World-famous Sharktooth Hill Bone Bed Finally Explained?

[Guest Inyo]

I posted a variation of the following over at the Newsgroup sci.bio.paleontology the other day.

It's all about that world-famous middle Miocene Sharktooth Hill bone bed in the western foothills

of the California's Sierra Nevada, several miles northeast of Bakersfield:

Over at http://www.sciencedaily.com/releases/2009/...90608131144.htm is an

article detailing recent scientific conclusions about how the world-famous

middle Miocene Sharktooth Hill bone bed near Bakersfield, California, came

to accumulate. Seems that the bone-bed, which contains on average some 200

fossil specimens per square meter (and the bone bed is exposed at the

surface along a ten-mile swath in the western foothills of the southern

Sierra Nevada) developed slowly, gradually, over at most a 700,000 year

period precisely 15.9 to 15.2 million years ago, coinciding with a major

transgressive-regressive marine cycle in what geologists call the ancient

Temblor Sea; probably currents helped divert clastic sedimentation, enabling

the bone-bed to accumulate with a paucity of sands, silts and muds.

The authors conclude that, contrary to many once-popular ideas over the years

the world-famous bone-bed is apparently not the wide-spread

product of a mass dying; neither is it the inevitable result of red-tide poisoning,

nor the remains of animals killed by volcanic eruptions, nor the

preservations of vertebrates through the concentrating action of turbidity

currents (which had been the most popular explanation for many years),

nor the myriad disarticulated remains of loads of critters killed by crazed Megalodons--

not even what's left behind after beaucoup geologic years of whales calving while sharks hunted.

According to the latest multidisciplinary research, the Sharktooth Hill Bone

Bed came about over many thousands of years due to slow, sure, steady bone

accumulations during a period of geologic time when very little clastic

sedimentation (sands and silts and muds) occurred.

http://inyo1.110mb.com/sb/sharkbonebed.html

"A Visit To The Sharktooth Hill Bone Bed, Southern California"

[FossilForKids]

I have always believed this since I've dug there. It has more of a "trash dump" feel to it rather than a mass mortality or feeding pattern. In layman's terms I use to say that if just one shark, sea lion or what ever died once a year in a square mile area and multiply that times about 1 million years you would have alot of remains. In a bay type area 1 death per square mile per year is more than realistic. Not to mention teeth loss by feeding.

[Guest Inyo]

I have always believed this since I've dug there. It has more of a "trash dump" feel to it rather than a mass mortality or feeding pattern. In layman's terms I use to say that if just one shark, sea lion or what ever died once a year in a square mile area and multiply that times about 1 million years you would have alot of remains. In a bay type area 1 death per square mile per year is more than realistic. Not to mention teeth loss by feeding.

Actually, I don't believe any serious scientific investigator has given credence to a mass mortality hypothesis for the formation of the Sharktooth Hill bone bed in many many years; and the obvious lack of juvenile specimens certainly mitigates against a calving area--again, most investigators threw that idea out "eons" ago. Plus, there are no volcanic constituents within the bone bed, so volcanic activity could not have played an obvious role. So, the authors of the recent paper have engaged, really, in some silly strawman arguments, at the very least, suggesting that they've finally "disproved" any number of presently popular theories, when those ideas have long since been discarded as viable possibilities.

The sticking point, the primary area of controversy, surrounds the mechanism that caused the bones to accumulate in a narrow bed to the exclusion of sandstones, silts and muds that comprise most of the middle Miocene Temblor Formation, both above and below the bone bed. The latest research, in that Science article, merely says that there was a 700,000 year period of sediment starvation, or by-pass, in which clastic sediments contributed negligibly to the formation of the bone bed. OK, great. So, what caused that sediment by-pass? The authors claimed it coincided with a major trangressive-regressive cycle. Great. How does that produce a geologic regime of sediment starvation, or so-called by-pass? Transgression of course merely refers to marine waters encroaching on an area where the sea had once receded. But lots of sections in lots of geologic exposures around the world reveal transgressive-regressive cycles of sedimentation without bearing bone-beds, or even major layers of invertebrate fossils, for that matter.

