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Isisford had ocean views 100 million years ago

10/11/2016

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Geology is an important component of any taphonomic investigation. This helps to 'set the scene' when considering potential taphonomic pathways: what environment did these ancient creatures live in? What happened to their bodies after they died? What environment were they buried in, and how quickly were they buried?

To this end, my colleagues and I have just published a paper about the ancient environment of the Winton Formation at Isisford to better understand the taphonomic history of crocodyliforms, osteichthyan fish, and dinosaur fossils uncovered there. We propose that around 100 million years ago, Isisford lay in the middle of a river delta that flowed into the nearby Eromanga Sea. 

We came to this conclusion by studying the sandstone concretions that encase each of the fossils found at Isisford. These concretions formed when sand grains were cemented together with calcium carbonate (calcite) from calcium-rich groundwater. The majority of fossils appear not to be distorted or warped in any way, and as fossilisation occurs under high temperature and high pressure, it seems the concretions surrounding the buried bones and afforded them some protection. If this calcite cement, and therefore the concretions formed before fossilisation, then information about groundwater quality where the bones were buried would be locked away in the calcite minerals themselves - specifically, isotopes of carbon and oxygen. 
Picture
Images showing a slice of sandstone concretion under a microscope. A, photograph of the concretion through a microscope, with darker coloured sand grains and light coloured calcite cement; B, the same area under cross-polarised light (helpful in finding more different types of minerals); C, cathodoluminescence showing the calcite cement fluorescing in bright orange with patches of bright yellow manganese ions; D, electron-dispersive diffraction (EDS or EDX) indicates the orange and yellow areas from C, shown in blue in this image, are all made of calcite. Image from Syme et al., 2016.
​We tested the carbon and oxygen stable isotopic values of the calcite cement, and by comparing our results to what is ‘typical’ for fresh water and sea water, found that it formed in a brackish water environment. This wasn’t terribly surprising: the regional geology indicates that the Eromanga Sea was nearby Isisford at this time, but until now we hadn't known whether it affected the environment at Isisford or not. When we examined geological core logs from near Isisford, and compared it to the types of rocks we found at the site, we concluded that these animals were buried in a river delta that flowed into the sea, with fresh river water mixing with salty ocean water. Unfortunately, there aren’t large exposures of Winton Formation at the site that would allow for typical facies analysis, but we worked with the material we had and came up with some pretty solid conclusions.
Picture
Figure showing the typical carbon and oxygen stable isotope values for fluvial (freshwater rivers), deltaic, estuarine, and marine water. The stable isotope values of the calcite cements from Isisford are shown by the purple area, and overlaps values typical for fluvial, deltaic, and marine waters. Coupled with all the other information from the site, we determined that Isisford cements were most likely deltaic in origin. Image from Syme et al., 2016.
Our next question is: if these animal's carcasses were buried in a river delta, did they die there or were their bodies washed in from far away? Did they even live nearby at all? That is the subject of future papers that we will be publishing mid to late next year.

References
Syme, C., Welsh, K., Roberts, E., and Salisbury, S. 2016. Depositional environment of the Lower Cretaceous (upper Albian) Winton Formation at Isisford, central-western Queensland, inferred from sandstone concretions. Journal of Sedimentary Research, 86: 1067-1082. DOI: 10.2110/jsr.2016.67
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Size range of theropod dinosaurs

7/11/2016

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Let's compare the sizes of these two dinosaurs: a hummingbird versus a Tyrannosaurus rex tooth. T. rex was only, oh, about 2 million times heavier than a hummingbird! Just look at the variation the vertebrate bauplan can achieve, just within theropoda! Brilliant!
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Image via The Witmer Lab
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Bite marks on fossil bones: what they can and can’t tell us about ancient crocodylians

1/11/2016

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​The powerful bite of a crocodile can cut through flesh and bone. Their teeth puncture, scratch, scrape and crush the skeletons of their prey. They leave behind tooth marks etched into bone, taphonomic traces which have been identified in the fossil record, and can tell us about the feeding behaviours and morphologies of ancient crocodylians. Or so we thought.

A new study by Drumheller et al. (2016) found that crocodile bite marks on bones do not seem to differ between crocodiles young and old, or male and female, or long-snouted and short-snouted. They collected bones bitten by 21 species of modern crocodylians and studied the types of bite marks left on bones surfaces – whether they were pits, punctures, scores, or furrows, and whether these marks were bisected (with extra notches or scoring from serrated teeth) or hooked (marks that changed direction). They compared the types and shapes of these bite marks to the snout shape, size, sex, age, feeding behaviour (the famous ‘death roll’), and captive or wild status of each individual crocodylian that created them. They found no significant difference between the shape of the bite marks and the individual who made them – even for crocodylians of different ages, with different snout shapes, or different feeding behaviours!
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Defining bite-marks: illustration of a bone in cross-section, showing that pits and scores do not penetrate beyond the cortical bone, but punctures and furrows are much deeper and penetrate to the trabecullar bone. Image from Drumheller et al. (2016).
However, they did determine that bisected bite marks – those showing extra notches or scoring – were indicative of the teeth of crown Crocodylia. And as bisected bite marks have been found on fossil bones, the culprits can be successfully identified as a crocodylian (or perhaps a non-crocodylian crocodyliform – that is, animals not related to modern crocodylians, but instead an ancient offshoot of crocodyliforms that are now extinct). But not every individual crocodylian creates bisected bite-marks every time they feed, depending on whether their teeth are freshly erupted and sharply serrated, or old and worn down.
​
So, it seems you might be able to identify a crown Crocodylian as the culprit of a bone bite-mark, but cannot predict its age, sex, snout shape, or which method it chose to dispatch of its prey.  
Picture
The difference between regular bite marks and bisected bite marks. Sharp teeth create bisected bite-marks with extra notches or scores at the base of the mark (A) as opposed to the 'blunt' base of marks created by old, worn teeth (B). The photographs C to F show bisected bite marks. Images from Drumheller et al. (2016). 

References
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Drumheller, Stephanie K., and Brochu, Chris A. 2016. Phylogenetic taphonomy: a statistical and phylogenetic approach for exploring taphonomic patterns in the fossil record using crocodylians. PALAIOS, 31: 463-478. [Paywalled]
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    About the author

    Dr Caitlin Syme is a palaeontologist studying the taphonomy (preservation state) of fossil non-avian dinosaurs, crocodiles and fish from the Winton Formation, Queensland, Australia. Think forensic science or CSI for fossils, and you're on the right track!

    Posts on this blog focus mainly on vertebrate palaeontology and taphonomy, as well early career researcher (ERC) productivity tips and insights.


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