Maryland: Miocene Seals Vs. Dolphins

[cowsharks]

In 18 years of collecting the Miocene along Calvert Cliffs, MD, I have only found a few Seal teeth (1 molar and two canines), no seal bone material (that I know of), compared to the hundreds of Dolphin teeth and numerous dolphin bones (verts, phalanges, rostrum psections, etc). I know it is not wise to draw any conclusions from "death assemblages" as a wise fellow collector friend from NJ would say, but I'm stumped as to why the large collecting disparity between the amount of Seal material compared to Dolphin material. A similar bias/disparity exists in my collection of material from Lee Creek, again having only a few Seal teeth/bones, but a vast amount of Dolphin teeth and bones.

So, resisting the temptation to draw conclusions from these "observations", what might be possible explanations for this? Is it possible that there were just more dolphins than seals in the area(s) where I collected the specimens?

It's amazing how much more a fossil Seal tooth is valued compared to a common dolphin tooth. I seem to find a dolphin tooth probably every other outing, and don't even pick up the ones that aren't in at least good or better condition. Yet, I would be so excited to find another seal tooth, especially a molar or piece of jaw bone.

Just curious if anyone has similar observations with regards the amount of Seal vs. Dolphin material they have collected?

Daryl.

[obsessed1]

I too have found way more dolphin material than seal. While I do have a seal humerus and a partial pelvis from the cliffs. I have found that the dolphin to seal material ratio isn't even close to equal. I have wondered if the seals were smaller and became "one bite meals in many cases and therefore less scraps containing bones settled to the bottom to be fossilized. Also the seal bones seem to be less dense and may not be preserved due to that? It will be interesting to here others views on this.

[Auspex]

The simplest answer would be that there were far more dolphins present than seals.

[Boesse]

I had similar results from my master's work in the Purisima Formation:http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0091419

"The simplest answer would be that there were far more dolphins present than seals."

This may seem straightforward, but it ignores the problem of preservation potential. For starters, odontocetes tend to have triple or even quadruple the number of teeth than a pinniped. Since they have differing bone histologies, it's not reasonable to assume they have equivalent preservation potential.

However, amongst the odontocete sample from the Purisima Formation, they consistently exhibited higher frequencies of abrasion, fragmentation, and polish than the pinniped sample - suggesting that they had a somewhat lower preservation potential. Any way, that would indicate that the true ratio would have been even more extremely biased towards high numbers of dolphins.

Lastly, my impression is that pinniped remains are generally fairly uncommon in comparison with cetacean remains. That being said, there is an enormous collection of fossil pinniped bones from Lee Creek (Yorktown Fm.) at the USNM.

[siteseer]

Regarding the Miocene of MD, isn't part of it just that it was a time early to very early in the evolution of seals especially in North America? Seals might be expected to have been not very diverse and uncommon. Whales had been around for more than 20 million years by the early Miocene - good headstart.

Have any of the known sites been considered as near a rookery of the time? Maybe the Pliocene Lee Creek seal remains were from an area nearer to a rookery or is it just that if four hundred collectors each find one seal specimen and donate it to USNM, then USNM ends up with at least four hundred specimens?

In my own west coast collecting, primarily in the STH Bonebed, I haven't found a lot of pinniped bones/teeth (a few phalanges and teeth mostly) either, though Allodesmus (not technically a seal),is considered common in the STH Bonebed. I'm not sure if anyone has proposed a rookery was nearby in that case but juvenile specimens are very rare at best (if any have been found at all).

Jess

In 18 years of collecting the Miocene along Calvert Cliffs, MD, I have only found a few Seal teeth (1 molar and two canines), no seal bone material (that I know of), compared to the hundreds of Dolphin teeth and numerous dolphin bones (verts, phalanges, rostrum psections, etc). I know it is not wise to draw any conclusions from "death assemblages" as a wise fellow collector friend from NJ would say, but I'm stumped as to why the large collecting disparity between the amount of Seal material compared to Dolphin material. A similar bias/disparity exists in my collection of material from Lee Creek, again having only a few Seal teeth/bones, but a vast amount of Dolphin teeth and bones.

So, resisting the temptation to draw conclusions from these "observations", what might be possible explanations for this? Is it possible that there were just more dolphins than seals in the area(s) where I collected the specimens?

