Decorah gastropods (and some things that look like gastropods)

The snails* of the Decorah Shale are a lesser component of the fauna than bryozoans, brachiopods, or crinoids. My personal experience is that snails are uncommon except for certain beds, which feature abundant and diverse snails. One of these beds is perhaps 20 ft (6 m) above the top of the Carimona in St. Paul; this bed produced the plate in the "Equatorial Minnesota" box near the top of the page. For whatever reason, this bed also hosts abundant trilobite pieces, particularly of Eomonorachus. There's probably a facies thing going on, such that the original depositional environment was favorable to snails and trilobites; it's not quite as stark as, say, McKee (1938)'s mollusk and open marine facies of the Kaibab Formation (the very durable rock at the top of the Grand Canyon stack), but there's certainly some kind of difference. It might be a carbonate thing; both the underlying Platteville and overlying Cummingsville/Prosser, which have more limestone and dolomite, also have more diverse mollusks than the muddy Decorah (Sloan and Webers 1987). On the other hand, the relatively limited diversity and abundance of snails, combined with most genera having visually distinctive appearances, make it possible to summarize them in a reasonably brief guide.

*and things that look a whole lot like snails, and things that people argue about, such as Sinuites

Several snails in a small section of a slab: C for Clathrospira, S for Sinuites, and L for "lophospiroid".

Regarding forams

Life started out microscopic (at least to humans) and most of it has stayed that way. Of course, many microscopic organisms have poor fossil records, due to factors like lack of hard parts and the whole "microscopic" thing (finding and studying microfossils takes special equipment and expertise that aren't used for collecting, say, brachiopods). However, a subset of microscopic organisms have very significant fossil records. We saw the ostracodes a few years ago, but there are also a number of groups of single-celled organisms that produce hard parts suitable for fossilization. Among the most important are: coccolithophores, phytoplankton which form skeletons of scale-like objects known as coccoliths, micron-scale structures that make up chalk (and which are sometimes called nannofossils because they're so darn small); diatoms, phytoplankton with cell walls made of silica; dinoflagellates, which form organic-walled cysts; radiolarians, protozoans that form body structures of silica; and the subjects of today's entry, the foraminifera, which can be described glibly as "amoebas with shells".

A living foram, the brackish-water benthic calcareous species Ammonia tepida, showing strands of pseudopodia surrounding the coiled test. What do all these terms mean? Read on! (Photo from Wikimedia Commons; unfortunately, no scale, but you'll get an idea of the size of what we're dealing with in the photos to come.)

Near-dinosaurs

The Triassic was an experimental time for large tetrapods. The Permian–Triassic extinction event had eviscerated the prevailing communities of diverse therapsids (relatives of mammals), various early reptiles, and large temnospondyl amphibians (for more on them, Tetrapod Zoology has made them a cottage industry since 2007). Into that vacuum the survivors seem to have gone with the strategy of "throw everything at the wall and see what sticks". Some lineages stuck quite well. Early turtles and sphenodonts (tuatara) showed up during the Triassic, as well as forerunners of crocodilians and mammals. Lizards are probably in there too somewhere. Frogs may predate the Triassic, but the first good fossils are Triassic. Famously, the dinosaur line, which eventually produced birds, appeared in the Triassic as well. Long-lived but now extinct groups that got their start in the Triassic include the ichthyosaurs, plesiosaurs, pterosaurs, and our friends the choristoderes. There was also a veritable heap of briefly successful groups that for whatever reason went extinct by the close of the Triassic. Among them: aetosaurs (armored herbivores that look vaguely like ankylosaurs), drepanosaurs, nothosaurs and other plesiosaur cousins, placodonts, phytosaurs (crocodiles before there were crocodiles), rauisuchians (carnivores with theropod-like skulls), rhynchosaurs (big beaked herbivores), tanystropheids (necks plus neck delivery systems), thallatosaurs (lanky marine reptiles), and all kinds of other strange one-hit wonders.

