What are the differences between marine mammals and land mammals?

[Digi]

In places where you can find both land mammal bone fragments and marine mammal bone fragments, how can you tell the difference? Would marine mammals have denser bones? Would hollow bones (marrow eaten out or other) be an indicator of land mammals?

[FossilNerd]

I think it’s a matter of knowing the skeletal structure of the animals in the area and what the bones look like. A specific bone from one species to another will look a lot different to the trained eye whether it’s land or marine. 

If you are talking about small fragments that you can’t tell exactly which bone it is, that is going to be harder, and may be impossible. The location that the bone was found in may give you a hint. 

 

If you are talking about fossilized bones... Those are the same as any other fossil. Knowing the age of the rock will let you know what type of animals lived during that time and will narrow down your choices. Then it goes back to knowing the bones, what they look like, and things to look for that will tell them apart from other animals. 

 

I don't think that density of the bones is a sure fire way to tell the difference between land and marine mammal. Land and Marine mammal bones can differ in density based on environment, food sources (or lack of), nutrients, and many other factors. They can vary from one individual to another even in the same species. It is hypothesized that marine mammal bones can differ in density greatly depending on diving depth of the animal to help control buoyancy.  This paper speaks about...

Gray, N. , Kainec, K. , Madar, S. , Tomko, L. and Wolfe, S. (2007), Sink or swim? Bone density as a mechanism for buoyancy control in early cetaceans. Anat Rec, 290: 638-653. doi:10.1002/ar.20533

 

Marrow being eaten is a sign of scavengers. Marrow can be eaten out of land and marine mammal bones...

[Digi]

That is incredibly interesting. Are there any ways to date fossils without lab equipment and technology? I’m assuming it would be knowing the area well enough.

[Ludwigia]

4 hours ago, Digi said:

That is incredibly interesting. Are there any ways to date fossils without lab equipment and technology? I’m assuming it would be knowing the area well enough.

The only other way for relatively exact dating is to find the fossil in situ within a specific and ascertainable geological horizon. Otherwise, if found lying around, then as you assume, a knowledge of the geology of the area can at least narrow down the estimate to within a particular geological time frame.

[siteseer]

On 7/19/2019 at 1:12 PM, Digi said:

In places where you can find both land mammal bone fragments and marine mammal bone fragments, how can you tell the difference? Would marine mammals have denser bones? Would hollow bones (marrow eaten out or other) be an indicator of land mammals?

 

Almost always, you get one or the other but you can get land mammal bones in a marine deposit because bones get washed into rivers and out to sea but those actions often wear down or break up bones and teeth.  Collectors have found land mammal remains at the Calvert Cliffs, Lee Creek Mine, and sites for the Sharktooth Hill Bonebed but they are very rare finds.

 

To confidently identify a limb bone piece, or at least eliminate possibilities,  you really need at least one mostly complete bone end.  A piece of skull might be identifiable as land or marine mammal but it has to have enough surface bone to see what part of the skull it was and have enough to get an idea of the size.  If you are talking about bone pieces less indicative of the size, shape, and texture of the complete bone, and as FossilNerd indicated, then maybe a mammal specialist could help but he or she is most likely going to say you need more of it to say more of it.

 

You can't always go by whether a bone is hollow.  Bird bones are hollow but then the shape is going to tell you that it's a bird bone rather than a mammal.  Mammal bones can rot out on the inside while the outside is still intact.