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Introduction The Hell Creek Formation (HCF) is a geologic unit deposited during the Maastrichtian of the Late Cretaceous (ca. 66 million years ago) in North America, recording the last page in the chronicle of non-avian dinosaurs. It represents a paleo-ecosystem that hosted some of the most charismatic animals ever, including T. rex, Triceratops, and others. While its dinosaurs certainly deserve attention, the HCF was burgeoning with overshadowed diversity. A large dinosaur skeleton provides an excellent picture of a single animal, but it alone tells us very little about its world. By looking at small fossils, (“microfossils”) en masse, we can open a larger window into the past, and better appreciate that fuller diversity of life. Over the past year I’ve been on a meticulous campaign, searching through a copious volume of HCF matrix collected on private land in Montana. So far, I’ve amassed a comprehensive collection of nearly four dozen distinct species from thousands of cm-micron scale fossils that span the whole fauna, including mollusks, bony and cartilaginous fishes, crocodyliforms, turtles, salamanders, frogs, lizards, mammals, and of course dinosaurs. In this thread I will detail my initial findings, and provide updates as I find new things. I’ve gotten a lot of joy in discovering, learning about, and documenting these fossils; hopefully this reading will similarly be an enjoyable venture into the Late Cretaceous, and be helpful to fellow micro-hunters. A family of Triceratops at the bank of a river channel. Art by Donna Braginetz. Collage of microfossils and silhouettes, a sampling of a few of the animals present in this complex community (not to scale). Below is a working faunal list of animals I have yet found, in not-so-formal categories in no particular order. In making my identifications, I’ve used a myriad of resources from peer reviewed literature, books, Dr. Dave DeMar’s ID guide, etc. It’s fortunate for me that the HCF is very well studied and published on - it’s been collected and researched intensely for over a century. I will cite the respective resources where appropriate (please pardon any inconsistent or incorrect formatting, I simply don’t have the patience for this non-academic work). I also provide a rough sketch of identification for some fossils that is in no way wholistic, but should suffice for most amateurs. And as always, I welcome informed corrections. Click on an underlined group to skip to that section. Invertebrates: Gastropoda cf. Viviparus Lioplacodes tenuicarinata Unionoidea indet. Sphaerium beckmani Plants: Carbonized plant matter & amber Wodehousia spinata (palynomorph) Osteichthyes (Bony Fishes): Lepisosteidae (Lepisosteus occidentalis) Cyclurus fragosus Holostei indet. Teleostei indet. Perciformes cf. Priscacara Estesesox sp. Acanthomorhpa type HC-2 Acipenser “erucifer” Chondrichthyes (Cartilaginous Fishes): Myledaphus pustulosus Lonchidion selachos Restesia americana Galagadon nordquistae Lissamphibia (Amphibians): Anura indet. Urodela indet. ?Habrosaurus sp. ?Scapherpeton tectum Non-dinosaurian reptiles: Crocodyliformes indet. Brachychampsa montana Champsosaurus sp. Trionychidae indet. Basilemys sp. Varanoidea indet. Polyglyphanodontia indet. Mammalia (Mammals): Metatheria indet. Multituberculata indet. ?Meniscoessus robustus Mesodma sp. Cimolodon sp. Gypsonictops sp. Dinosauria: Ornithischia: Ceratopsidae cf. Triceratops Leptoceratops gracilis Edmontosaurus annectens Nodosauridae cf. Denversaurus Thescelosaurus sp. Pachycephalosauridae Ornithischia indet. Saurischia (Theropoda): Zapsalis abradens (=Dromaeosauridae) Richardoestesia isosceles ?Richardoestesia gilmorei Paronychodon sp. ?Ornithomimidae Theropoda indet. Acheroraptor temertyorum Dromaeosauridae indet. Pectinodon bakkeri Tyrannosaurus rex I have also roughly been keeping track of the quantity and diversity of specimens to have some crude statistical sense of the deposit’s demographics. There is some ambiguity here in how I’m counting fossils (e.g. a jaw with teeth and an isolated tooth are each counted as one), however I don’t have the time to figure out a better way to keep track, and anyhow the purpose of this is just to provide a ballpark summary. There is also unrealized diversity, since I couldn’t identify some fossils very precisely. Keep in mind there are significant preservation and human biases here, so these charts don’t accurately reflect the true abundance of animals in the environment, merely the relative quantities of fossils I recovered. By quantity of identifiable fossils, the deposit was dominated by fishes, namely Myledaphus. Following in abundance were ornithischian dinosaurs and non-dinosaur reptiles. Osteichthyes is also undercounted since I get tired of counting broken scales and shed Cylcurus teeth (keep in mind at this point I’ve had to record many hundreds of specimens). I also considered the relative abundance of dinosaurs. Following the results of many other studies, hadrosaurid and ceratopsid fossils were quite common compared to theropods. The rarest dinosaurs represented were the ankylosaurians. Of course since e.g. ornithomimids and oviraptorosaurs lacked teeth or small identifiable parts, they are under-represented. Digression on Lithology and Taphonomy Geology is a crucial tool for understanding these fossils and how they were preserved; in the interest of providing greater