Mystery Monday, Tax week edition

Mystery Monday, Tax week edition

For today’s Mystery Monday fossil, we are looking at an animal who is being celebrated this week. No, it’s not a rabbit, Easter is next week. For scale, the bar on top is marked in millimeters, so we are looking at a wee little creature. If getting your taxes done is driving you crazy, spend some nice relaxing time with a fossil.

Fossil Friday

On Monday the fossil shown below was put up. How did you do? We got some very good suggestions about it possibly being a horn or a tooth. It isn’t a horn, not exactly. It does also bear a remarkable resemblance to some spinosaur teeth I’ve seen, which was one of the guesses. But it’s not a tooth either. Congratulations to Laurenwritesscience, who really knows her fossils for correctly identifying it as a horn coral.

rugose coralModern day coral are pretty much all in the group Scleractinia, meaning stony rays, known as the stony corals because they make an external skeleton out of aragonite, a form of calcium carbonate, very similar to calcite. They are the major reef builders in modern oceans. But it wasn’t always that way. Before scleractinians, there were tabulate corals and rugose, or wrinkly corals. Rugose corals, like modern corals formed both large colonies and solitary structures. The solitary rugose corals, like this one, are called horn corals, obviously for their commonly stated similarity to horns.

Rugose corals first appeared in the Ordovician Period roughly 47o million years ago, give or take 20 million years.  They became important reef builders in the Devonian Period and remained a highly visible presence until the end of the Permian about 250 million years ago. They, like almost everything else, succumbed during the Great Dying, as the mass extinction of that time is called. As a result, if you want to find them in Arkansas, you should try the marine limestones in northern Arkansas, throughout the Ozarks. The best place to find them are the Mississippian rocks, such as the St. Joe, Boone, and Pitkin Formations.

Life cycle of a rugose coral. Picture from the Kansas Geological Survey

Life cycle of a rugose coral. Picture from the Kansas Geological Survey

Rugose corals, like all corals, are more than just the stony skeleton most people think of when they see coral. Inside, the animals themselves grow as polyps sitting, in the case of horn corals, a stony cup called a calyx, which they constantly build up as they grow. When alive, the pointy end went down, permanently attached to the surface upon which it grew. Inside the calyx, they grow septae, little walls used to provide structural support for the skeleton and to hang themselves on (while this may cause one to picture a very depressed polyp, a better visual image would be hanging pictures or cabinets on a wall). Rugose corals typically grew six main septae, forming six compartments. Four of the compartments were further subdivided by minor septae. All of these septae radiated from a central axis, or rod supporting the whole structure. The four main compartments is one reason why they are sometimes called Tetracorallia.

Growth of septae in rugose and hexacoral. Paleo.cortland.edu.

Growth of septae in rugose and hexacoral. Paleo.cortland.edu.

This is one of the key characteristics that separate them from modern day coral, which are members of a group called Hexacorallia, noted for having six main septae and six main compartments, all of which are furhter subdivided with minor septae, thus having six-fold symmetry (you can draw six lines through the center of the organism in which one side is a mirror image of the other). Hexacorallia also include the sea anemones, also noted for having six lines of symmetry. The Rugosa, on the other hand, due to the four compartments, often exhibited only bilateral symmetry, meaning there was only one plane you could draw to find a mirror image.

Coral polyp anatomy. Fossilmuseum.net.

Coral polyp anatomy. Fossilmuseum.net.