Until the 1960s, dinosaurs seemed to have more or less just appeared in the Late Triassic, with theropods and prosauropods (in the sense of "all them sauropodomorphs what ain't sauropods")  recognized as present. What came before those theropods and prosauropods wasn't known, although there were a few guesses and extrapolations. One popular option of the time was that, technically speaking, there weren't really any basal dinosaurs because "dinosaurs" themselves were an artificial group "united" by some coincidental bits of anatomy related to being large land-living animals. This view is practically extinct, although I cannot say completely extinct. One of the great truths of humanity is that there is someone who will believe any proposition. One of the great truths of the Internet is that now you can find that person (or oftentimes, they will find you, if you are holding an opposing position). Other researchers drafted in various poorly known Triassic reptiles. The most enduring may have been Teratosaurus, which people who got into dinosaurs before the late 1980s will probably remember as a sort of megalosaur-like thing stalking the wilds of Late Triassic Europe. It was actually based on jaw material from a rauisuchian, with prosauropod skeletal remains misattributed to it. The misidentification of Teratosaurus, though illustrative, serves mostly as a lesson in the honesty of bonebeds. More recent work with Reveultosaurus, Shuvosaurus, and others shows that it can be darn hard to separate true early dinosaurs from the various wacky archosaurs of the Triassic if you've only got a few remains. The terrestrial Triassic still has more fools to make.

Of all the various bits and pieces put forward as early dinosaurs in the days before the 1960s, the only one that actually is both vaguely useful and does not easily slot into any of the known clades of true dinosaurs is Saltopus elginensis, described in 1910 by von Huene. It got to be in all the best dinosaur books as an archetype, overcoming the significant handicap of being a terrible specimen, which just goes to show that sometimes all you have to do to succeed is show up. The first useful basal dinosaurs to be described, Herrerasaurus and Ischisaurus, were described in 1963, followed by Staurikosaurus in 1970. They were followed by Lagosuchus, Lagerpeton, and Lewisuchus in 1971 and 1972, which were underappreciated at the time but eventually were shown to be dinosaurian cousins once we got that whole "unnatural Dinosauria" thing worked out of the collective scientific system. At the present, there are around 20 species of near-dinosaurs, from Dinosauromorpha to Dinosauria. This doesn't compete with, say, Titanosauria, but it's not bad for about 45 years of serious work. There's a little wiggle room built in depending on how charitable you feel toward Pseudolagosuchus, how you handle "Thecodontosaurus" alophos and borderline cases (e.g. Agnosphitys, Alwalkeria, Teyuwasu), and the occasional analysis that pulls Herrerasauridae or Eoraptor out of Saurischia. Some of them can be grouped as lagerpetids, diminutive bipeds, or as silesaurids, larger animals which could reach roughly the size and shape of Fred Flintstone's pet Dino. At least one of these, the namesake Silesaurus, was equipped with a little can-opener of bone at the tip of its lower jaw, perfect for being confused with the ornithischian predentary. A few others either don't slot comfortably into either group, or are poorly known (and I tend to the conservative when it comes to where I slot, which in this case mostly affects Lewisuchus/Pseudolagosuchus). The chart below lays them out with age and continent denoted. One thing to note is the abundance of species for South America and Africa, which not only suggests a Gondwanan origin but also provides a partial explanation for why these animals have only come to light in the past few decades: there haven't been a lot of paleontologists in those areas until recently.

Click for further enlightenment

Another important thing to keep in mind is to avoid the trap of turning extinction and evolution into a morality play. "Near-dinosaurs" were not merely a sideline, or waiting hopefully to eventually evolve into dinosaurs, or a bunch of saps that got pushed out of the way by their cousins. They were their own creatures, diversifing into several lineages and living alongside their more famous cousins for twenty or so million years, at least. We currently have named examples from four continental landmasses, and it would hardly be surprising to add a few more landmasses, five to ten million years, or additional lineages.