context, I will offer some brief comments on the rock these fossils were found in and their taphonomy (how they were fossilized). The host rock these fossils come from is a loose sandstone conglomerate. It was deposited by an ancient river channel, like many HCF deposits. These river channels laced through forested floodplains and swamps, draining into the receding Western Interior Seaway that cut through the middle of North America during the Late Cretaceous. They carried within them a load of sediment sourced from the then-nascent Rocky Mountains in the West. Gill & Cobban (1973) found that the sediment specifically came from the volcanic Elkhorn mountains region. The river was likely freshwater and further inland with little to no marine influence (discussed later). It’s possibly a so-called lag deposit, where sediment settled out of the current on the inside of a river bend, where the slower flow allows material to drop out and accumulate in a small volume. The disaggregated matrix itself is about half sand/silt and half claystone/ironstone pebbles by volume, the pebbles being ~ cm’s in size, the largest about 10 cm. The claystone pebbles are essentially rip-up mud clasts, where high currents in the river eroded pre-existing mud and transported it some distance downstream. Most have rounded forms and easily fracture, revealing a very fine-grained interior. The geology of the ironstone pebbles I’m less confident about; they are either preformed, part of the river’s load and secondarily deposited, or are diagenetic, forming after deposition of the sediment (or both - clarification would be appreciated). They similarly have rounded forms, but are very solid and more dense; when broken they show their concretionary structure. Also present are typically small (< 1 cm), river-polished, extra-formational metamorphic pebbles - persevering fragments of the Rockies not yet ground to sand. The matrix is inundated with pulverized fragments of iridescent unionoid mussel shells. Following in abundance are bone fragments, and lastly are identifiable vertebrate fossils. I estimate the vertebrate fossil volumetric yield to be less than 1-2% (including unidentifiable bone fragments), similar to Rogers & Brady (2010). However, as you’ll see, that seemingly minimal yield has produced spectacular diversity. Map of North America ca. 66 million years ago, showing the approximate location of the deposit in the context of its paleo-geography. Rivers transported sediment from the young Rockies in the West to the receding seaway in the East. Map adapted from Tyler Carpenter. Sediment accumulation in the Williston basin during the Late Cretaceous (Gill & Cobban 1973). Roughly following the gradient of the contours (a right angle to the tangent of the lines) gives the direction of sediment transport. These data support the sediment of the HCF coming specifically from the volcanic Elkhorn mountains region. The Elkhorn mountains viewed from the north. It is in part thanks to these mountains that we have fossils of Judithian/Lancian dinosaurs in Montana. Unprocessed matrix. There’s nothing apparently special upon first glance; just sand, pebbles, and shell slivers Wetting the matrix reveals the darker fossil bones. Typical claystone pebbles - essentially mud balls that were rolled by the river current. Getting an idea of the size distribution of these pebbles might say something about the speed of the river’s flow, a task for a later date. Various mud clasts Mudstone pebbles with unidentifiable plant matter. The leftmost chunk has an orange piece of amber visible. Many planty mudstone pebbles show laminations (fine layers) and cleave in planes, indicating they preserve the original sedimentation undisturbed by river action. The foremost portion of the left piece may preserve a seed. Metamorphic pebbles, looks like mostly quartzite? These pebbles look the same as they did 66 million years ago. Sand with flecks of mica and mollusk shell. The rare remaining vertebrate fossils at this scale are mere crumbs of bone that aren’t worth collecting. Channel deposits are particularly valuable microsites since they can capture a large cross section of the fauna and concentrate fossils into a small volume, still there are biases to be considered. For example, larger fossils are often broken up in the current, only fossils of a certain size range are captured, and more durable fossils are favored for preservation, while fragile ones are more commonly destroyed. Teeth and scales are both strongly selected for in this mode of preservation; as we’ll see they make up a large portion of the finds. Because this rock was once an active river, many fossils show signs of significant water transport and chemical corrosion pre-fossilization (as is typical of channel-hosted microsites, see Rogers & Brady 2010). Physical weathering erases detail and smooths sharp features, chemical weathering dissolves the bone and leaves shallow pitting on the surface. Some bones are heavily tumbled into rounded forms (some are nearly polished) and are referred to as “bone pebbles” in the literature. Most smaller bones show greater degree of tumbling, which makes intuitive sense, since they could be carried farther down the river before settling out. Much of the material here is from aquatic life, which is natural for a river deposit. The terrestrial material would’ve found its way to the river due to rains and flooding washing things in, or