The polyp forming the living organism was a tiny creature with a simple sac-like digestive tract, meaning there is only one opening. Food passes into the mouth and waste comes right back out the same way. This is the typical pattern seen in almost all of the Cnidarians, the group in which corals are a part, along with animals like the jellyfish. Like the jellyfish, coral polyps have nematocysts, stinging cells they use to capture prey. Surrounding the mouth, and where one would find the nematocysts, are a ring of tentacles, forming a flower appearance, the reason the group containing all corals is called Anthozoa, the flower animals. A lot of group names and you might be getting lost, so I’ll summarize the names: Horn corals are a part of the Rugosa, or rugose corals, also called Tetracorallia, which are a part of Anthozoa, along with all the other corals and sea anemones, which are a part of Cnidaria, which also includes the jellyfish among others. The modern corals are mostly scleractinians, or stony corals, which are a part of Hexacorallia, which are Anthozoans and thus Cnidarians (which you may also see referred to as coelenterates). Like all members of the Anthozoa, rugose corals almost assuredly reproduced asexually by budding or sexually by releasing both sperm and eggs. In many corals, most of the polyps in an area will do this at or near the same time, called broadcast spawning, creating a massive cloud of eggs and sperm floating around the reef. I have read this is a fascinating sight, if rather disturbing to some. Why do they do this? Because everything else in the reef considers them good eating, so the only way to ensure that some escape long enough to survive and grow is to simply overwhelm them with sheer numbers. This idea is often called predator satiation. It also gets around the fact that coral can’t move, they are the ultimate homebodies. The only hope they have to meet a good mate is to just put the seeds of their offspring into the wind, or in this case, currents, and hope for the best.

Most corals do not depend just on what they can capture or filter out from the water. They also have a mutualistic relationship with photosynthetic dinoflagelates and zooanthellae, or single celled algae. They algae get a safe place to live and nutrients, while the coral gets the products of the photosynthesis, such as oxygen, sugars and amino acids. Normally the coral don’t depend upon the extra energy derived from the photosynthesis, but when times are tough, the added supply can see them through, allowing both the coral and the algae to survive longer. The mutualism certainly provides a much more abundant resource supply for the coral, allowing them to grow more efficiently. We don’t know if rugose coral did this, but it is reasonable to assume they did for now. Of course, this means that corals who use this relationship are restricted to shallow, clear waters conducive to the growth of algae. We do know that they formed close relationships with stromatoporoid sponges. The sponges formed the backbone of the reefs, with the coral growing within the sponge reefs. What sort of association beyond that is not known though.

Life in the Devonian seas. UMich Mus. Natural History.

Life in the Devonian seas. UMich Mus. Natural History.

 

 

Mystery Monday

Mystery Monday

Time for another Mystery Monday fossil! See if you can figure out this wrinkly rock.

Fossil Friday, Having a Blast

It’s Friday, which, along with the anticipations of the weekend, means it’s time for the reveal of this week’s Mystery Monday fossil. We’ve had guesses of starfish, aka sea star, and sea urchins. Both are close. Were you able to guess it?

Image

This fossil is of Pentremites, a echinoderm in the group Blastoidea, so a relative of both sea stars and sea urchins. Like them, you would have found them in shallow marine communities in fairly clear water, if they still lived. Blastoids are what you might get if you crossed a crinoid (another echinoderm) with a sea urchin, but we’ll get to that. This particular image was taken by Dr. Richard Pasilk, of the Humboldt State University Natural History Museum. You can find it and many more fascinating images at the Paleoportal.org fossil galleries.

Echinoderms, or “spiny skin,” have been referred to as walking castles because most of them form plates and spines of calcium carbonate that lock together, forming a mobile fortress. Echinoderms include starfish, sea urchins, crinoids, and holothuroids, or sea cucumbers. Echinoderms are known for having tube feet, a part of their water vascular system. If anyone has seen hydraulic mechanical systems, you know how these work, by pumping water in and out of tubes to change the water pressure, allowing the tubes to extend or contract. They don’t have much in the way of nervous or sensory systems, although sea stars do have rudimentary eyes allowing them to see, albeit very poorly. At least some sea stars can turn their stomachs inside out to eat, and sea cucumbers can basically eviscerate themselves, ejecting their guts through their anus, to gross out potential predators. Sea cucumber poop is also very important for coral reefs, so be a hero, save the sea cucumber, save the ocean.

Image

Blastoids grew on long stalks like crinoids formed of many flat disks, but instead of having fairly disordered plates that formed a rough ball-shaped shell called a theca, the plates forming the blastoid thecae were nicely ordered, arranged in a shell that many have thought resembled a hickory nut. This ordered, integrated theca is much more similar to the echinoid sea urchins than it is the crinoids. It has the advantage for fossil hunters that it held together better, meaning that they are much easier to find than crinoid thecae, which pretty much scattered across the sea floor as unidentifiable calcite crystals as soon as the animal died, unless they were killed by being buried.