Dromomerom romeri, by Nobu Tamura (from https://commons.wikimedia.org/wiki/File:Dromomeron_BW.jpg). For whatever reason I've always found this restoration charming.

Algae and not-algae

The last of the fossils to be covered to complete the Upper Ordovician Twin Cities set (barring some microfossils and real rarities) are a nebulous group of oft-times enigmatic organisms that either are algae, or have been classified as algae. "Algae" is a much more problematic a term than you might suspect. To put it simply, "algae" is more of a state of mind than a formal classification. At its broadest, "algae" covers basically anything that does photosynthesis and doesn't have the obvious distinct tissues of derived plants, like leaves and roots. This would include anything from diverse microbes to seaweeds. While this has some utility for back-of-the-envelope things, it is not the most useful term for serious classification. It should not be surprising that untold numbers of fossils have been been classified as algae, and that many of these "algae" belong to disparate groups, some of which still have unsettled classifications. About a half-dozen taxa from the Platteville, Decorah, and basal Cummingsville formations have fallen into the "algae" bin at one time or another. They include the following:

Conodonts

A common yet easily overlooked type of fossil is the conodont (or "conodont element", should we choose to be picky and/or technical). They are easy to overlook because they are sub-millimeter-scale fossils, which makes them just barely visible to the naked eye if you look really hard. Most of the time, though, all you can do is to take bulk samples of rock and run them acids to liberate these tiny fossils; conodont fossils are made of calcium phosphate, so they will not be dissolved by some acids that do dissolve calcium carbonate, i.e. limestone. Having dissolved your limestone, you can then inspect the remainder under a microscope. Ideally, you will have some little pointy toothy bits, which are the conodont elements.

Graptolites: gone yesterday, here today?

Graptolites are one of the less-heralded members of the local Ordovician menagerie, probably because they don't seem to be all that common around here. Graptolites were colonial animals (and I use "were" and other past-tense terminology with implied quotation marks, because of the shocking twist yet to come). The colonies, called "rhabdosomes", are made up of branches called "stipes", which support cup-like structures called "theca" that housed the individual animals. (See also here for official terminology.) The overall effect is that stipes often resemble saw blades, with the "teeth" being the theca. Early graptolites were apparently attached to the seafloor and formed densely branching (dendritic) colonies (see for example several of the specimens here and here), whereas later forms were apparently planktonic and attached to floats of their own device or other floating things, such as seaweeds [2020/11/11: no! It appears that there is no evidence for floats or attachments; see Maletz 2015 for the gory details]. The rapid taxonomic turnover of graptolites, coupled with the ease of distribution for planktonic forms, make them excellent index fossils. Planktonic distribution also has the neat side effect of getting graptolites into rock formations that otherwise lack much for fossils, usually because of low oxygen levels in the water column precluding a great deal of life while the formation was being deposited. While low oxygen would limit bottom dwellers, it wouldn't stop things from drifting through higher up the water column. Fossils of graptolites are found from the Cambrian into the Carboniferous. This may not be where the story ends, though: it has been known for a while that graptolites are most similar to pterobranchs, a living group of tube-dwelling often-colonial worm-like hemichordates. There is evidence that pterobranchs are, in fact, living graptolites (e.g. Mitchell et al. 2013; Discovery Magazine ran an article on this topic back in 1993). Should this be the case, it would appear that only the planktonic forms truly disappeared; seafloor forms persisted, albeit in much reduced circumstances (only a handful of pterobranch genera and species are known). All of the species reported from the Twin Cities appear to be of the planktonic persuasion.

