Shark week 4: megalodon tooth colonized by boring clams

November 9, 2022

Alert readers probably noticed that I titled the first post in this series “Matt’s first megalodon tooth“, implying that there would be other megalodon teeth to follow. Here’s my second one.

At first glance, this is a pretty jacked-up megalodon tooth. It is pocked with circular and ovoid craters, and has a big fat hole drilled right through it. Hardly collector grade! And in fact that’s what first caught my attention about this tooth — it’s a 6-incher that was being offered for an enticingly low price. But I got even more excited when I clicked past the thumbnail image on the sale site and saw precisely how this tooth was damaged. This is not random, senseless taphonomic battery (ahem); this tooth was colonized by a bunch of boring clams.


Like Adam Savage — and, I suspect, most collectors-of-things — I am fascinated by objects and the stories that they tell. And this tooth tells several stories. First, it’s a huge tooth from a huge shark, a truly vast, multi-ton animal heavier than a T. rex and longer than my house. Second, it’s a fossil that’s millions of years old, evidence of an extinct species from a vanished ecology, one where gigantic sharks and macroraptorial sperm whales hunted small baleen whales, early seals and sea lions, and manatees and sea cows. And third, it’s a relic of another, entirely different ecology, one in which this shed tooth sank to the sea floor and was colonized by a host of smaller organisms, including most obviously hole-boring clams. In effect, this one tooth was a miniature reef, supporting multiple species of invertebrates. The traces left by those invertebrates are themselves ichnofossils, so this tooth is a body fossil with ichnofossils dug out of it. It’s turtles all the way down!


Can we figure out what any of those invertebrates were? Just a few years ago that would have been a challenging task for a non-specialist, but fortunately in 2019 Harry Maisch and colleagues published a really cool paper, “Macroborings in Otodus megalodon and Otodus chubutensis shark teeth from the submerged shelf of Onslow Bay, North Carolina, USA: implications for processes of lag deposit formation”. That paper is very well illustrated, and the figures basically serve as a field guide for anyone who wants to identify similar traces in rocks or teeth of equivalent age. I will take up that sword in a future post.

Incidentally, this is now the biggest tooth in my little collection, just slightly — but noticeably — bigger than my first megalodon tooth: 157mm on the long side, vs 155mm, and 112mm max root width, vs 107mm.

Bonus goofy observation: I strongly suspect that no other megalodon tooth in the world beats this one in simulating a Star Trek phaser.

Reference

Maisch IV, H.M., Becker, M.A. and Chamberlain Jr, J.A. 2020. Macroborings in Otodus megalodon and Otodus chubutensis shark teeth from the submerged shelf of Onslow Bay, North Carolina, USA: implications for processes of lag deposit formation. Ichnos 27(2): 122-141.

Posted by Matt Wedel
Filed in freakin sharks, hands used as scale bars, ichnofossils, megalodon, stinkin' appendicular elements, stinkin' every thing that's not a sauropod, taphonomy, teeth
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Shark week 2: great white teeth redux

November 6, 2022

Cast (white) and fossil (gray) great white shark teeth, lingual (tongue) sides.

Something cool came in the mail today: a fossil tooth of a great white shark, Carcharodon carcharias. The root is a bit eroded, but the enamel-covered crown is in great shape, and it’s almost exactly the same size as my cast tooth from a modern great white.

The labial (outer or lip-facing) sides of the same teeth.

I got this for a couple of reasons. One, I wanted a real great white shark tooth to show people alongside my megalodon tooth (for which see the previous post). Extant great whites are quite rightly protected, and their teeth are outside my price range when they are available at all. Fortunately there are zillions of fossil great white teeth to be had.

Also, the cast great white tooth has been kind of a disappointment. It’s so white that it’s actually a letdown, visually. Tactilely it’s great, with all kinds of subtle features on the crown especially, but those features are almost impossible to see or photograph. In the photo above, you can make out some of the long, smooth wrinkles in the enamel of the cast tooth, but the median ridge, which is dead obvious on the fossil tooth, only shows up under very low-angle lighting on the cast. The fossil tooth is just a more interesting and more informative specimen, material and origin aside. Now that I have it, I might try either staining or painting the cast tooth, to see if I can rehabilitate it as a visual object.

This fossil tooth is also noticeably thicker than the cast tooth. I don’t know if that’s serial, individual, population, or evolutionary variation. In the last post I contrasted the proportional thinness of the cast tooth with the robustness of the megalodon tooth; this fossil tooth might fare a little better if subjected to the same comparison. I should have thought to do that when I was taking these photos.

Speaking of comparisons, here’s megalodon to remind everyone who’s boss. There’s no scale bar here, but the cast great white tooth is 65mm from the tip of the crown to the tip of the longer root, and the meg tooth is 155mm between the same points.

Now I have a gleam in my eye of assembling a couple of sets of fossil teeth: one to illustrate the evolution of the modern great white from its less-serrated ancestors, like this diagram from the great white Wikipedia page, and one to illustrate the evolution of megalodon from its side-cusped ancestors, like this diagram from the megalodon page — presuming that current hypotheses for the two lineages are accurate. If I ever get either set done, I’m sure I’ll yap about it here.

Posted by Matt Wedel
Filed in freakin sharks, hands used as scale bars, megalodon, stinkin' appendicular elements, teeth
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Matt’s first megalodon tooth

November 5, 2022

 

I got this thing a while back. I’d always wanted one, and it really does spark joy.