say if an animal died or otherwise directly deposited material in the river (e.g. a dromaeosaurid shedding a tooth while feeding on a carcass on the riverbank). Now, there is some dispute as to the true origins of many of these microsite bone beds. Rogers & Brady 2010 proposed that channel-hosted microsites actually sourced their fossils by eroding pre-existing lacustrine (lake) deposits. They claim this scenario explains the similar quality of preservation and rich diversity in both kinds of deposits. It’s an interesting idea, but I don’t know enough about sedimentology to contest it or test it. All of the material here was deeply buried and only recently exhumed last Summer (after ca. 66 million years of waiting) near the HCF type locality, so there is little environmental degradation like sun bleaching or plant root etching. All of the bones are chocolate browns in color, some nearly black. The quality of preservation varies substantially, from heavily corroded and worn to incredibly pristine. Most fossils I find are just chunks of bone, but every once in a while something more substantial pops out. The vast majority of what I find looks like this - chunks of bone and fish bits. In leaving “no stone unturned”, I’ve picked out thousands of pieces of this stuff just to recover a handful of exquisite specimens. A sampling of unidentifiable bone fragments, ranging from cm-mm in size Well-tumbled “bone pebbles”; I have an odd appreciation for these rounded and smoothed bits of bone. Some might’ve gone on long journeys to get so smooth. Pitting on the surface of bone, evidence of pre-fossilization corrosion. Illustration of channel weathering effects: physical and chemical. A) Trionychid turtle shell; B) holostean fish (Cyclurus) maxilla. Fastovsky, David E. and Antoine Bercovici. “The Hell Creek Formation and its contribution to the Cretaceous–Paleogene extinction: A short primer.” Cretaceous Research 57 (2016): 368-390. Rogers, Raymond R., and Mara E. Brady. "Origins of Microfossil Bonebeds: Insights from the Upper Cretaceous Judith River Formation of North-central Montana." Paleobiology 36.1 (2010): 80-112. Gill, J. R. and William Aubrey Cobban. “Stratigraphy and geologic history of the Montana group and equivalent rocks, Montana, Wyoming, and North and South Dakota.” (1973). Processing Methods I knew I wanted to search this matrix as thoroughly as humanly possible; my goal was to recover every single fossil hidden within it. Though experienced at searching matrix, I’m by no means professional or optimal when it comes to processing. The goal of preparing matrix is to make fossils easier to find. You want to remove as much rock as possible to concentrate the fossils, and for ease of searching, you want to separate fossils by size. This is usually done with metal screens, which accomplish both. I follow the method of “screen washing” practiced by paleontologists for over a century - simply running water over matrix on screens of varying mesh sizes. As we all know, water is excellent for separating things like dirt and rock because it’s a polar molecule, and it I think better mediates collisions between objects during screening so delicate fossils are less likely to break. As mentioned earlier, roughly half the volume is sand, so a good deal of “fossil-empty” matrix can be easily winnowed out this way. For screens to wash the matrix through, I used food strainers of a couple different sizes from the local grocery store. I caught the sediment that fell through with cut up old plastic jugs sitting in cheap storage tubs to catch overflow. It’s not ideal, but it’s cheap and it works. Very few fossils were broken, and my yield is maxed out - the two things that matter most. (You check yield by looking at the finest grain size and make sure there’s nothing left in it.) At the cm scale, I searched through the clay pebbles by eye under good lighting against a white background to maximize contrast, and I picked out particularly cool or delicate fossils I managed to spot as I washed the matrix. It helped that when wet, the fossils were dark brown-black and stood out from the lighter matrix. At the mm-scale, I used a binocular microscope with 20x magnification, sorting through a spoonful of material at a time with a small paintbrush. I always enjoy seeing “as found” pics, so here are a few, That was a good day - a large Edmontosaurus dentary tooth 99 times out of 100, the small shiny triangular thing is a fish scale, the other time it’s a troodontid tooth - as in this case Mammals are always a joy to find - a rooted marsupial lower premolar A salamander caudal vertebra in association with a fragment of amiid fish palate An uncommon microscopic carpet shark tooth - Galagadon nordquistae - just a mm or two in size A marsupial upper premolar, hiding among mollusk shell flakes Now, on to what I actually found!

Hello, I'm sorting though microfossil matrix from the Permian Texas red beds. I'm not very familiar with this area, but understand that there are shark, reptile and amphibian teeth in it. Could you point me to a guide or other literature relevant to this location or help me to understand most of what I'm looking at? I've found specimens as large as 5 mm and the smallest can't be picked up with my tweezers. This matrix has been sorted under a cheap microscope, so I'm sorry the pictures are rather blurry. Thanks for the help. (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)