Image

Reconstruction of Asteroblastus volborthi by Ernst Haeckel, 1899-1904.

The mouth is located at the top, surrounded by five grooves called ambulacra. Coming off the ambulacra were a series of feathery appendages called brachioles, which would filter particles from the water, much like the feathery arms of the crinoids. Between the start of each ambulacra sat an opening. Four of them led to the respiratory system, consisting of complexly folded structures called hydrospires. Loosely fold a piece of paper a couple of times, then roll it up and you will get an idea what it looked like. Water would flow from the brachioles into your paper hydrospire between the edges of the paper and out the top of the tube. The other one was the anus, so the digestive system was U-shaped, with the mouth and anus adjacent to each other.

Image

The fossil record of echinoderms is extensive, starting in the Cambrian over 540 million years and possibly as far back as the Ediacaran around 600 million years ago. The fossil record of the blastoids is somewhat debated. Whereas some sources say they originated in the Ordovician, most put the oldest confirmed blastoid in the Silurian, roughly 425 million years ago. They became abundant in the Mississippian Period and were persistent members of a diverse shallow marine community until they died out by the end of the Permian Period a little over 250 million years ago, along with most of the world in “The Great Dying.” In Arkansas, as in the rest of North America, blastoids were common and diverse in the Mississippian Period, also known as the Lower Carboniferous Period, although they became rare in the Pennsylvanian, the Upper Carboniferous Period. Arkansas has some of the only Pennsylvanian blastoids in North America.

Look for rocks like this. Pitkin Limestone along Hway 65. www.geology.ar.gov

Look for rocks like this. Pitkin Limestone along Hway 65. http://www.geology.ar.gov

If you want to look for them in Arkansas, the best places to go would be the Mississippian age limestones in the Ozark Plateau, such as the Pitkin Limestone and the Boone Formation, and the early Pennsylvanian age limestones, such as the Brentwood Limestone of the Bloyd Formation. Follow Highway 65 north towards Leslie and Marshall and stop at any roadcut through the Ozarks showing whitish rocks and you stand a decent chance of finding them. Just don’t collect in the National Forests and watch the traffic.

 

Mystery Monday

Mystery Monday

I hope you had a blast over Spring Break. Here is a fossil you might find in Northwest Arkansas. See if you can figure out what it is.

Fossil Friday, Written in Stone

On Monday, I said that the mystery fossil this week takes us back to the beginning and ties us to the present. I also said the answer was written in stone. Could you figure it out? Katharyn D. was the first to get it this week.

Image

 

The picture shows a graptolite. This particular one is likely a species of Cyrtograptus. Most specimens of this type of graptolite are from Europe, they do appear in Canada and parts of the United States, including Oklahoma, according to Fossilworks.org. Did it appear in Arkansas? We don’t know. Other graptolites did, but then the book of Arkansas graptolites has not yet been written, so the true diversity of graptolites within the state is not really known.

Acorn worm. Eol.org.

Acorn worm. Eol.org.

So what are graptolites? The name means  “written in stone” because they reminded people of hierogylphs or petrographs, writing or pictures scratched into stone. The first graptolites I remember seeing in the Quachita Mountains reminded me of nothing more than pencil scratches. Others, better preserved, look like saw blades or little tubes. For a long time, no one really knew what to make of them, regarding them as cnidarians, or plants, or even inorganic mineral formations. But with the advent of electron microscopy, most workers have come to the conclusion they are actually members of the group Hemichordata. This conclusion puts them at the very beginnings of all vertebrates. Vertebrates evolved from a group called urochordates, the first animals with a stiffened rod for support, an ancestral spine. Today, urochordates are tiny animals called tunicates, or sea squirts. But even before these animals evolved, there were the hemichordates, meaning “half-chordate.” They share branchial opening, or “gill slits,” a collar-like pharynx, and the beginnings of a notochord, called a stomochord. The main hemichordates alive today are called enteropneusts, or acron worms. If you go any earlier than this, you find yourself in echinoderms.