As mentioned, graptolites are an uncommon component of the local fauna. They do pop up from time to time, though, and their thin dark saw blade fossils are quite distinctive if you get a large enough chunk. Reports of graptolites in the Twin Cities go back almost to the beginning of local geology, actually. Shumard (1852) reported graptolites from a bluff about a half-mile downstream from Fort Snelling, in what would now be considered the lower Platteville Formation. Winchell and Schuchert (1895) provided illustrations for three species, Climacograptus typicalis, Diplograptus pristis?, and Diplograptus putillus. All of their cited specimens came from outside the metro, and apparently from higher stratigraphically than virtually all metro rocks, with C. typicalis from what would now be the Cummingsville Formation and the other two from what would now be the Dubuque Formation, per the stratigraphic table in Winchell and Ulrich (1897). Stauffer (1930) found abundant black fragments of graptolites in a particularly calcareous layer low in the Decorah Shale in rocks recovered from a heating shaft dug for Northrop Auditorium. Stauffer and Thiel (1941) observed C. typicalis in what would now be the Mifflin Member at Lock and Dam 1, on the Ramsey County side. Their lists reported three following species in the Platteville Formation and Carimona Member (their Spechts Ferry). All three were listed under Hydrozoa, the group that includes relatives of corals and jellyfish, which was a reasonable enough hypothesis at the time (before the discovery of pterobranch affinities, graptolite relationships were a free-for-all):

Climacograptus typicalis (Pl, Ca)
Climacograptus (Mesograptus) putillus (Pl)
Diplograptus amplexicaulis? (Pl)

(More graptolites are known from older rocks a bit farther afield in Minnesota. Ruedemann [1933] described several species from the Upper Cambrian St. Lawrence Formation of Afton.)

From Winchell and Schuchert (1895). This looks kinda like at least one of the examples in the following photo, but you should always be wary of diagnosing your fossils via photos and plates.

A University of Minnesota piece with a few graptolites (look for the things that look like saw blades on sticks. Mouse-sized hacksaws.).

References

Maletz, J. 2015. Graptolite reconstructions and interpretations. Paläontologische Zeitschrift 89:271–286.

Mitchell, C. E., M. J. Melchin, C. B. Cameron, and J. R. Maletz. 2013. Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite. Lethaia 46(1):43–56.

Ruedemann, R. 1933. The Cambrian of the upper Mississippi Valley, part III, Graptolitoidea. Milwaukee Public Museum Bulletin 12(3):307–348.

Shumard, B. F. 1852. Geological report of local, detailed observations, in the valleys of the Minnesota, Mississippi, and Wisconsin rivers, made in the years 1848 and 1849, under the direction of David Dale Owen, United States Geologist, by B. F. Shumard, head of subcorps. Pages 481–531 in Owen, D. D. Report of a geological survey of Wisconsin, Iowa, and Minnesota; and incidentally of a portion of Nebraska Territory. Lippincott, Grambo & Co., Philadelphia, PA. Available at http://archive.org/details/mobot31753000174885 (plates not included), https://archive.org/details/reportofgeologi00owen (full plates) or http://books.google.com/books?id=Y_ZYAAAAYAAJ.

Stauffer, C. R. 1930. Conodonts from the Decorah Shale. Journal of Paleontology 4(2):121–128.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, MN. Bulletin 29.

Winchell, N. H., and C. Schuchert. 1895. Sponges, graptolites, and corals from the Lower Silurian in Minnesota. Pages 55–95 in Lesquereux, L., C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(1). Johnson, Smith & Harrison, state printers, Minneapolis, MN.

Winchell, N. H. and E. O. Ulrich. 1897. The lower Silurian deposits of the Upper Mississippi Province: a correlation of the strata with those in the Cincinnati, Tennessee, New York and Canadian provinces, and the stratigraphic and geographic distribution of the fossils. Pages lxxxiii–cxxix in L. Lesquereux, C. Schuchert, A. Woodward, E. Ulrich, B. Thomas, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

