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It is the unfortunate fact of life that volunteer efforts are all too often derailed by other pursuits. Such is the case for last week’s Mystery Monday fossil. Nevertheless, the answer shall be forthcoming. If you have been paying attention to the Facebook feed, you will know that the fossil presented last Monday was identified. Were you able to figure it out?
This is a large, very well preserved piece of tabulate coral. Corals are colonial species that are very important in modern ecosystems. A fourth of all ocean species live within these reefs. They form the backbone of reefs that are among the richer areas of biodiversity on the planet. Billions of dollars each year are pumped into local economies across the world.
What we think of as coral is mostly the calcareous homes they form, within which the animals live. The actual animal is a tiny animal in the Phylum Cnidaria. Cnidarians are soft-bodied animals, the best known of which are the jellyfish and sea anemone. Cnidarians take two general forms. Medusae are free-floating forms like the jellyfish. Coral and sea anemones are polyps, mostly stationary, or “sessile”, forms that remain in place their entire lives. Corals, like other cnidarians, are predatory, catching their prey with tentacles armed with nematocysts, cells containing potent poisons to immobilize or kill their prey. Of course, since corals are tiny creatures themselves, they prey on even tinier prey. The tentacles surround an opening which serves as both mouth and anus, basically making the animal a living, carnivorous sack. This is not the only way corals get food though. Most modern corals also have a symbiotic relationship with single-celled algae called zooxanthellae, which provide essential nutrients for the coral in which they live. Unfortunately, when the coral gets too stressed from increasing temperatures or other causes, they tend to respond by evicting the zooxanthellae. Because the zooxanthellae are what gives corals their bright colors, this is known as coral bleaching.
While there are several different kinds of coral, most of the coral people are familiar with are the stony corals, or Scleractinia, because these are the ones that build the reefs. They are part of the larger group of corals known as Hexacorallia (at least, if you are talking to modern biologists, paleontologists often restrict Hexacorallia to scleractinians), known for often having the individual coral homes partially divided with six partitions, or septa (although you may be hard pressed to identify the three axes forming the six partitions even if they are present in that number).
The scleractinians have only been around since the Mesozoic however. They did not build the coral reefs of the later Paleozoic Era. That distinction goes to the rugose, or horn, corals and the tabulate corals, such as the example above. Tabulate corals are known for the corals being aligned in horizontal stacks. The image above should really be rotated 90 degrees to get the life position. This stacking always reminds me of apartment building, particularly cheap tenement housing, or wire mesh. According to phylogenetic studies on modern corals, it appears that the earliest scleractinians did not have zooanthellae, the symbiotic relationship evolving later, so it seems likely tabulate corals didn’t either. Tabulate corals appeared in the Ordovician Period roughly 450 million years ago. They started dying out in the Permian and finally succumbed to extinction at the end of the Permian period 252 million years ago, along with most other life on the planet. However, it is a bit misleading to say they went extinct. It is thought that the modern scleractinians that arose in the early Triassic are descended from tabulate corals, so they appear to have evolved, rather than just died out.
If you want to find corals such as this in Arkansas, one need only travel anywhere in most of the northern part of the state. The Ozark Mountains are predominantly formed from shallow marine Paleozoic rocks. Anywhere you find limestone in the Ozarks, keep your eyes peeled for samples of this type of coral. They are invertebrates, so as long as you are not collecting in a National Forest or private property without the owner’s permission, you are free to collect them.
The answer to last week’s Mystery Monday fossil was supposed to be posted last Friday, which didn’t get done. I am going to have to make some changes in the schedule or change the way I do things because I simply don’t have the time to post a new fossil and give a full discussion of it every week and do anything else. So if you have any suggestions on how you think changes would be best done, let me know. I could cut back to every two weeks; still give a fossil every week, but not go into much discussion of what it is each week; or some other possibility. Let me know your preferences.
At any rate, for the last Mystery Monday fossil, I posted this little fossil.
What we have here is a little goniatite ammonoid that has been pyritized, meaning that the shell has been replaced with pyrite. This type of fossilization is pretty common. As the bacteria eat the organism, some of them will release sulfur, which then combines with the hydrogen in the water to form hydrogen sulfide. When it precipitates out of the water, it usually does so as pyrite. In some cases, like in this one, the pyrite crystals can replace the organism so well that it makes a detailed copy of the original. For obvious reasons, this type of fossilization is called replacement, in which the original oranism is replaced with a mineral, be it calcium carbonate, iron, opal, quartz, or in this case, pyrite.