I just don't believe that the authors have adduced enough evidence to throw out the turbidity current hypothesis (the most-favored explanation for over a decade now)--that major underwater currents, periodically, swept carcasses and bones into a subterranean trench, thereby concentrating them through the ages. The trench would naturally have been cut off from normal sedimentation--producing a localized area of bone accumulations. This is key, because sedimentation was not disrupted over the entire paleogeographic area where the Temblor Formation accumulated. Witness the fact that that bone-bed finally pinches out laterally and that strata deposited elsewhere, during the same 700 thousand year period when the bone bed formed, bear no layer of concentrated fossil bones--only barren sandstones, or scattered invertebrate remains, with an occasional, rare marine mammal bone thrown in for good measure.

Anyhow, Sharktooth Hill continues to amaze with its prolific old bones and its prodigious numbers of geologic mysteries.

http://inyo1.110mb.com/latham/latham.html

Trilobites In The Marble Mountains, Mojave Desert, California (the place the first inspired my life-long fascination with fossils and the stories they can tell of us of long-vanished ages).

[Auspex]

Inyo, I applaud the style and presentation of your argument :applause:

This kind of discourse is illuminating, and useful.

[siteseer]

Thanks for the link and your comments. The idea that the bonebed formed over many thousands of years as a result of slow sedimentation is not a new one (e.g. Barnes, 1972: 38) but it appears researchers, including long-time STH guys (Mitchell and Lipps), finally decided to put the catastrophic explanations to rest.

Barnes, L.G. 1972.

Miocene Desmatophocinae (Mammalia: Carnivora) from California. University of California Publications in Geological Sciences Volume 89.

[mommabetts]

Thanks for sharing.

[Boesse]

Hey Inyo,

Understanding the geologic processes involved in Pyenson et al. 2009's article requires a knowledge of up-to-date sequence stratigraphy.

First off, sediment is delivered to the shelf via river systems. The exact nature of the river-marine interface is paramount to understanding sediment starvation. Sediment starvation is caused during a transgression. Basically, during transgressions river mouths form estuaries - during regressions, or during sea level highstands, they form a delta that progrades into the basin.

When river mouths form estuaries, all the sediment gets trapped in the estuary, and very little makes it onto the shelf. This can result in a basin or shelf wide depositional hiatus, and in many cases allows the winnowing of shelf sediments. In most cases a transgressive surface of erosion or transgressive lag is formed. Based on the available evidence, I think the authors are very close to the mark, with the exception of not identifying reworking of bones and transport on the sea floor via transport processes and submarine erosion.

There are several major problems with these fossils being deposited by submarine turbidity currents -

1). There is no sedimentologic evidence (i.e. Bouma sequences, turbidites) of turbidity currents - i.e. no turbidites have been identified within the Round Mountain Silt.

2). The San Joaquin Basin was not a trench, and was deposited above 200 m water depth, which is more or less 'shelfal' in depth.

3). The deep sea still receives sediment - in fact, a LOT of sediment. Submarine fans constitute the second largest depositional environment on earth (the first largest being the basin floor beyond the limits of clastic sedimentation, where suspension fallout of mud-size particles occurs). Submarine fans are enormous, and nearly completely composed of sand/sandstone. As mentioned above, there is no evidence that any deep sea processes are occuring (although I should mention that turbidity currents are known to occur rarely on the shelf - i.e. hyperpycnal flow, although these typically generate a completely different suite of sedimentary structures 'unique' to the shelf, such as hummocky cross bedding).

4). Unlike the shelf, there are very few processes that can form laterally extensive bonebeds along the slope and continental rise. Bonebeds are caused by changes in sea level driving sediment supply, submarine erosion, and local tectonics (i.e. uplift of the seafloor).

5). Invertebrates (shallow water Pectinids) and trace fossils (i.e. Ophiomorpha) indicate shallow shelf waters.

