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I am fortunate enough to have such a huge amount of Middle Devonian Givetian material that I thought it best to put the older Middle Devonian stage, the Eifelian, in its own thread. There are some spectacular fossils here as well though! I thought a good place to start would be in the Formosa Reef, which I believe is quite early Eifelian. This tabulate coral and stromatoporoid reef continues similar complexes found from the Middle Silurian, see my: https://www.thefossilforum.com/topic/84678-adams-silurian/page/3/ thread from page three onwards for details. All these Formosa Reef specimens come from a delightful gift from my good friend @Monica who is a tad busy with life at the moment but is fine and still thinking of the forum. This outcrop can be found on Route 12 near Formosa/Amherstburg, Bruce County, Ontario, Canada. This beautiful-looking specimen came to me with only a third of it revealed but I managed to get it this far after nine days of painful pin prepping. Monica found another one and posted it for ID here: https://www.thefossilforum.com/topic/105528-weird-circular-imprints-formosa-reef-lower-devonian/#comment-1172285 The specimen was identified by another Canny Canadian @Kane to be the little stromatoporoid sponge Syringostroma cylindricum. Hardly a reef-builder, but gorgeous nonetheless. It does have a little thickness to it, but not much. Beautiful! Pretty thin, actually. I love this Monica, thank you!

The following content is purely based on applying a mathematical equation from Ferron (2017), who obtained it from Sambilay (1990). No other source besides a few used for length measurements have been used besides Ferron (2017). I am not a mathematician and my calculations may have mistakes, which is why I've shown all of my work in case any of you want to check my math. The formulas themselves may possibly be updated/invalidated by later studies. Please take this with the finest grain of salt. Apparently, Cretoxyrhina may possibly be the one of the, if not the fastest shark known so far according to a set of equations. When reading through the scientific paper Ferron (2017), I found out that the equation he used to make his speed calculations are ones that I might be able to do myself, so here I am making a post that consists of a wall of algebraic nonsense to try to see for myself on things. Ferron (2017) used an equation that was developed by Sambilay (1990) (which I unfortunately cannot access) to calculate the maximum burst speed of a large fish: Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(PCL) + 0.3478 ⋅ Log10(AR) AR stands for aspect ratio, which is the ratio between the height and surface area of the caudal fin. PCL stands for post-caudal length and can be calculated from a shark's total length (TL) using this equation: PCL = -0.9195 + 0.8535 ⋅ TL When Ferron (2017) inputted the equation for Cretoxyrhina mantelli using Shimada (2006)'s conservative estimates for maximum size of 640 cm as total length (TL) and an unspecified aspect ratio (AR), a burst swimming speed of ≈70 km/h was calculated. This is essentially one of the highest confirmed calculated speeds of any shark ever recorded. I decided to try the equation for myself, which is shown below. I assumed that the AR used would be the conservative aspect ratio of 4.3 (The aspect ratios for C. mantelli were 4.3 and/or 4.9 depending on the type of angle used). AR = 4.3 PCL = -0.09195 + 0.8535 ⋅ TL PCL = -0.09195 + 0.8535 ⋅ (640) PCL = 546.14805 Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(PCL) + 0.3478 ⋅ Log10(AR) Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(546.14805) + 0.3478 ⋅ Log10(4.3) Log10(546.14805) = x1 x1 = Log(546.14805)/Log(10) x1 = (2.73731038736)/(1) x1 = 2.73731038736 Log10(4.3) = x2 x2 = Log(4.3)/Log(10) x2 = (0.6334684556)/(1) x2 = 0.6334684556 Log10(Sb) = -0.0659 + 0.6196 ⋅ (2.73731038736) + 0.3478 ⋅ (0.6334684556) Log10(Sb) = 1.84995107 Sb = 101.84995107 Sb = 70.7866028 Sb ≈ 71 km ⋅ h-1 Sb ≈ 44 mi ⋅ h-1 My result is a burst swimming speed of 70.8 km/h, 71 km/h when using proper rounding. Sort of off, but considering that the aspect ratio could have been different, I think this is good enough. But Ferron (2017) only used Shimada (2006)'s conservative estimates. Here, I redid the equation but used the maximum size of 700 cm as total length (TL) and a mean estimate of 4.6 for aspect ratio (AR)- AR = 4.6 PCL = -0.09195 + 0.8535 ⋅ TL PCL = -0.09195 + 0.8535 ⋅ (700) PCL = 597.35805 Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(PCL) + 0.3478 ⋅ Log10(AR) Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(597.35805) + 0.3478 ⋅ Log10(4.6) Log10(597.35805) = x1 x1 = Log(597.35805)/Log(10) x1 = (2.7762347206)/(1) x1 = 2.7762347206 Log10(4.6) = x2 x2 = Log(4.6)/Log(10) x2 = (0.6627578317)/(1) x2 = 0.6627578317 Log10(Sb) = -0.0659 + 0.6196 ⋅ (2.7762347206) + 0.3478 ⋅ (0.6627578317) Log10(Sb) = 1.8847622067 