Graptolites from the Womble shale. www.geology.ar.gov

Graptolites from the Womble shale. http://www.geology.ar.gov

Graptolites from the Womble shale. www.geology.ar.gov

Graptolites from the Womble shale. http://www.geology.ar.gov

Graptolites appeared in the Cambrian Period over 500 million years ago and were thought to have died out in the Mississipian Period (Early Carboniferous), roughly 320 million years ago. They are useful as index fossils, meaning they can be used to date rocks with a fair degree of precision, because they have a worldwide distribution and are common in the rocks, as well as evolving quickly so they have many species, many of which did not last long.

armapGraptolites started off growing on the ocean floor, but later ones floated freely in the ocean. When they died, they would sink to the bottom, becoming entombed in the deep ocean sediments. So today, graptolite fossils are usually found in shale formed from the deep ocean mud. Sometimes they can be found in marine limestone or chert, but they are less commonly found there. As a result, the best place to find them is in the shales of the Ouachita Mountains in west-central Arkansas. The best place to find them is in the Womble Shale, a black shale with thin limestone layers and a few small silty sandstone and chert layers. The Womble Formation was named after the town of Womble, which is now Norman, in Montgomery County, AR, so that makes that area a fine place to look. You can also find conodont fossils commonly in those rocks, which are tiny, tony, early chordates. Conodonts were the first animals in the vertebrate lineage with mineralized tissue. They had teeth, but no bone. Other places you can find them are the Arkansas Novaculite in the Ouachitas, although you will have better luck in the older rocks, such as the Mazarn or Collier Shales. You can also find them in any of the Ordovician or Silurian aged Ozark limestones in northern Arkansas, but they will be harder to find as those rocks were formed in shallower water, with many more fossils of many other animals which are far more common, whereas the graptolites will be much rarer than in the Ouachitas. For a full listing and description of the appropriate rocks and maps to their location, try the Arkansas Geological Survery website here.

Cephalodiscus, a type of pterobranch. http://metazoan.auburn.edu/halanych/lab/projects.html

Cephalodiscus, a type of pterobranch. http://metazoan.auburn.edu/halanych/lab/projects.html

I mentioned that graptolites were thought to have died out in the Mississippian Period. That is because no fossils are found after this date. However, it is thought by most workers that graptolites may still be living today. We just call them pterobranchs, another type of hemichordate and are acorn worm-like animals with plant-like fronds used to filter out plankton from the water.

The Worst Headline in the World

“Sea Anemones are Half-Plant, Half-Animal, Gene Study Finds.”

Sea anemones are not a real life version of Swamp Thing from DC Comics.

Sea anemones are not a real life version of Swamp Thing from DC Comics.

Ok, maybe calling it the worst headline in the world is a little hyperbolic, but it is a terrible headline. I have seen this headline over and over again, phrased exactly the same way, indicating that many, many sites do not bother even attempting to write their own versions, but simply cut and paste from other sources. But that is not the problem I have with it. The headline is woefully incorrect, seriously misleading, and damages all the hard work educators have spent trying to get people to understand genetics and how it affects our evolutionary understanding. The people who wrote this headline should really have known better and the people who wrote the articles should certainly have known better. For an example of a well written article that really explains what the research says, yet has this horrid title to it, try here. The information in the article is excellent if one can get past the horribly misleading title. In many cases, the author of the article is not to blame because the headlines are written by completely different people, who don’t necessarily either read nor understand what the article really says. That is a serious problem because many people, if not most, only read the title, or if they read the article, only remember the spin imposed by the title. Thus, titles matter (for more information on the effects of headlines, try here,  here, here, here, and here).

The headline and many of the articles imply that a new study on the genetic code of the sea anemone revealed a mixture of animal and plant DNA. The research does not in fact say that. So I will attempt here to explain what it really DID say. To summarize, the genome of the sea anemone is very similar to other animals because it is, you know, an animal. However, there are certain key aspects of its gene regulation that are similar to plants. This is not as bizarre as it might sound though.

What's  in your genome? by Steve Cook.

What’s in your genome? by Steve Cook.