6). IF bones and teeth had been deposited by debris flows, they would show a much different pattern. For example, unless this hypothesis would utilize only one turbidity current, fossils would occur not at a single horizon but at many, each of which would pinch out laterally at a far smaller scale. Complete or partial crania and delicate elements would not occur within these due to the extremely violent sedimentary transport events. Skeletons would especially not occur within the bonebed; the skeletons would become dissassociated.

In any event, the sum of the evidence points to shallow water processes concentrating these elements over a long time span via winnowing and reworking elements from older deposits, with sedimentation rate eventually increasing. Deep water sediment gravity flows (i.e. turbidity currents) would produce a deposit with a completely different taphonomic architecture, and sedimentary structures indicative of turbidite deposition, which have never yet been identified in the Round Mountain Silt.

By the way, you state that the turbidity current hypothesis has been the leading hypothesis over the last 10 years. I haven't seen much mentioned of this in the literature. Where is this published, or is it a footnote in Mitchell's papers? Either way, just because a hypothesis is popular doesn't mean jack - science doesn't progress via popular vote.

Bobby

[FossilForKids]

Bobby

Good logical dissertation on your part but I have a problem with # 6 on two points. The layer where you find skeletons is usually 8-12 feet seperate from the boney layer and that represents what I would think would be a singnificant amount of geological time and possibly the current conditions where different. Additionally when an animal dies, settles on the bottom and then gets covered and starts the fossilization process. It would then be protected from these currents that possibly are depositing debris. There is ver little occurance of invertabrates at least in the Bakersfield area. Only a few shells have ever been found on Bob's land and across the river I have only seen two small shell beds. The Olces Sand has plenty of shells and crabs. Just a thought. I really want to take you out there some time some you can work it as see how it lays out. Another thought you might consider which goes with your last statement. Larry Barnes is like anybody else he makes hypothesis and conjecture based on his best SUBJECTIVE and objection information and experience. He is by no means the end all authority on Sharktooth Hill. STH is an anomaly and like all anomalies they keep their secrets well. By the way I tend to agree with the shallow water processes as well but as related to a near perfect enviromental zone where mammals and large fish thrived and sharks DINED :-)

John

[fig rocks]

Inyo, I love your mind!!!!!!!!!!!!!!!!

[tracer]

i feel that the lack of normal "seafloor" organisms and the presence of a lot of vertebrate material in such an area just kind of forces me to consider that perhaps there was, for whatever reason, a long period in which the lower levels of water in the area did not have enough oxygen to properly support life. in that situation, you wouldn't have shells and gastropods - you would only have the remains of whatever sank and died, was killed and sank, died and sank, etc.

depending on the diagenetic processes at work and the reducing bacteria in the environment, sedimentation rate wouldn't seem to be all that huge a factor in the preservation process, because the normal function of sedimentation in creating a reducing/anoxic environment wouldn't be necessary.

let's face it, if there were no oxygen in your garage, how often would you clean it out?

(i threw that last part in there to hyperbolicize my layperson's perspective, so that when i finally get around to presenting to ya'll a doctoral thesis entitled "our friend, the ammonite", you won't have a conniption fit.)

<singing> sometimes, all i need is the air that i breathe, and to love you...

[Auspex]

That, tracer, is an excellent and thought-provoking observation.

This whole thread has been outstanding.

[Boesse]

I'll post more in response to Tracer's and John's observations/talking points later on tonight. I'm about two hours off from finally submitting my first publication ever.

Bobby

[FossilForKids]

I'll post more in response to Tracer's and John's observations/talking points later on tonight. I'm about two hours off from finally submitting my first publication ever.

Bobby

Congratulations Bobby on you publications!! It's seems like only yesterday we were sifting in Scotts Valley!

John

[tracer]

I'll post more in response to Tracer's and John's observations/talking points later on tonight. I'm about two hours off from finally submitting my first publication ever.

Bobby

be gentle with me - i can't even spell mioseen.

[Auspex]

Congratulations Bobby on you publications!! It's seems like only yesterday we were sifting in Scotts Valley!