All of the other echinoderms

If you thought that echinoderms have a variety of seemingly unrelated body plans today (sea stars, crinoids, sea urchins, sea cucumbers...), you should have seen them during the Paleozoic, when several now-extinct classes populated the oceans. The group with probably the most recognition are the blastoids, or sea buds, which had stalks like crinoids but with a nut-like structure instead of a cup-like structure as the business end. The rocks of the Twin Cities area are not known to have produced blastoids, but they have produced rare examples of four other groups not including the crinoids we saw before. They are: asteroids (sea stars), rhombiferan cystoids, edrioasteroids, and stylophorans (a.k.a. carpoids, a.k.a. homalozoans). Three of these groups are extinct, and it doesn't take much to guess which. This diversity of echinoderms is not particularly unusual; Cincinnatian rocks have all the same classes as well as brittle stars and cyclocystoids (extinct and known mostly from their ring-like outer structures), and the Bromide Formation of Oklahoma, of comparable age, outdoes the Minnesota rocks handily. But for the luck of the cosmic draw, any of these extinct groups might be around today.

Crinoids

For the fossil enthusiast in Minnesota, there are few groups with a bigger gap between "what they show in the books" and "what you actually find" than crinoids. The ideal of a crinoid fossil is a long segmented stem or stalk connected to a cup-like structure crowned by a group of delicate feathery arms. The Digital Atlas of Ordovician Life and the UGA Stratigraphy Lab's pages on Cincinnatian fossils have many images of these ideal crinoid fossils, if you're feeling like living vicariously. The reality in Minnesota is a bit more like this:

Of course, they are not always found loose, but you get the idea.

The noble ostracode

One of the most diverse fossil groups in the Paleozoic rocks of Minnesota is also one of the least familiar to the layperson. This group is the Ostracoda, or "seed shrimp", a class of crustaceans that is still thriving. They owe their lack of familiarity to their diminutive size; they aren't called seed shrimp for nothing. Although found practically anywhere you can rub a couple of water molecules together, they are often more or less invisible to humans because most of them are on the order of 1 mm long. They owe their abundance in the fossil record to their durable shells, being equipped with a pair of valves not unlike a clam. Inside the valves is the tiny crustacean itself, with several pairs of appendages and distinct internal organs. With around 8,000 living species, there's a lot of diversity to go around. Ostracodes (also spelled ostracods) inhabit marine, freshwater, and moist terrestrial settings, are free-swimming or bottom dwellers, and include predators, herbivores, and detritivores. Definite ostracodes appeared during the Ordovician; there are some suspiciously ostracode-like creatures in the Cambrian, such as bradoriids, but they are not currently considered ostracodes. Fossil ostracodes are of particular interest for paleoenvironmental studies (depth, temperature, salinity, etc.) and biostratigraphy. There is one slight drawback to appreciating them as fossils, though: their size. Without a good microscope and the equipment to extract ostracodes, their charms will necessarily be vicarious for most people. Nothing against microfossils (I personally have a fondness for forams, because one of my first projects involved them), but those are just the practical breaks. There is, however, one exception in the rocks of the Twin Cities, a giant among ostracodes, the form commonly known as Eoleperditia fabulites or Leperditia fabulites. This species tops out around a centimeter long and resembles a bean. It can be both common and recognizable in the Platteville Formation.

This is the fabled giant ostracode Eoleperditia (or Leperditia) fabulites, from the University of Minnesota collections.

And here are examples in the wild, in the Platteville Formation (probably Mifflin Member).

Stauffer and Thiel (1941) have their usual list, featuring several dozen species, but it probably won't do much for you unless you have the requisite facilities or are just really good at spotting sesame-seed-sized fossils. Also, there are many publications specifically dedicated to the ostracodes of the Platteville and Decorah, and they're going to be more useful than an uncited faunal list. If you want the true Minnesota Ordovician ostracode experience, a selection of citations includes Ulrich (1890, 1892, 1897), Kay (1934, 1940), Hansen (1951), Cornell (1956), Swain et al. (1961), Swain (1987), Swain and Cornell (1987), and Johnson et al. (1991).

