C. Carcharias & Isurus Hastalis (xiphodon)

[fossilselachian]

Just add some serrations and morph into a new species.

[Auspex]

That's pretty cool! To me, it begs the question "what change in the prey base led to the adaptation?". (This question is just as valid if it is a case of species replacement rather than species derivation).

[Guest bmorefossil]

wow very nice and fresh teeth

[Boesse]

Thats a really interesting question, one which I've thought about quite a bit. Right about the same time serrations develop, C. megalodon goes extinct, several groups of cetaceans go extinct, there is massive turnover in the pinniped fauna in the Pacific (i.e. Desmatophocids, or pseudo-sea lions, go extinct, and true sea lions/fur seals and walruses evolve and some convert to molluscivory). Much of the food chain more or less gets restructured at that point in time (between 5 and 8 Ma).

Bobby

[jbstedman]

Thats a really interesting question, one which I've thought about quite a bit. Right about the same time serrations develop, C. megalodon goes extinct, several groups of cetaceans go extinct, there is massive turnover in the pinniped fauna in the Pacific (i.e. Desmatophocids, or pseudo-sea lions, go extinct, and true sea lions/fur seals and walruses evolve and some convert to molluscivory). Much of the food chain more or less gets restructured at that point in time (between 5 and 8 Ma).

Bobby

Fascinating topic, but I'm really confused about this and need some explanation. Here's what confuses me. C. megalodon certainly appeared before 5-8 MYA and was serrated at the outset. So, I'm not sure how serration is associated with it, in particular, going extinct. Further, there are serrated and non-serrated species that apparently co-existed in the same environment. It's not as though the only successful sharks were the ones with serrated teeth, or the ones with smooth teeth.

[Auspex]

Fascinating topic, but I'm really confused about this and need some explanation. Here's what confuses me. C. megalodon certainly appeared before 5-8 MYA and was serrated at the outset. So, I'm not sure how serration is associated with it, in particular, going extinct. Further, there are serrated and non-serrated species that apparently co-existed in the same environment. It's not as though the only successful sharks were the ones with serrated teeth, or the ones with smooth teeth.

I always look to a natural model for insight. Look at the variety in bird's beaks, for example. They differ primarily to exploit different food resources, even to the extent that some species are sexually dimorphic (reducing competition between the sexes for resources). When related critters are different, they probably make a living in different ways, and when unrelated critters share similarities, they share resources. At close range, it is too convoluted to see. "Forest for the trees" and all that. When you step back for a wider view, some of the details become invisible. Things persist because they work; when they no longer work, they cease to be. The Devil is in figuring out why.

[Boesse]

"Fascinating topic, but I'm really confused about this and need some explanation. Here's what confuses me. C. megalodon certainly appeared before 5-8 MYA and was serrated at the outset. So, I'm not sure how serration is associated with it, in particular, going extinct. Further, there are serrated and non-serrated species that apparently co-existed in the same environment. It's not as though the only successful sharks were the ones with serrated teeth, or the ones with smooth teeth."

C. megalodon is not in the genus Carcharodon; it is in the genus Carcharocles. Isurus hastalis is the same exact lineage as Carcharodon carcharias, the only difference being the development of serrations in the latter. Serrations appear on C. carcharias teeth between 6 and 7 MYA; the best chronostratigraphic control on this transition is in the Purisima Formation of central California.

Right about this time is when Carcharocles megalodon goes extinct; there are probably no credible occurrences of C. megalodon much younger than this, and it certainly faced extinction not long after the Mio-Pliocene boundary. Outside of the atlantic seaboard (i.e. the Yorktown Fm) there are few other early Pliocene occurrences worldwide (there *may* be some in western Europe). In the pacific realm, C. megalodon appears to be gone by 5 MYA.

Bobby

[isurus90064]

The point Auspex is making is fundamentally relevant. It is essentially asking 'why' not 'how' something evolved into something new. I have often asked myself that exact same question. During the 'great white' evolution there is evidence of pinniped populations that contain individuals of decreasing size over time.