John

For all we know, he was

Congratulations Bobby, and "break a leg"!

[Boesse]

Hey Guys,

John:

If you're talking about the skeletons that contain skeletons above the bonebed, then I know what you're talking about. The authors of the article reported that skeletons also occur in the upper part of the actual bonebed itself, and that's what I was referring to.

As far as the invertebrates go, I simply mean by the types of invertebrates that do occur, not by the occurrence of invertebrates themselves (i.e. their ecology, not presence).

As far as Larry Barnes goes - he has seen and studied more marine mammal fossils than probably any other person alive, and he has seen and studied more marine mammal fossils from sharktooth hill than any other person. As far as research on the anatomy and evolution of STH marine mammals goes, he is the world's expert. However, he isn't a taphonomist. He conducted research back in the early 1980's, which is how he ended up on the paper, but most of the research was conducted by Pyenson, the lead author on the study.

Tracer, as far as anoxia goes - anoxic conditions usually produce extremely fine grained, organic-rich, black shale. The Round Mountain silt is fairly 'organic poor' (the obvious exception being the vertebrate fossils). Anoxic 'rocks' usually contain completely articulated skeletons, and as anoxic bottom waters have very little current, no transport or reworking (via submarine erosion) of bones/teeth. Many teeth and bones in the STH bonebed are worn and broken, indicating probable reworking and transport on the seafloor, which pretty much requires submarine erosion (which jives with the sedimentology and trace fossils, all indicative of a shallow marine, highly oxygenated, medium energy environment).

I'm extremely interested in this paper, because my master's thesis is on a similar topic. Let's keep this discussion going; I'm going to be spending my entire summer doing marine bonebed taphonomy, and writing a couple papers on it... I appreciate the discussion, guys! This is my favorite topic in paleontology, even more so than cetacean and pinniped paleo...

By the way, if anyone wants a PDF, pm me...

Bobby

[Boesse]

Thought I'd mention - my paper is on Miocene whale jaws from Scotts Valley. It still has to go out for review, then I'll make the required edits, then resubmit it for publication (many months away). So don't congratulate me yet...

I'm still in Montana right now - I got a job as a teaching assistant for our department's geology field camp - we're off to Dillon MT tommorrow for the final mapping project, doing a Paleozoic thru Cenozoic section at a place called Frying Pan Gulch.

Bobby

[tracer]

"anoxic conditions usually produce extremely fine grained, organic-rich, black shale."

i have not read that the conditions under which anoxia and shale exist must be mutually inclusive. fine clay/silt deposition and compaction may be indicative of a certain water-column depth and stillness that also facilitates retention of anoxic conditions, but i'm unaware of any requirement for those two variables to be present to have anoxia. i am aware that the conditions as you describe were somewhat prevalent apparently in paleozoic and mesozoic strata, but we're dealing (well, you are, i'm staying well east of the issue) with miocenozoic stuff here. you could have all the turbulence in the world in the water, and if the atmosphere above the water is anoxic, then there won't be any oxygenation occurring, which leads me to the secondary and tertiary components of my hypothesis, which i had been reluctant to mention.

oxygenation of air is more difficult than many people realize, particularly in the summers when agricultural fertilizer molecules are wafted by trade winds into the off-gassing from legume cultivars and channelled toward the coast. Did you ever see that movie "transformers"? well, when you get...you know, nevermind, there's no way ya'll are going to be able to follow the rest of this.

suffice it to say that a bunch of hard parts got thrown into that bone bed without enough sediment to account for how they got all collected together mechanically. so you can either come up with mechanical explanations by analogizing how some gravel beds in rivers come to be without a bunch of sediment in the gravels, or you can go with my highly sophiscatated chemical/biological/cosmopolitan theories, which, although somewhat complex, cannot possibly be disproven through earthly means.

your choice, buckos and buckolettes.