So what are goniatite ammonoids? Ammonoids are part of the group (often called phylum, but for various reasons the specific rank of the group is often no longer used) Mollusca; which includes snails, (gastropods), clams (Bivalvia, meaning two shells, or less commonly, Pelecypoda, meaning hatchet foot), and the Cephalopods.
Cephalopods include the squids and octopuses, as well as the Nautilus, which is what concerns us here. If you aren’t familiar with a nautilus, think of a squid inside a spiral shell. Squids used to have shells, either long, straight ones or curved and coiled ones. The only one left of these shelled squid is the nautilus. However, you can still see the remnant of the shell in an internal structure called a squid pen, or in the case of cuttlefish, the cuttlebone. In either case, they are the last vestiges of the external shells we see in the nautilus and the ammonoids.
During the Paleozoic and Mesozoic Eras, ammonoids were much more common and much more diverse. During the Jurassic and Cretaceous Periods during the Mesozoic, the type of ammonoids that were most common were the more familiar fossils known as ammonites. Goniatites were much earlier and lived during the Paleozoic.
How does one tell the difference between the different types of ammonoids? Look at the internal partitions. These partitions, called septae, separate the chambers within the shell. As the animal grows, it adds material to the edge of the shell, making it larger and larger at that end. Once the shell gets big enough, the animal will create a new partition in the back, separating the current body chamber from the earlier, smaller one. A small hole is left in the septae so the siphuncle, a thin tube, can pass through, connecting all the previous chambers. The siphuncle could then be used to pump water or gas in and out of the chambers so they could be used as ballast, allowing fine control of their buoyancy.
The septae in the nautiloids (the group of ammonoids containing the modern nautilus) are all very smooth, forming a nice curve. The nautiloid septae curve inwards, whereas the ammonoids curve outwards to some extent. Ammonoid septae are also much more complex. Goniatites, the earlier forms, had simple wavy septae. Ceratitic ammonoids created septae in which the waves were more jagged, with what often looks like little saw-toothed crenulations. The later ammonites (many people call all of them ammonites, but this term more properly only refers to the more derived subset) had very complex septae, showing several smaller wave patterns overlaid upon the larger wave.
Ammonoids were very common in the Paleozoic and Mesozoic and were found throughout the oceans of the time. They typically lived in shallow marine environments all over the world. They also evolved rapidly, so new species tended to appear and disappear on a fairly regular basis. This abundance, diversity, and rapid turnover make them prime index fossils. Index fossils are those fossils which are useful for dating the rock layer and correlating the layer from one spot to another. Using index fossils allows us to piece together a complete sequence of events even if there is no one place that has the entire sequence of rocks preserved. As a result, any fossils that can be used as index fossils become very important to people trying to figure out the history of life on earth, such as this little ammonoid. If you want to find one for yourself, look in almost any of the limestone or chert formations in the Ozark mountains. There are plenty.
The creature in the picture is a representative of an early shark called Falcatus. It was a very strange shark, in which the males had a forward-pointing spike on its head. This is one of the earliest examples of sexual dimorphism in the fossil record. Sexual dimorphism is when the male and females look different. Humans are sexually dimorphic in a variety of ways, but the classic example is the peacock with its extravagant tail. They are so dimorphic that many people do not even realize the term peacock only applies to the male of the peafowl species. The females are called peahens.
Falcatus was chosen this week because of an interesting shark fossil found in 325 million year old limestone near Leslie, AR. The fossil was recently published and got a lot of press. Pictures of the fossil itself show what appears to be little more than a couple of lumpy concretions stuck together. There is little there that resembles anything like a shark. At least, until you look inside. A concretion is an inorganic structure consisting of layers of minerals that have been precipitated around a central core, much like a rocky onion. Oftentimes, a fossil lies at that core and served as the basis upon which the mineral precipitation got started. So one never knows what one will find cracking open a concretion. It may be nothing more than concentric mineral bands, or it may be an exquisite fossil preserved for the ages. In this case, the scientists got lucky. Not only was there an exquisite fossil, but one in which few had ever seen before: the skeletal structure of an early shark’s gill basket.