References cited:

Cornell, J. R. 1956. The Ostracoda zones of the Decorah Shale. Thesis. University of Minnesota, Minneapolis, Minnesota.

Hansen, D. L. 1951. Distribution of Ostracoda in the Decorah Shale Formation at St. Paul, Minnesota. Thesis. University of Minnesota, Minneapolis, Minnesota.

Johnson, J. D., L. V. Benolkin, and F. M. Swain. 1991. Ostracoda from the Glenwood Shale (Ordovician-middle Caradocian) of Minnesota. Revista Espanola de Micropaleontologia 23(2):141–152.

Kay, G. M. 1934. Mohawkian Ostracoda: species common to Trenton faunules from the Hull and Decorah Formations. Journal of Paleontology 8(3):328-343.

Kay, G. M. 1940. Ordovician Mohawkian Ostracoda: lower Trenton Decorah fauna. Journal of Paleontology 14(3):234-269.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Bulletin 29.

Swain, F. M. 1987. Middle and Upper Ordovician Ostracoda of Minnesota and Iowa. Pages 99–101 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Swain, F. M., and J. R. Cornell. 1987. Ostracoda of the superfamilies Drepanellacea, Hollinacea, Leperditellacea, and Healdiacea from the Decorah Shale of Minnesota. Pages 102–130 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Swain, F. M., J. R. Cornell, and D. L. Hansen. 1961. Ostracoda of the families Aparchitidae, Aechminidae, Leperditellidae, Drepanellidae, Eurychilinidae and Punctaparchitidae from the Decorah Shale of Minnesota. Journal of Paleontology 35(2):345–372.

Ulrich, E. O. 1890. New and little known American Paleozoic Ostracoda. Journal of the Cincinnati Society of Natural History 13(3):104–137.

Ulrich, E. O. 1892. New Lower Silurian Ostracoda, no. 1. The American Geologist 10(5):263–270.

Ulrich, E. O. 1897. The Lower Silurian Ostracoda of Minnesota. Pages 629–693 in E. Ulrich, W. Scofield, J. Clarke, and N. H. Winchell. The geology of Minnesota. Minnesota Geological and Natural History Survey, Final Report 3(2). Johnson, Smith & Harrison, state printers, Minneapolis, Minnesota.

Trilobites

To start it off, this being Father's Day as I write, I shamelessly link to my father's Flickr photostream.  He's got photos of landscapes and landmarks of places in Hawaii, Arizona, Utah, Washington D.C., South Dakota, Virginia, Idaho, Alaska, and Minnesota. I recommend the eagles on Grey Cloud Island.

Where were we? Yes, the majestic trilobite, the three-lobed former denizen of the deep, the oval with antennae, bearer of the first compound eye, shameless vandal of pristine Paleozoic sediment. Trilobites are one of the most famous types of extinct organism, perhaps not on par with certain vertebrates, but certainly the most renowned fossil invertebrates. (In second place, with plenty of daylight intervening, are ammonites. In last place, maybe edrioblastoids? Some kind of echinoderm, probably.) You can find online information on practically any aspect of trilobites you may want to know about, often lovingly illustrated with photos. You'll probably end up at Trilobite.info sooner rather than later. The American Museum of Natural History also has a good guide, with photos of several of the species mentioned below. Wikipedia and the Kansas Geological Survey have detailed entries, and more photo guides particularly relevant to our slice of time can be found at the Atlas of Ordovician Life, the Dry Dredgers, fossilid.info, and University of Georgia pages on the Cincinnatian and Nashville.

Snails (and not-snails?)

I am posting this on May 24, 2015, which also happens to be Bob Dylan's 74th birthday. I considered titling this "A Hard Shell's A-Gonna Crawl", but even I have standards. "Shell" doesn't sound a bit like "rain".