To begin with, let’s clarify what the article is referring to. Most of the time, when people talk about genetics, they are referring to the nucleic acid sequences, specifically Deoxyribonucleic Acid, or DNA, that code for genes. This is actually only a small part of the DNA. The rest is taken up with a variety of other things, including a variety of gene regulatory elements, junk DNA (broken genes, viral sequences, etc.), and chromosome structural sequences. The genome, or total genetic sequence, collected into a set of chromosomes, can be thought of as a book. The text includes all the gene sequences, but most of it has been marked out and written over ( what is often referred to collectively as “junk DNA”). The table of contents, page numbers, chapter headings, introductory publishing information, and index would be the regulatory bits; and the spine, cover, pages themselves, and everything holding it all together would be the structural elements. All of these parts are included in the complete DNA sequence, along with various proteins like histones that help package all the DNA.

Pretty much all of this DNA in the sea anemone, according to the study, is very similar to other animals, even vertebrates like us. The genes are similar, the regulatory sequences are similar, pretty much everything is similar. This is interesting because it speaks to the relative closeness of relationships between all animals. The genes used by sea anemones are simply variations of genes found within us. The regulatory sequences are variations of the same sequences we use.

MicroRNA at work. by Steve Karp

MicroRNA at work. by Steve Karp

However, not everything is the same. There are small sequences called micro-RNAs. Ribonucleic acid, more commonly just called RNA, is most commonly known as the molecules that help translate DNA gene sequences into working proteins. But that is not all they can do, Unlike DNA, RNA can do more than act as a codex of information. RNA can act as an enzyme at the same time. Enzymes are typically proteins that act on other proteins or on the DNA and RNA themselves. When micro-RNAs act on the DNA and RNA, they regulate expression of the genes, altering how much and what kind of proteins are created. This is where sea anemones differ from other animals. The micro-RNA of sea anemones is more like plants than animals. It had always been supposed that plants and animals evolved their versions of micro-RNAs separately, because they use different sequences and use different molecular pathways. They act differently. For instance, in animals, micro-RNAs bind to multiple proteins and inhibit their functions. Plant micro-RNAs are much more specific and slice up the protein, they don’t just inhibit them. Thus, it may be that sea anemones are telling us that the system developed before plants and animals split off from the ancestral organisms from which both plants and animals evolved. Not only that, but that the original pattern was kept by plants and substantially altered in animals.

The other part of this story that is not made clear is just what relationship sea anemones have with other animals, as well as plants for that matter, which in this case is fairly important. When looking at the early history of eukaryotic (with a nucleus containing DNA), multicellular animals, sea anemones are really, really early. Sea anemones are part of a group called cnidarians. Cnidarians, so named for cells called nematocysts, or stinging cells, also include what are generally called jellyfish, along with coral. On the tree of animal life, you can’t get much more primitive. The only animals thought to have evolved earlier than cnidarians are sponges and ctenophores, commonly known as comb jellies. They are so close to the base of the animal tree of life, one might say if they were lower, they would be only a step away from being fungi. Fungi, according to the genetic research, branched off from the animal lineage shortly after the split between animals and plants, which indicates just how closely related these early forms are.

AnimalPhylo_morph

Cooperation rules. Wikimedia.

Cooperation rules. Wikimedia.

In addition to being really close to the the plant/animal split, there is one last thing to consider. Sea anemones, like coral, depend on a mutualistic symbiosis with green algae, which, obviously, are plants. The algae collect sunlight to make sugar and oxygen, which gets shared with the sea anemone. The sea anemone in turn provides the algae a secure home, guarded by venomous tentacles. This relationship is similar to the well-known symbiosis with clownfish (remember Finding Nemo?), in which the clownfish gets protected from predators while providing food for the anemone. This is one case in which having messy tenants is actually a good thing.

Now, considering that the anemone relies on algae for its survival, linked as it is metabolically to the algae, and evolutionarily is not that far from being a plant itself, is it any surprise then to find that some of its metabolic regulation uses a system seen in plants? Sea anemones are not half animal/ half plant. They are extremely primitive animals that have their metabolisms linked directly to a plant, unlike the indirect link that other animals have (eating plants or animals that eat plants is a strong, but indirect link, wouldn’t you say?).

Sea anemones: not real plants, not even by half, they just play them on TV.

Sea anemones: not real plants, not even by half, they just play them on TV.

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