<scowling - looking pretty hurt that he can't just be taken seriously for once>

[Boesse]

<scowling - looking pretty hurt that he can't just be taken seriously for once>

Haha, Don't say that Tracer - we are having a serious discussion! Anyway, I'm off to the field, if I can make it to the UM Western Campus today or tommorrow I'll address some of your salient points.

Bobby

[siteseer]

Bobby,

I've often wondered what's been found at Scotts Valley other than the isolated teeth and worn bones normally found. Back in the 80's, I heard about the occasional big find. There's a cast of a dugong skeleton (found in a local sand quarry) hanging from the ceiling of that little natural history museum in Santa Cruz (I like that place).

Jess

Thought I'd mention - my paper is on Miocene whale jaws from Scotts Valley. It still has to go out for review, then I'll make the required edits, then resubmit it for publication (many months away). So don't congratulate me yet...

I'm still in Montana right now - I got a job as a teaching assistant for our department's geology field camp - we're off to Dillon MT tommorrow for the final mapping project, doing a Paleozoic thru Cenozoic section at a place called Frying Pan Gulch.

Bobby

[siteseer]

As a general comment, I think it's great that there is more interest in the STH bonebed these days. It seemed to pick up a 5-6 years ago when Nick Pyenson, Jim Parham, and a couple of others from UC Berkeley visited the area. Recent articles on the pseudodontorn and hardshell sea turtle from there came out of that and a few other abstracts promise more in the near future. I'm just sorry that Bob Ernst didn't live to see all this happening though he knew papers would be coming out (Prothero's co-authored articles regarding magnetic stratigraphy and the land mammals). He would have gotten a kick out of it.

Jess

Hey Guys,

John:

If you're talking about the skeletons that contain skeletons above the bonebed, then I know what you're talking about. The authors of the article reported that skeletons also occur in the upper part of the actual bonebed itself, and that's what I was referring to.

As far as the invertebrates go, I simply mean by the types of invertebrates that do occur, not by the occurrence of invertebrates themselves (i.e. their ecology, not presence).

As far as Larry Barnes goes - he has seen and studied more marine mammal fossils than probably any other person alive, and he has seen and studied more marine mammal fossils from sharktooth hill than any other person. As far as research on the anatomy and evolution of STH marine mammals goes, he is the world's expert. However, he isn't a taphonomist. He conducted research back in the early 1980's, which is how he ended up on the paper, but most of the research was conducted by Pyenson, the lead author on the study.

Tracer, as far as anoxia goes - anoxic conditions usually produce extremely fine grained, organic-rich, black shale. The Round Mountain silt is fairly 'organic poor' (the obvious exception being the vertebrate fossils). Anoxic 'rocks' usually contain completely articulated skeletons, and as anoxic bottom waters have very little current, no transport or reworking (via submarine erosion) of bones/teeth. Many teeth and bones in the STH bonebed are worn and broken, indicating probable reworking and transport on the seafloor, which pretty much requires submarine erosion (which jives with the sedimentology and trace fossils, all indicative of a shallow marine, highly oxygenated, medium energy environment).

I'm extremely interested in this paper, because my master's thesis is on a similar topic. Let's keep this discussion going; I'm going to be spending my entire summer doing marine bonebed taphonomy, and writing a couple papers on it... I appreciate the discussion, guys! This is my favorite topic in paleontology, even more so than cetacean and pinniped paleo...

By the way, if anyone wants a PDF, pm me...

Bobby

[tracer]

Haha, Don't say that Tracer - we are having a serious discussion! Anyway, I'm off to the field, if I can make it to the UM Western Campus today or tommorrow I'll address some of your salient points.

Bobby

bobby - don't say "we" unless you've got an isurus planus in your pocket. i always reserve the right to segue to non sequitur when out-gunned...it's my style, especially when i'm light-headed from hunting in shale...

and by the way, i did some more research, and i think i'm on to something. i've found evidence that an extant mammal's evolution was apparently affected through exposure to the same environmental factors that created the STH bonebed. and the organism's response was both physiological adaptation and future avoidance of that particular environment. please view the link for the evidence....

adaptation at work