Nautiloids: cephalopod overlords of the Ordovician

The cephalopods include a wide variety of tentacled friends. Today, they are represented by a few species of Nautilus and Allonautilus, and an array of squids, octopuses, and cuttlefish grouped under the subclass Coleoidea. Aside from the nautiloids, with their chambered shells, the modern cephalopods are not great for making fossils. The external shell fell out of cephalopod fashion coincidentally not long after the Cretaceous–Paleocene extinction event eliminated almost all of the practitioners, and although there is a history of internal shells in coleoids, those have been reduced or lost altogether by the smart modern coleoid. They still have more resistant beaks, feeding organs (the radula), and tentacle hooks, with some preservation potential, but otherwise you're looking at a lot of soft tissue. What this means is that most of the groups of cephalopods we see today have wimpy fossil records. On the other hand, we know of a great diversity of extinct shelled cephalopods, from three major lineages. The most famous are the ammonites, best known for coiled forms. They appeared by the middle Paleozoic and left the scene at the end of the Cretaceous. Going extinct at the same time, but apparently only extending back to the Triassic, are the belemnites, a subgroup of the coleoids represented by their bullet-shaped internal shells. Finally, there are abundant extinct forms, primarily from the Paleozoic, lumped together as "nautiloids". These early nautiloids had the run of things from the Ordovician to the Devonian, following which they declined until the lineage that includes modern nautilids was the last branch remaining unpruned at the end of the Mesozoic.

Bivalves

The clam. The oyster. The scallop. The mussel. These are not just names suitable for a low-rank themed superhero or supervillain, but they are also common terms used for some of the abundant and diverse members of the class Bivalvia (also known as Pelecypoda in some references, and Lamellibranchiata if you go back far enough or have a desire to be "that person"). We've already seen the difference between the functional and technical usage of the term "bivalve" with brachiopods, now we get the bivalves that "are" bivalves.

Hyoliths and Scenella

One of the aspects of Paleozoic fossils you learn to appreciate is just how many groups had a brief moment in the sun and then disappeared. I'm not just talking about famous things, like trilobites or ammonites (which saved their best for the Mesozoic, of course). You couldn't wade around in an Ordovician sea without crushing or disturbing something that has no living relatives. In fact, given the diminutive size of most of these things, you would probably endanger an entire fauna with each step, so by all means be careful next time you happen to swing by the Ordovician. Today's spotlight shines on two groups that hang around the fringes of Mollusca, and which show up in small numbers in the Ordovician of the Twin Cities: the hyoliths and scenellids.

Brachiopods

Technicalities. Say you've got this thing, it's got two halves to its shell, the halves are called "valves", "bi" is two, "bivalve". Simple, right? Not so fast! There are bivalves, in the simple "it's got two half shells" sense, and then there are bivalves, in the "mollusk that belongs to Class Bivalvia" sense. Why bring this up? Aside from bivalved mollusks, we've also got another group of invertebrates with two half shells that is quite common in the fossil record, with abundant examples in the Decorah and Platteville formations. This group is Brachiopoda.

Rostroconchs: Paleozoic taco shells

The Paleozoic was full of invertebrate groups that didn't quite hack it. The kings of extinct invertebrates are, of course, trilobites. The next tier down, widely known to paleontologists, geologists, and fossil collectors, is where we find things like tabulate corals, rugose corals, and eurypterids ("sea scorpions"). Then there are some that linger in obscurity; the aglaspids (Cambrian critters that were something like trilobites with poor-quality horseshoe crab disguises), for example, or cyclocystoids (disc-like echinoderms), which of this writing don't even have a Wikipedia article. Rostroconchs are another group that hangs out near the aglaspid/cyclocystoid end of the scale.