First up: what should we call this critter? AFAIK, the species name has never been in doubt, it’s always been [Somegenus] megalodon. That genus has variously been argued to be Carcharodon (same as the extant great white shark, Carcharodon carcharias), Carcharocles, Otodus, Megaselachus, and probably others. From my limited reading, the current consensus seems to be converging on Otodus, for reasons that seem reasonable to me, but I’m hardly an expert on this problem. It’s not that I think it’s unimportant, more that the generic identity of [Somegenus] megalodon has been historically labile, and as a non-expert I hesitate to come down firmly behind any of the hypotheses. If it’s still Otodus megalodon in another decade, I might take a stand. If you want to do a deep dive on this, check out Kent (2018: 80-85). In the meantime, I’m going to refer to it informally as ‘megalodon’, without italics. Although the actual genus name Megalodon was tragically wasted a fossil clam (true story), I’m confident that no-one, scientist or layperson, will misunderstand when I refer to the humongous extinct megatoothed shark as megalodon.

With that out of the way: wow, that’s a big freakin’ tooth! Here it is again with a scale bar.

The serrations on the sides are very cool. The edges are worn a bit in places, and that plus the visible notch on one side of the tooth (upper left in the photo above) suggests that this tooth was used, as opposed to being a replacement tooth that rotted out of the jaw before it ever had a chance to be deployed. Where ‘used’ means ‘used to punch and then tear immense holes in other animals’. Pretty wild to think about ancient whales dying on this very tooth.

I use this thing at outreach events, and I got a cast tooth of a modern great white shark for comparison. Those great white teeth are 10 bucks at Bone Clones, so I got a bunch of them and gave them to nieces and nephews as stocking stuffers.

Here’s a labeled version. From what I’ve been able to determine (i.e., shark people, please correct me if I’m wrong!), most shark teeth ‘lean’ away from the body midline. Upper teeth of megalodon tend to be very wide, with wide, shallow angles at the base, whereas lower teeth are more dagger-shaped and have a more pronounced basal angle. I’m pretty sure this meg tooth is a lower, and we’re looking at the lingual (tongue) side in this photo (more on that in a bit), so the tooth is facing the same way we are. I think that makes it a left lower tooth. The great white tooth is a probably a left upper, although great whites apparently have one tooth position that leans mesially instead of distally, so I could have that one wrong-sided. The ‘bourlette’ is an area of exposed orthodentine between the root and the enamel that covers the tooth crown (Kent 2018: 86). This tooth is not in perfect shape, there’s been some peeling of the enamel just above the bourlette. 

I think this photo makes the size-comparison point even more clearly.

Worth noting: if the hypothesis that megalodon belongs in Otodus is correct, the similarities between megalodon and the great white shark are convergent; megalodon teeth are Otodus teeth that lost their side-cusps, and great white teeth are basically wider, serrated mako teeth. That level of convergence shouldn’t be surprising to anyone who has seen a thylacine skull. Still, this photo makes it very obvious why Louis Agassiz assigned megalodon to Carcharodon, the great white shark genus, when he named the species back in 1843: the two look a lot alike. (Also: Agassiz didn’t have all the transitional fossils that we do now.)

Boomerang thought, added in post: at least, megalodon teeth look a lot like the upper teeth of great whites. The lower teeth of great whites are much narrower and more mako-esque. 

A couple of features worth noting here. The mesial margin has a little wrinkle, which cannot be damage because the serrations follow the in-folded contour. This seems to be a minor developmental anomaly that is pretty common in megalodon teeth. The distal margin has a distinct notch, also mentioned above, which probably represents feeding damage sustained in life.

Arguably this side-view is even more striking; the megalodon tooth is 2.38 times the length of the great white tooth (155mm vs 65mm on the long side), but more than three times as thick (29mm vs 9mm max thickness), and the blade of the tooth stays proportionally thicker over more of its length. This tooth was built to do some work.

Am I fanboying here? Sure, a little (and not for the first time). Giant extinct monsters are exciting, and I’m happy to celebrate that while also wanting to know more about how they lived.

The thing that surprised me the most while reading up on shark teeth is how they are oriented in the jaws. I’d always assumed that the convex faces (toward the bottom of the above photo) faced outward (labial or lip-facing), and the flat faces (toward the top of the above photo) faced inward (lingual or tongue-facing), but it’s actually opposite. In the photo above, the labial or outward faces are up, and the lingual or inward faces are down. I’m sure this is old hat to shark people, but it hurts my head. Most teeth I know of have their convex faces outward, like human incisors and tyrannosaur premaxillary teeth. Plus, instinctively it seems like predator teeth should curve toward the back of the mouth, but with their flat labial faces and convex lingual faces, most shark teeth seem to curve toward the front (I realize that they may have been placed in the jaws so that they still pointed backwards overall). I was so surprised by this that I did a lot of checking before bringing it up in this post, but it’s clear even in really good photos of live great white sharks with their mouths open. There’s no bigger story here, just me confronting my own misapprehension about animal morphology. Still seems weird.

If you want to know more about how megalodon lived, I’ve included links below to some papers on its size (Shimada 2019, Shimada et al. 2020, Cooper et al. 2020, 2022, Perez et al. 2021), breeding habits and life history (Miller et al. 2018, Shimada et al. 2021, 2022), evolution (Shimada et al. 2016, Kent 2018, Perez et al. 2018), and paleobiology (Maisch et al. 2019, Ballell and Ferron 2021, Miller et al. 2022, Sternes et al. 2022). This is a highly idiosyncratic collection based on like one evening of messing around on Google Scholar. I’m sure I missed tons of important work, so feel free to recommend more refs in the comments.

Oh, like virtually everything else on this site, these photos are freely available under the CC-BY license, so if you want to use them, modify them, etc., go nuts.

References

Posted by Matt Wedel
Filed in freakin sharks, hands used as scale bars, megalodon, stinkin' appendicular elements, stinkin' every thing that's not a sauropod, teeth
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