The researchers named the fossil Ozarcus mapesae, for the Ozarks in which it was found and for Royal and Gene Mapes, geology professors at Ohio University. The Mapes are also paleontologists and have collected a large number of fossils, one of which happened to be this curious-looking concretion. Small teeth on the exterior pointed to possibly more interesting material inside, so they CT-scanned it, which revealed the remains of the head of the shark. But to get better detail, they had to use a synchotron. This technology is new and expensive enough that it was not possible to look at the fossil this way until recently (a good example of why we preserve fossils in museums, you never know when future technology will allow examination in ways never thought of before).
The gill basket goes by many names, the branchial basket, branchial arches, gill arches, etc. but they all refer to the skeletal supports for the gills. To the shark, they are important for holding up the gills and forming the path for water to flow over the gills so the shark can breathe. For us, they are important because the arches evolved into a variety of structures, primarily the jaws and hyoid, a small bone in the throat to which the back of the tongue is attached. Because of the importance of jaws, the evolution of those structures has been a big topic of interest. The traditional view has been that the first jawed fish, the placoderms, evolved into the early precursors of the sharks, which then evolved into the early bony fish, the osteichthyans. The sharks then were expected to have a more primitive structure than bony fish. This story makes sense, given that the order of appearance in the fossil record pretty much matches what we would expect and the skeletal structures look more primitive in sharks than bony fish. Of course, despite the common view that sharks are relics of a bygone age, they have had over 400 million years to evolve after they split off from bony fish. How likely is it that they would have retained such ancestral characteristics for all this time? To answer that question, we can look at the fossil record to tell us what they were like at the earliest stages.
The big problem with examining the fossil record of sharks is that sharks are chondricthyans. They do not make bone. Other than the teeth, the skeleton is supported by a simpler set of calcium phosphate crystals. Unlike bone, which has a very structured arrangement of crystals and connective tissue, the bones in sharks are made of cartilage and a haphazard set of disorganized crystals, which fall apart shortly after the animal dies. Thus, finding any fossils of sharks that contain more than the teeth is extremely rare. Finding ones in which everything is still in place is almost impossible. Fortunately, over a long enough period of time, even the almost impossible will happen eventually. That is the thing with large numbers and vast amounts of time, our perception of what is unlikely doesn’t really work. For instance, given a 1 in a million chance that a tweet on Twitter will have something requiring the security team to deal with, that still gives them 500 tweets every single day. That “almost impossible” fossil was found with Ozarcus. This fossil provided our first look at what the throat of a primitive shark actually looked like.
Let’s have a bit of background to cover what we have known of early jaw evolution up to this point. Placoderms, armored fish from the early Paleozoic Era, were the first animals with jaws. The jaws themselves appear to develop from the first gill arch, according to a lot of embryological studies on modern animals.
The studies haven’t really answered where the bone came from though. In placoderms, it is pretty clear the armor came from modifications of the dermis, the basal layer of the skin. But they also have internal bone forming their skeleton. Modern sharks have no bone other than teeth and bony fish have jaws made from that dermal bone. In addition to the origins of the bone, there is the matter of how the bones are attached to the skull. The upper jaw is formed by a embryological structure called the palatoquadrate (the top part of the first gill arch), so named because bones called the palatine and quadrate form from it. The bottom jaw forms from what is called Meckel’s cartilage (the bottom part of the first gill arch). In modern fish, the palatoquadrate is braced against the skull only at the front, with the back unattached. The jaw joint itself is attached to a bone called the hyomandibular, which forms from the second gill arch. The hyomandibular acts like a swinging pivot, allowing the jaw to open very wide. When the jaw joint gets pulled forward, the back of the upper jaw can move down, using the point where the front part is braced as the pivot point. Sharks take this to an extreme, not bracing the upper jaw on anything at all, with the jaws attached solely by the hyomandibular bone (the “hyostylic” joint) allowing both the upper and lower jaw to move forward when they open their mouths. So when you see that shark opening its jaws and it looks like they are coming right out at you, they really are. For a great example of a hyostylic jaw joint, check out the goblin shark.
Unfortunately, when we look at the earliest fossil sharks and bony fish, both of them show a jaw in which the upper jaw is braced against the skull in both front and back (the “amphistylic” joint). So it doesn’t tell us much about how the sharks fit into the sequence. One might say that it seems logical to think the bony fish came first, loosening the jaw in the back and then the chondrichthyans took this one step farther. But surely, others might say, the fact that bony osteichthyans have a more advanced bony skeletal structure means they would have to have come later, right? Here again, the fossil record doesn’t help here because the earliest representatives of both groups appear close enough in time that it cannot be strongly stated which came first.