Serrations make a very distinct visual change to the appearance of a tooth but seem to easily develop as a tooth character within Carcarodon (likely a minor diagnostic detail that underscores the nature/relevance of chrono-species' rather than a collection of separate steps in evolution meaning the difference between a splitter and a lumper when it comes to naming species).

Another critical measurement rarely if ever taken is tooth volume (cc's). What used to be called C. sulcidens seems to be a more robust and stout version of the modern great white (also compare to Pleistocene C. carcharias; much thinner and more delicate). A more robust tooth structure does either or both of two things 1. it compensates for the previous! lack of serrations by being bigger, thicker, and essentially stronger teeth, previously required considering its prey (possibly bigger and stronger), 2. and/or serration development and/or prey evolution cause a reduction in tooth volume over time by taking advantage of a more effective cutting surface.

Because it seems relatively safe to assume that prey evolution/prey change (environmental factors etc.) drove morphological changes in GW's, we have to then also assume that serrations were required! in this 'new' situation leading us to believe that primary dietary changes required more effective cutting. You can draw all sorts of conculsion from this such as: prey became more muscular, prey tissues became tougher, bones in prey may have been closer to the skin, less blubber, smaller prey etc. etc.

Marcel.

[isurus90064]

Quick comment, C. megalodon started its long evolution as a chrono species with unserrated Otodus obliquus. From there it kept evolving:

C. aksuaticus (MENNER, 1928) [late Early Eocene]

C. auriculatus (BLAINVILLE, 1818) [early Middle Eocene]

C. sokolovi (JAEKEL, 1895) [late Middle Eocene to Early Oligocene]

C. angustidens (AGASSIZ, 1843) [Early Oligocene]

C. chubutensis AMEGHINO, 1906 [Late Oligocene]

C. megalodon (AGASSIZ, 1835 or 1837) [Miocene-Pliocene]

This "evolution" is entirely based on tooth morphology in terms of increase in size over time, serration count per inch, less serration size gradation (to the tip), less serration variability over time, loss of side cusps over time, and changes in enamel and root design). The problem with this list of species is that you're not just dealing with tooth characters within a single tooth across time, but the same changes take place within a given individual shark from birth to adulthood. F.i. Late Miocene juvenile C. megalodon resembles C. chubutensis because it has side-cusps. Clearly a meg cannot be a chub at the same time. Herein lies the big problem in naming and separating species ... hence the concept of a chrono species.

[jbstedman]

"Fascinating topic, but I'm really confused about this and need some explanation. Here's what confuses me. C. megalodon certainly appeared before 5-8 MYA and was serrated at the outset. So, I'm not sure how serration is associated with it, in particular, going extinct. Further, there are serrated and non-serrated species that apparently co-existed in the same environment. It's not as though the only successful sharks were the ones with serrated teeth, or the ones with smooth teeth."

C. megalodon is not in the genus Carcharodon; it is in the genus Carcharocles. Isurus hastalis is the same exact lineage as Carcharodon carcharias, the only difference being the development of serrations in the latter. Serrations appear on C. carcharias teeth between 6 and 7 MYA; the best chronostratigraphic control on this transition is in the Purisima Formation of central California.

Right about this time is when Carcharocles megalodon goes extinct; there are probably no credible occurrences of C. megalodon much younger than this, and it certainly faced extinction not long after the Mio-Pliocene boundary. Outside of the atlantic seaboard (i.e. the Yorktown Fm) there are few other early Pliocene occurrences worldwide (there *may* be some in western Europe). In the pacific realm, C. megalodon appears to be gone by 5 MYA.

Bobby

Thanks for your response. I certainly wasn't clear in my posting. I, too, believe that Megs and Great Whites aren't from the same genus (though there are a few who would argue that point). Actually, I initially thought you were making a point about the general efficacy of serrated teeth as instruments of predation. Given how this thread started, I should have understood that your point was that the specific appearance of C. carcharias with serrated teeth may have been a precipitating event for the changes you identify. Right, or did I miss it again?

[Auspex]

...that the specific appearance of C. carcharias with serrated teeth may have been a precipitating event for the changes you identify.