Bryozoans

Bryozoans make up one of the "B"s of the Decorah BBC (brachiopods, bryozoans, and crinoids), and may be the most abundant; I have seen rocks that are more or less bryozoan hash. Bryozoans are known colloquially as "moss animals", which is fair enough in terms of some of them encrusting things in the manner of carpets of moss, and also being animals. They are often compared to corals, but are quite different. A coral animal is a sack; food goes in and waste goes out the same passage. Bryozoans have a one-way flow with two holes. Coral animals are much larger than bryozoan animals; for example, for the Ordovician fossils of the Twin Cities, coral apertures typically measure several mm across, while the apertures in bryozoan colonies are sub-mm in scale. Finally, bryozoans have never gone in for the algal symbiont trick like corals have done. Not having symbionts, bryozoans must rely on filter-feeding, which the individual bryozoan animals do via a structure called a lophophore, a sort of ring of tentacles around the mouth. This structure is also found in some soft-bodied marine critters with very poor fossil records and the other "B" of the BBC, the brachiopods, showing that they were related, making up a group called the Lophophorata. Various extinct groups known from "worm tubes", like our old friends the cornulitids, may also have been lophophorates.

The Amazing Four-Sided Herringboned Ice Cream Cone (with creamy polyp center?)

The farther back you go, the less familiar the lifeforms. Obvious, no? And yet the pattern is not a simple one to one relationship. For example, the Cambrian was a relatively brief time in which the invertebrates collectively decided that "anything goes" and did their darnedest to fulfill that maxim. Many groups didn't hack it and disappeared before the Ordovician. From the Ordovician to the Permian, the shallow seas were filled with bryozoans, brachiopods, and crinoids, with growing numbers of rugose and tabulate corals, mollusks, and fish. The dominant groups of the Paleozoic were greatly reduced or wiped out altogether at the end of the Permian. Almost all of the enigmatic or otherwise difficult-to-classify groups kicked the bucket by the Permian–Triassic extinction, with a few exceptions such as the bellerophont snails (or monoplacophorans), the conodonts, and today's entry, which all persisted into the Triassic for reasons known only to them. The post-Cambrian bryo-brach-crinoid seafloor communities were replaced by Mesozoic communities dominated by mollusks, stony corals, echinoids (sea urchins), and cartilaginous and bony fish. With the end-Cretaceous subtraction of the ammonites, belemnites, certain groups of bivalves, and marine reptiles, and the addition of marine mammals, this becomes the typical modern marine assemblage. Today, many of the extinct Paleozoic groups appear strange, which is a bit unfair because they were just being the best filter feeders/detritivores/algal symbionts they could, and because there are plenty of unusual things alive in the ocean this very instant. Some of them, however, seemingly went out of their way to stand out. One example follows below the fold.

Corals of the Twin Cities

The Ordovician seas of the Twin Cities would have been unfamiliar in a lot of ways. There were no sharks, no bony fish, no marine mammals, no seabirds. No driftwood bobbed in the water. There were no octopuses or true squids, no lobsters or crabs scuttling about. The kings of the echinoderms were not sea stars, brittle stars, and sea urchins, but crinoids. A diver would see a Lilliputian sea-scape featuring cm-scale brachiopods, profusions of bryozoans, and "forests" of sea lilies, traversed by trilobites. Snails, of course, would provide a common point of reference. "Are there no corals?" you ask, thumbing through your waterproof guidebook as you try to figure out if you have just stepped on an inarticulate brachiopod, a bivalve, or a monoplacophoran. Well, yes, there are corals; it's just they are all representatives of groups that have been extinct since the end of the Permian (that pesky Permian–Triassic extinction).

Sponge detective: when faunal lists go bad

I set out to do something simple, really I did. All I wanted to write was an introduction to sponges and a quick description of the forms known from the Twin Cities region. I already had a list of appropriate species, and I knew that most of the original forms weren't actually sponges, which I thought would make things easier. "There's only a couple left, that's not too bad." Then I made the mistake of checking into those leftovers. It turns out that you can never assume a classification for early Paleozoic sponge-like things. There's always room for an argument. In paleontology, the answer to any question always includes "start digging," whether it be rocks or research, and, frankly, isn't some mystery more interesting than a list?