That is where things were, until 2013, when the fossil record started answering these questions. A placoderm called Entelognathus was published in 2013. This fossil was of great interest because it showed the structure of the jaws in great detail. Entelognathus had a jaw that looked very much like an osteichthyan. This fossil was 419 million years old, so it was likely too late to be ancestral to either chondrichthyans or osteichthyans, but old enough to be very close to the ancestral form. What Entelognathus tells us is that the bones forming the jaws in placoderms was already like those seen in modern bony fish, indicating that sharks would have started out with bone, but lost it during their early evolution. Of course, since sharks don’t have bone, this was hard to demonstrate on sharks, so the question was still unsettled.
Outside of the actual jaws themselves, there are all the other support structures around the jaw, such as the hyoid bone. Many of these structures are formed from the next few gill arches (earlier jawless fish had at least seven gill arches, so using a few to make the jaws and throat still leaves plenty for gills). The skeletal structures in bony fish that support the gill arches form a fairly simple chevron, or wide v shape, whereas the sharks have a slightly more complicated structure. But if finding fossils of shark skeletal structures is rare, finding one with tiny gill arch supports still in position is almost impossible. There is where Ozarcus comes in, because it is here that the almost impossible becomes reality. Once the researchers were able to see inside the fossil with the synchotron, they were able to see preserved gill arch supports in position. That position resembled the simple chevron shape of modern osteichthyans.
Thus, between Entelognathus and Ozarcus, we can confidently assess the development of jaws as having started in placoderms with primitive, osteichthyan jaws. We have evidence of the bony origins of the jaws from Entelognathus and evidence from the gill arches from Ozarcus, so we now have plenty of evidence to strongly support the claim that the chondrichthyans are not in the evolutionary pathway to modern jaws at all. They are an offshoot that actually lost bone to form a more flexible skeleton for some reason. Whether it was mechanical advantage for their lifestyle, such as increasing flexibility, or a reduction in mineral storage, who can say.
This new view of jaw evolution demonstrates that the common view of sharks as being evolutionary relics is wrong. Far from being primitive, they evolved throughout the millenia to the modern sharks we see today, changing the shape and development of their jaws and throat as they evolved from the primitive condition seen in early bony fish into the efficient predators of today. But then, they have been separated from the lineage that led through bony fish up through the early tetrapods all the way to us for over 400 million years. Why would anyone think that in all that time, they stayed still evolutionarily? Even if they look much more similar to their ancestors than we do, they have evolved too. Nothing stays still.
Were you able to solve Monday’s mystery fossil? They aren’t little poop balls, nor are they clams, although they are often mistaken for them.
This photo can be found at the Arkansas Geological Survey website under “Brachiopod.” They look a lot like clams. Brachiopods, often called lamp shells, have two shells and live in shallow marine environments just like clams and the occupy the same niche, feeding on organics filtered from the water. But unlike clams, which are molluscs, just like snails and squid, brachiopods are lophophorates, most closely related to bryozoans, the “moss animals.”.
So what is a lophophorate? Lophophorate means “crest or tuft bearer, so named for their feeding apparatus called a lophophore, which is shaped like a roughly circular or semi-circular ring of tentacles. These tentacles lazily wave through the water passing through the lophophore, catching small particles of food suspended in the currents. Thus, everything in this group are what is known as suspension feeders. These lophophores serve not only to collect food, but for gas exchange as well. In addition, the animals are headless, with the lophophore surrounding the mouth. The food enters the mouth and passes through the digestive tract, which makes a U-turn and dumps out what it can’t digest just outside the ring of tentacles. Clams do essentially the same thing, only they use an entirely different apparatus to do so.
Clams attach themselves to surfaces by secreting a collection of what are called byssal threads. Most brachiopods, on the other hand, form a pedicle, a stalk that holds them in place. Some do not make pedicles, instead just gluing themselves down directly onto the rock.
Another difference that can usually be seen between clams and brachiopods is the symmetry of their shells. Brachiopods are symmetrical from side to side, their left side is the same as their right side. Clams follow a different pattern. They usually have two identical shells, but the shells themselves are not symmetrical. This is not always true though. The Cretaceous oyster, Exogyra ponderosa, has an huge, thick shell on one side and a thin lid for a shell on the other. But as a general rule, this usually works. Another difference that is sometimes stated is that brachiopods use their muscles to close their shells, while clams use their muscles to open their shells, closing them by the use of ligaments; thus making brachiopods more susceptible to predators. This, however, is not true. In truth, brachiopods use their muscles to both open and close their shells. Clams have large adductor muscles that function to close the shells and they have ligaments that open them when the muscles relax.