Forgive me for butting in...

The evolution of predator & prey is one of "antagonistic symbiosis"; a geeing and hawing of advantage through physical adaptation and behavior modification. A specialized predator that eats all of its prey base will go extinct at the same moment; what usually prevents Armageddon in this arms race is the "loose" genetics of the prey, allowing for comparatively rapid adaptation. (In general, prey species reproduce much more quickly than predators, resulting in more variability in their gene pool. Prey populations will also thus recover much more quickly from a crash). Keep in mind is that, for each step up the food chain, there is a 10 to one ratio: it takes 10 pounds of prey to make one pound of predator. A useful rule-of-thumb, to keep the best perspective on this chicken-and-egg problem, is that (contrary to popular belief) it is the prey population that controlls the predator population, not vice-versa. This sort of "rolling change" will result in the illusion of equilibrium. Then all of a sudden, some environmental factor will change too quickly and upset the apple cart. The dominoes can topple, resulting in extinctions throughout the system. Rate of change vs. adaptive flexibility is the key.

[Boesse]

Marcel: I have never encountered anywhere in the literature the idea that pinnipeds decreased in size during the latest Miocene/Pliocene. I cannot vouch for the Atlantic or the southern oceans at this time, but I can say that in the Pacific pinniped record, one lineage (a fur seal) decreased in size (Thalassoleon --> Callorhinus gilmorei; C. gilmore is approx. 2/3 or 3/4 the size of the former). Otherwise, there are some walrus lineages that do not change perceptively.

One thing that does happen just before this, however, is the adaptation of walruses to molluscivory; indeed, the first molluscivorous pinnipeds appear about this time, and more walruses switch from piscivory (fish eating) to molluscivory during the Pliocene. Molluscivorous pinnipeds need to feed in predominantly shallow water; whereas piscivorous and teuthivorous (squid eating) pinnipeds can pretty much feed anywhere (and preferably down deep, offshore).

There is also a distinct change from the late Miocene to the latest Miocene, and that is the extinction of medium sized long-beaked "kentriodontids", and the proliferation of phocoenids (true porpoises) and delphinids (true dolphins). For example, in California, the majority of middle and late Miocene odontocetes are medium sized, poorly echolocating fish and squid eaters, while in the latest Miocene there becomes an abundance of small bodied fish eaters with excellent echolocating abilities. Additionally, 'river dolphins' such as iniids and platanistids become scarce, and sperm whales so typical of the middle Miocene also tend to disappear, to a degree. Baleen whales also get a whole lot larger.

jbstedman: Ya, we're both on the same page now, I think.

Auspex: I like your reasoning, and I wrote a term paper for my macroevolution class on this subject, and I ended up arguing that this faunal turnover around the Mio-Pliocene boundary is all bottom up change; i.e. predator-prey competition was driven by the evolution of the latter, which was driven by climate change, etc.

My hypothesis (which holds true for the Pacific realm, but needs better data on the first occurrences of C. carcharias and the last occurrences of C. megalodon in the atlantic) is that evolution in baleen whales caused the extinction of C. megalodon, and the void left by C. megalodon allowed C. carcharias to take over its niche, and then opportunistically developed serrations. Thoughts? Otherwise, if C. carcharias developed serrations prior to C. megalodon kicking the bucket, than C. carcharias outcompeted the latter.

And then comes the whole problem of the Globicephalinae (i.e. killer and pilot whales), which appear during the early Pliocene and have a C. megalodon-size macrophagous niche.

Phew!

[Auspex]

Ain't it cool?! You get a flash of insight about "why", and sort of look that direction for new puzzle pieces. Each new piece is studied in a slightly new light and tested in the growing mosaic. In the process, the mosaic is sometimes subtly altered, and all the prior pieces, so carefully lain, must be reexamined. Conclusions can be very fleeting, especially when the top specialists are so into their own few pieces that they can't see the whole picture. It is hard for a Big Picture paleoecologist to defend his theories against attack by offended specialists because he cannot marshal proof in the form of specimens; his is a realm of near infinate interrelationships. It's like fractal geometry, without the comfort of reliable numbers. Darwin didn't get much traction until he grasped the simplicity of "island ecology" in the Galapagos, where there are far fewer pieces strewn over a much shorter time-line.