It's amazing how much more a fossil Seal tooth is valued compared to a common dolphin tooth. I seem to find a dolphin tooth probably every other outing, and don't even pick up the ones that aren't in at least good or better condition. Yet, I would be so excited to find another seal tooth, especially a molar or piece of jaw bone.

Just curious if anyone has similar observations with regards the amount of Seal vs. Dolphin material they have collected?

Daryl.

[Boesse]

The Calvert Fm. is indeed, IIRC, the earliest occurrence of phocid seals in the NW Atlantic (those Oligocene femora from Charleston don't really count).

[caldigger]

I know that digging Shark Tooth Hill area , the number of Allodesmus (about as close to a sealion as we get here) body parts and teeth far out number any of the dolphin findings. I have however found a few dolphin teeth while on the digs. I have gotten a few of the Allodesmus cheek teeth and have personally yet to find a canine, but people do find them regularly.

[cowsharks]

Thanks for the informative replies folks.

Another thought that occurred to me was just how often I hear fellow collectors talk about yet "another dolphin skull" being found somewhere along Calvert Cliffs. I seem to hear this sort of thing at least a few times a year, and read about some of the excavations by the Calvert Marine museum. I don't recall ever hearing about a seal skull (or skeleton) being found. So, I've always known that the Seal material was rare, just didn't realize how rare compared to the other fossils I find.

When I get a chance I'll post a pic of a partial Seal jaw with teeth that I traded for recently.

Daryl.

[Boesse]

There are two skulls of Leptophoca lenis that have been found: one is a partial skull at the USNM, and the other is a complete skull at the CMM. The latter was described by Irina Koretsky in 2001, and the former was figured in her Ph.D. thesis. A couple of fragmentary skulls are known from the Yorktown Fm. at Lee Creek... and that's almost it for the east coast, as far as skulls go.

[dozer operator]

Does anyone know in more detail of what time in the miocene the fossils from calvert cliffs are?(early middle late). Miocene spanned alonggg time. Im sure there were climate changes there during the miocene. Maybe seals at that time prefered a certain climate which may not have lasted long? Which would mean maybe they didnt thrive for as long as the dolphin in that area at that time. If the dolphin thrived for a much longer period than that seal then that would mean we would find much more bone/teeth from that animal.

[siteseer]

Based on what I read the last time I checked on what is found at the Calvert Cliffs, the base of the cliffs has rocks about 17 million years old (Fairhaven Member of the Calvert Formation) and the rocks range at least to about 12-13 million years old, the base of the Choptank Formation, which overlies the Calvert Fm. That's a range from about early-middle to late-middle Miocene. This is an interesting interval because right about 15 million years ago was the warmest time in the Miocene (second warmest time of the Cenozoic Era - second only to the "hothouse" Early Eocene). Within three million years, climates had cooled noticeably. Forests gave way to expanding grasslands on all the continents except Antarctica. Browsing mammals experienced a decline (many extinct by the end of the Miocene) as grazing forms diversified.

I should add that does not mean that the Calvert is represented from the Fairhaven Member forward (some time unaccounted for at the cliffs but represented elsewhere in the region)..

Does anyone know in more detail of what time in the miocene the fossils from calvert cliffs are?(early middle late). Miocene spanned alonggg time. Im sure there were climate changes there during the miocene. Maybe seals at that time prefered a certain climate which may not have lasted long? Which would mean maybe they didnt thrive for as long as the dolphin in that area at that time. If the dolphin thrived for a much longer period than that seal then that would mean we would find much more bone/teeth from that animal.

[hokiehunter]

Daryl, I subscribe to the rookery explanation/the fact that cetacean's are/were more prevalent. Look no further than today's great white vs. seal relationship. Great whites hunt seals around rookeries like the Farallon Islands, Ano Nuevo, and False Bay. In the open water seals have an advantage over great whites in terms of maneuverability therefore the sharks wait for the seals to get close to the rookeries where they are more concentrated or apt to rest before ambushing.

This isn't to say the occasional ambush doesn't happen out to sea but that the attacks/predation is much more concentrated. Contrast this with a cetacean that swims and is fed upon in much larger less concentrated areas and you can come up with a quick and easy explanation for a lack of fossils in the Calvert formation. I think the fossils are somewhere in the formation in great abundance and maybe one day will be exposed but until we find a rookery or gathering area for miocene seals/sea lions they will remain quite scarce and prized. I have yet to find my first. :-(

Jason