Sb = 101.8847622067 Sb = 76.6941443789 Sb ≈ 77 km ⋅ h-1 Sb ≈ 48 mi ⋅ h-1 My results show that a C. mantelli that measures 7 meters in length can swim at a bust speed of almost 77 km/h. That is essentially faster than any ocean animal except some of the billfishes according to popular estimates. Amazingly, this may possibly not be too farfetched. It has already been predicted for a long time that C. mantelli is likely a fast swimmer, potentially one of the fastest. Shimada (1997) showed that Cretoxyrhina possess a scale pattern that has superb efficiency in drag reduction (such patterns have only been found in the fastest Lamnids today). Kim et al. (2013) noted that the caudal fin of Cretoxyrhina represents the most extreme case of a Type 4 fin, where both the Cobb's and hypochordal ray angles are higher than any known shark. Type 4s are ones that are the most efficient in building speed, so maybe the "most Type 4" of them all could be the fastest? For comparison, I also applied the equation to Carcharodon carcharias, which is most similar with C. mantelli in morphology and ecological role (I assumed that the largest confirmed length is 610 cm) AR=2.7* PCL = -0.09195 + 0.8535 ⋅ TL PCL = -0.09195 + 0.8535 ⋅ (610) PCL = 520.54305 Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(PCL) + 0.3478 ⋅ Log10(AR) Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(520.54305) + 0.3478 ⋅ Log10(2.7) Log10(520.54305) = x1 x1 = Log(520.54305)/Log(10) x1 = (2.7164566524)/(1) x1 = 2.7164566524 Log10(2.7) = x2 x2 = Log(2.7)/Log(10) x2 = (0.4313637642)/(1) x2 = 0.4313637642 Log10(Sb) = -0.0659 + 0.6196 ⋅ (2.7164566524) + 0.3478 ⋅ (0.431367642) Log10(Sb) = 1.7672462077 Sb = 101.7672462077 Sb = 58.5121703926 Sb ≈ 59 km ⋅ h-1 Sb ≈ 36 mi ⋅ h-1 As far as I know, the fastest recorded speed for a great white is about 56 km/h. Considering that they probably have been clocked from smaller, more average individual, this calculation seems reasonable. However, if I input the equation to Isurus oxyrinchus using the 2013 Huntington Beach catch record of 373 cm for total length (TL)- AR=3.3* PCL = -0.09195 + 0.8535 ⋅ TL PCL = -0.09195 + 0.8535 ⋅ (373) PCL = 318.26355 Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(PCL) + 0.3478 ⋅ Log10(AR) Log10(Sb) = -0.0659 + 0.6196 ⋅ Log10(318.26355) + 0.3478 ⋅ Log10(3.3) Log10(318.26355) = x1 x1 = Log(318.26355)/Log(10) x1 = (2.502786903)/(1) x1 = 2.502786903 Log10(3.3) = x2 x2 = Log(3.3)/Log(10) x2 = (0.51851399)/(1) x2 = 0.51851399 Log10(Sb) = -0.0659 + 0.6196 ⋅ (318.26355) + 0.3478 ⋅ (0.51851399) Sb = 102.086400122 Sb = 122.0113188 Sb ≈ 122 km ⋅ h-1 Sb ≈ 76 mi ⋅ h-1 -I get a calculated burst speed of 122 km/h, completely tearing the "Cretoxyrhina the fastest shark" claim to shreds. Mako sharks already are the fastest of all extant sharks, but having a calculated speed of this much is mind-blowing. As far as I know, the fastest recorded speed for a mako shark (and therefore any shark) is about 74 km/h, but my calculation is higher than that of any ocean animal ever recorded except the sailfish whose swimming speeds is up to 130 km/h according to some sources. However, it is possible that I either messed up the math, this equation might not work on smaller fishes, or this equation calculates the dire, dire fastest speed of a large fish. If the latter is true, I'm really eager to obtain an aspect ratio for the sailfish and run it through the equation and see if it tops the peregrine falcon. What do you guys think? *AR was obtained from Ferron (2017) EXTRAS

This has been bugging me for some time now, and I can't find any papers that answer these questions. I'm honestly surprised at how little research seems to have been done about these topics. 1. How fast were they compared to some modern-day benthic animals? Were they slow and cumbersome, or were they fast and agile? How did this change through time/environment? I assume that the earlier, Cambro-Ordovician trilobites would have been a fair bit slower than their later counterparts due to a lack of nektic predators that could easily pierce their armor, but this is just my assumption. 2. Did fish actually actively predate on them in any significant capacity? I often see it said that the rise of fish throughout the middle Paleozoic is a reason for the Trilobite's demise, due to increased predation, but I've never actually seen any significant evidence for this. I find it hard to imagine how Palaeozoic fish could have effectively preyed on them - their morphology just doesn't seem to have been suited to "breaking their defenses", so to speak. Even modern day fish seem to rarely actively hunt animals like crabs, which are somewhat analogous to the trilobites in terms of morphology and ecology. 3. Was their post-Devonian terminal decline due to environmental elimination (mainly, the destruction of the Tabulate-Stromatoporoid reefs which they seem to have inhabited in significant numbers), or due to something else?