[siteseer]

This is a very interesting topic which took on a broader scope.

Regarding the first topic, the acquisition of serrations, Marcel and I have talked about this before. I believe fossilselachian and I have discussed it as well in letters back in the ancient times before there was email. Marcel was saying something an ichthyologist also told me. Developing serrations is not a big stretch for a genetic expression especially in rather large teeth. A "ripple" in the fine cutting edge occurs randomly. Anyone with a good pile of fossil mako teeth has noticed a few specimens with an irregularity like that in the cutting edge. Marcel once summed it up something like, "the larger the canvas, the more likely wrinkles will develop." A ripple extended into a few ripples in some teeth of some individuals. Sometimes, the rippled cutting edge went nowhere (dying out with not just the individual but with the taxonomic group), but sometimes, it caught on in a lineage and was carried along many diverging branches over time.

Serrations started developing along the cutting edges of teeth not long after vertebrates developed teeth. You see them in edestid, xenacanth, and ctenacanth sharks and in early synapsids like Dimetrodon. Early carnivorous vertebrates preyed on animals they could swallow whole or could easily pull/tear apart. Serrations allowed teeth to cut or saw into larger prey and then saw their way out, taking a chunk of flesh in one bite and most likely causing a quick and crippling, if not mortal, wound. This would be a much more energy-efficient strike than biting and then having to wrestle the prey with the predator possibly getting injured (or simply fatigued to the point of the prey escaping) during a protracted struggle.

Isurus (or Cosmopolitodus) hastalis had an efficient set of cutting teeth already even being unserrated. The blade was rather thick and very sharp. The dentition allowed the mouth to chop into large bony fishes, other sharks, rays, and marine mammals. However, as anyone who has tried to cut a tough steak with a paring knife knows, a fine cutting edge has its limits. Carcharocles megalodon already had serrated teeth by the time of the appearance of I. hastalis. With its massive teeth it could cut into the bodies of large sharks, pinnipeds, and whales, even sawing through the thickest bones.

The second topic is the question of what was the change in the prey. Boesse answered that but also consider that worldwide climates were warmer in the Early to Middle Miocene - the warmest Earth since the Early-Mid Eocene (30-35 million years earlier). Climates cooled in the Late Miocene heading toward the ice ages. In that time of 5-10 million years ago, and as Boesse noted, the Pacific Ocean lost a group of sea lion-like pinnipeds (desmatophocids, relatives of Allodesmus), many dolphins, and some sperm whales. On top of that, desmostylians die out completely while dugongs also suffer declines in diversity and distribution worldwide. A group of baleen whales loosely-grouped as cetotheres, which appeared in the Late Oligocene, leave fewer fossils in Late Miocene rocks as well.

It is interesting that megalodon and the last cetotheres disappear sometime during the Pliocene. Another large shark, Parotodus, also died out during the Pliocene. It might be said that cetotheres were warm-temperate, shallow-water animals that could not adapt to a world trending toward predominately cool-temperate conditions. C. megalodon, as in many modern sharks, probably had a comfort zone in terms of temperature. With cooling climates various megalodon populations and its longtime prey groups may have become restricted to equatorial regions. With the formation of the Isthmus of Panama, there was no longer a warm-water passage between the Pacific and the Atlantic. This not only isolated marine animal populations (limiting the gene pool on both sides) but also affected warm and cool water ocean currents regionally and to some extent globally.

C. megalodon might have enjoyed a last gasp of survival in the Pliocene, catching the last cetotheres and the occasional newer-type (cool water-adapted) baleen whale as it migrated across the tropics. However, with continued climatic deterioration and a physiology unsuited for cooler water and perhaps insufficient for long bursts of speed, once-peerless megalodon found itself outpaced by new competition: pack-hunting killer whales and Carcharodon carcharias, racier models for a changing world demanding better energy efficiency and versatility of its apex predators .