Brachiopods are quite diverse, with many different types. They range in size from less than a dime to almost 40 cm (15″). There are two general groups, the Articulates, which have toothed hinges holding the shells together, and the Inarticulates, which do not have teeth, so they fall apart easily after death. Probably the most commonly found in Arkansas are spirifers, known for being somewhat wing-shaped , with a prominent sulcus, or depression in the center. Many brachiopods prefer solid substrates, like rock, others were adapted for softer substrates like sand or mud. Productids, like the ones in our mystery fossil, often grew spines, which helped secure them to muddy surfaces. Others, like strophomenid brachiopods, handled muddy substrates by developing large, very flat shells, which floated on the mud like a snowshoe. Still others, like the modern-day lingulids, developed long pedicles, allowing them to burrow down into the sediment.
Brachiopods have been around since at least the Cambrian, over 520 million years ago. They were most abundant in the Paleozoic Era, but suffered greatly during the Permo-triassic extinction event. They recovered to some extent, but never reached their previous abundance due to the appearance of clams, which began taking over some of the spaces they occupied. Nevertheless, there are still several different kinds in the modern ocean and can often be seen clinging to rocks near shore or buried in the sand. In Arkansas, you won’t find any living specimens, but you can find numerous fossil brachiopods in the Paleozoic rocks throughout the Ozarks and Boston Mountains, even in some places of the Arkansas Valley. Stop by any outcrop along Highway 65 between Conway and the north edge of the state, particularly limestone outcrops, and you are likely to find some. You can find a few in the Bigfork Chert in the Ouachitas, but they are not nearly so common as they are farther north.
Between classes and school appearances, I have not had the time to write up as complete a description as I would like, so I will do a more complete description of the fossil later. But for now, did any of you think you saw crinoids in the face? If you did, you are correct! This photo was originally published on the Arkansas Geological Survey‘s blog. If you haven’t checked them out, I encourage you to do so.
Crinoids are perhaps the most common fossil found in Arkansas. They can be found in many of the Paleozoic rocks in northern Arkansas in the Ozarks and Ouachitas, although they are most common in the Mississippian age limestones of the Ozarks. All those white rocks along Highway 65 towards Leslie and Marshall are good candidates, although watch out for cars along the highway, please.
Crinoids are often called sea lilies because of their resemblance to plants, but they are actually animals that are related to sea urchins and starfish, so they are far more closely related to you than to any plant. Even though they lived in shallow marine environments during the Paleozoic Era, you can still find them today in deep water along what is called the continental slope. If you swim out into the deep water a long way away from shore and you get to the edge of the continent, you will see a cliff or steep slope descending all the way down to the abyss of the absolute bottom of the ocean. Congratulations, you have reached the continental slope and the last refuge of the crinoids.
Our tour of Arkansas fossils and geology should begin, like any tour, at the beginning. The oldest rocks found in Arkansas in which fossils may be found were formed in the Cambrian Period, the earliest part of the Paleozoic Era. When the Paleozoic Era was first named, it began with the rocks containing the oldest known fossils. We now know of fossils far older than that. Nevertheless, it marks a good starting point for rocks in which fossils become commonly found and are easily recognizable. So while Arkansas does not have the earliest fossils, we do have fossils dating back through most of the history of life once hard parts developed.
The Cambrian Period started about 540 million years ago and lasted until 485 million years ago. During that time, while the land was mostly barren, the seas were full of life. Much of what people know about the Cambrian comes from the Burgess Shale in Canada, possibly the best known example of a lagerstätten, a fossil site rich in either fossil diversity or exceptional preservation, of which the Burgess Shale has both. From the Burgess Shale and other localities, we know that the Cambrian saw the rise of most of the major groups of animals we see today. In addition to the comb jellies, sponges, algae and anemones, brachiopods and bristle worms, velvet worms and crinoids; the Cambrian also arthropods of several kinds, most in particular the trilobites, the first chordates like Pikaia, and bizarre creatures like Anomalocaris and Hallucigenia.
