there was no competition. When plants got their act together, Prototaxites went extinct.

Some light has been shown on the origin of plants by the sequencing of Chlamydormonas reinhardtii-- a single celled soil dwelling algae. C. reinhardtii has many aspects of both plant and animal. It has flagella and it has a chloroplast. It can move and it can photosynthesize. And it shares in pretty close to equal measure some genes that are found in both plants and animals.

The first plants that hit the ground weren't trees, either. The Bryophyte Physcomitrella patens is a on the border between algae and plants-- presumably the sort of plant that would be best suited for land colonization. To quote John Timmer of Nobel Intent, "If you view Chlamy as lying on the border between algae and animals, you can view Bryophytes as on the border between Chlamy and trees." This moss has been sequenced and the results are interesting in that it shows how the moss is only partly adapted to life on land and what biochemical mechanisms were yet to be developed.

But trees did come later-- more than once, it turns out. The tree strategy-- base trunk, height, crown of material-- showed up in the middle Devonian as exemplified in the Gilboa trees. True trees that we might recognize didn't show up until the latter Devonian. These trees likely didn't use their crowns for photosynthesis but instead probably used them as a mechanism for seed dispersal. A later development, the addition of photosynthesis to the crown, would have outcompeted these organisms. It's not clear to me whether the Gilboa tree was a dead end or precursor to the modern tree. What is clear, however, is that the Gilboa trees were among the first true vascular plants were a component of a Paleozoic forest. They were part of the vanguard that forced poor Prototaxites out of the sky.

Moving forward to our favorite group of ancient animals, the dinosaurs, new species are being discovered right and left. A sauropodomorph (the ancestor group of both the sauropods and theropods) was found in Antarctica. Not only is the location interesting but it sheds light on the debate on sauropodomorph development. It looks like sauropodomorphs and sauropods coexisted for some time-- as did Neanderthals and human beings though for a much longer time. Fossils found in Niger were identified as belonging to a new species of Carcharodontosaurus. This guy would have been one of the biggest carnivores around: 13-14 meters long.

This year also showed the sequencing of the protein bits found in the T. Rex bones back in 2005. Collagen (expected in bone, after all) was sequenced and found to be essentially the same as modern collagen. This shouldn't be surprising since collagen is very conserved across species. But the sequencing itself is big news.

My own personal favorite animals from the saurian period are the great sea monsters: mosasaurs, pleisiosaurs, etc. These animals were just as evolved and interesting as dinosaurs but without their press agents. Pteranodons, another interesting non-dinosaur group, also have this publicity problem.

In 2006, the BBC has started reporting on a new collection of fossils. Big, big, mosasaurs were found. I haven't seen the National Geographic film but I'd like to.

And, finally, the mammals.

One of the continuing hypotheses of the mammalian radiation following the demise of the dinosaurs was that they just barely survived until the dinos died and then they took off. This turns out to be wrong. Apparently, according to sequencing evidence, mammals split between marsupials and placentals about 145 million years ago, went through a number of changes and then things got quiet. About 95 mYears ago, long before the Cretaceous extinction, mammals woke up and started changing again to the point that by 95 mYears ago most of the orders of mammals were in place. This seems to have taken only 10 mYears. Those orders survived the Cretaceous extinction but didn't do much until 15 mYears later. The spike in mammalian diversity coincides with the Paleocene-Eocene Thermal Maximum (PETM) when sea surface temperatures rose 5-8 degrees C in just a few thousand years. (Not re-creating the PETM is what we are trying to avoid by carbon conservation.)

What's odd about this study is it suggests that mammalian evolution was largely unrelated to saurian evolution. This seems strange to me-- after all, dinosaurs occupied virtually every niche except flight. It doesn't make any sense to me that there would be no sign of evolution pressure from competition at all. The scientists involved suggest that instead the driver for evolution in mammals was the global environment.

We live in paleontologically interesting times.