The favorite prey of I. hastalis may have been desmatophocids and other warm-temperate forms as well but it had an advantage that was likely lacking in megalodon. Isurus and other members of its family (Lamnidae) possessed a circulatory system that could keep the animal running hot even in cool water. It might have been standard equipment for early lamnids of the Eocene - perhaps something that developed to help newborns and juveniles be more energetic during a vulnerable time (their small bodies otherwise always losing heat without an offset) and/or allowed adults to venture into cooler waters avoided by most other large predators. The acquisition of serrations allowed hastalis to prey on the presumably thicker-skinned pinnipeds and small whales that appeared in the Late Miocene and also more efficiently scavenge large whale carcasses (fine-edged teeth would have gotten hung-up more often in thick hides, blubber, and bones), biting out clean chunks of highly-nutritious blubber.

That is how I have understood the hastalis-carcharias transition and the extinction of megalodon. However, as pointed out by Auspex and Boesse, these highly-visible events in the fossil record took place among numerous others less-easily discernible but of more fundamental impact including those at the microscopic level.

The Late Miocene is an interesting time to look at: many groups of organisms of previous epochs became rare or disappeared while others more familiar to us, including our human family, were just emerging.

Marcel: I have never encountered anywhere in the literature the idea that pinnipeds decreased in size during the latest Miocene/Pliocene. I cannot vouch for the Atlantic or the southern oceans at this time, but I can say that in the Pacific pinniped record, one lineage (a fur seal) decreased in size (Thalassoleon --> Callorhinus gilmorei; C. gilmore is approx. 2/3 or 3/4 the size of the former). Otherwise, there are some walrus lineages that do not change perceptively.

One thing that does happen just before this, however, is the adaptation of walruses to molluscivory; indeed, the first molluscivorous pinnipeds appear about this time, and more walruses switch from piscivory (fish eating) to molluscivory during the Pliocene. Molluscivorous pinnipeds need to feed in predominantly shallow water; whereas piscivorous and teuthivorous (squid eating) pinnipeds can pretty much feed anywhere (and preferably down deep, offshore).

There is also a distinct change from the late Miocene to the latest Miocene, and that is the extinction of medium sized long-beaked "kentriodontids", and the proliferation of phocoenids (true porpoises) and delphinids (true dolphins). For example, in California, the majority of middle and late Miocene odontocetes are medium sized, poorly echolocating fish and squid eaters, while in the latest Miocene there becomes an abundance of small bodied fish eaters with excellent echolocating abilities. Additionally, 'river dolphins' such as iniids and platanistids become scarce, and sperm whales so typical of the middle Miocene also tend to disappear, to a degree. Baleen whales also get a whole lot larger.

jbstedman: Ya, we're both on the same page now, I think.

Auspex: I like your reasoning, and I wrote a term paper for my macroevolution class on this subject, and I ended up arguing that this faunal turnover around the Mio-Pliocene boundary is all bottom up change; i.e. predator-prey competition was driven by the evolution of the latter, which was driven by climate change, etc.

My hypothesis (which holds true for the Pacific realm, but needs better data on the first occurrences of C. carcharias and the last occurrences of C. megalodon in the atlantic) is that evolution in baleen whales caused the extinction of C. megalodon, and the void left by C. megalodon allowed C. carcharias to take over its niche, and then opportunistically developed serrations. Thoughts? Otherwise, if C. carcharias developed serrations prior to C. megalodon kicking the bucket, than C. carcharias outcompeted the latter.

And then comes the whole problem of the Globicephalinae (i.e. killer and pilot whales), which appear during the early Pliocene and have a C. megalodon-size macrophagous niche.

Phew!

[Paleoc]

Another large shark, Parotodus, also died out during the Pliocene.

A note on this. Parotodus actually out survived megalodon. I have collected Parotodus specimens at 2 late Pliocene sites (1 Atlantic and 1 Pacific) where Carcharodon was common but Carcharocles was totally absent, not even a fragment.