The rise of such a diversity of animal life during the Cambrian has been termed the Cambrian Explosion, leading some people to assume it appeared suddenly and without precedent. In truth, the Cambrian “explosion” took tens of millions of years and was preceded by a diverse fauna known as the Ediacaran or Vendian fauna, which first appeared almost 100 million years earlier. The end of the “Garden of Ediacara” and the rise of the Cambrian fauna is thought to have come about due to the evolution of the first predators, necessitating hard shells for defense and hard claws and teeth to kill prey.
The only place in Arkansas to find Cambrian rocks is in the Collier Shale, which was formed in the Cambrian through the Lower Ordovician.
Outcrops for the Collier Shale are limited to a small set of ridges in the Ouachita Mountains, within Montgomery County between Caddo Gap and Mt. Ida, just to the east of state Highway 27. However, most of this area is part of the Ouachita National forest and is ILLEGAL TO COLLECT anything without a permit.
The Collier Shale is a large unit at least 1000 feet thick formed mostly of gray to black clay shale that was intensely crumpled during the formation of the Ouachitas. Interspersed within the shale are thin layers of black chert, which together indicate a deep water environment. However, there are also thin layers of dark gray to black limestone, which contain pebbles of chert, limestone, quartz, and even sandstone. It is thought that these layers initially formed in shallower water on the continental shelf before some event caused them to slide off the continental slope into the abyss.
The Collier Shale is not known for abundant fossils, but it does have some. In the Cambrian section of the formation, several genera of trilobites have been found, chiefly of the groups known as Asaphida and Ptychopariida. For more information on trilobites and the different types, try the Fossilmuseum.net and Trilobites.info websites. The trilobite genera found in the Collier Shale have been from what is known as the Elvinia and Taenicephalus Zones. These are specific groups of trilobite genera that, when found together, allow the age of the rocks to be determined using correlative dating. These groups, or assemblages, of genera have been found in other parts of the world in rocks that have been able to be dated using rigorous and independent methods, such as radiometric dating. We know that rocks elsewhere in the world containing these fossils are roughly between 490 and 500 million years old, indicating the rocks forming this part of the Collier Shale are the same age. This conclusion is supported by fossils in the rock units overlying this part of the Collier matching those found in rock units over similar rock units of known age elsewhere. The trilobites in the Collier are found in the lower part of the formation. The upper part of the Collier contains fossils known as conodonts, but they are Ordovician in age and will be discussed later.
Trilobite images from www.fossilmuseum.net and www.trilobites.info. The Cambrian painting by Miller can be found at http://paleobiology.si.edu/burgess/cambrianWorld.html, along with more Cambrian information. The map of the Collier Shale can be found at www.geology.ar.gov and the continental shelf image is from kids.britannica.com.
Hart, W. D., J. H. Stitt, S. R. Hohensee, and R. L. Ethington. 1987. Geological implications of Late Cambrian trilobites from the Collier Shale, Jessieville area, Arkansas. Geology 15:447–450.
Hohensee, S. R.; Stitt, J. H. 1989. Redeposited Elvinia zone Upper Cambrian trilobites from the Collier Shale, Ouachita Mountains, west-central Arkansas. Journal of Paleontology 63(6): 857-879
Loch, J.D. and J.F. Taylor. 2004. New trilobite taxa from Upper Cambrian microbial reefs in the central Appalachians. Journal of Paleontology 78(3):591-602. Online publication date: 1-May-2004.
UPDATE: I thought I would add a little more information about the “Cambrian Explosion,” or as Dr. Donald Prothero calls it, the “Cambrian slow fuse.” The reason for this is because of how long it really took for multicellular life to develop. We have evidence for the earliest life going back over 3.5 billion years, but the earliest agreed upon multicellular life appeared in the Ediacaran fauna (Grypania is a possible multicellular organism dating back 2.1 billion years, but may not be a true multicellular organism and really a colonial organism). The diagram to the right (click to enlarge) is from Prothero’s book, Evolution: What the fossils Say and Why it Matters, and reproduced on a review he wrote of another book. In the diagram, he shows the Ediacaran as starting about 600 million years ago, but now most researchers peg that to about 635 million years ago, so the slow fuse is actually even longer than he shows. The Collier Shale in Arkansas is in the late Cambrian, so as you can see, several other groups are already present. The fact that we have thus far only found trilobites means that we may yet find more diverse types of fossils, so keep looking (and if you find anything, let us know)!