Nature playing tricks on me, or could it be?

[RCD]

This rock caught my eye in a stream bed (SF East Bay area, CA, USA), so I took a couple pics and left it. Okay, my mind is blown how the curvatures of this match the femur head of various ancient beasts. Even the different sides seem to match, but it can't be right? Just a rock right? Do you think its worth going back to get some better pics? Thanks in advance. 

 

P.S. - Sorry no scale reference, it was maybe 6 inches across. 

[DPS Ammonite]

It has no bone texture; it is a well worn piece of silt to sandstone. BTW, nice piece of red Franciscan jasper underneath.

[Rockwood]

On the bright side, you didn't already lug it home. 

[Ludwigia]

Rocks can do amazing things, as I'm sure you are learning.

[digit]

5 hours ago, RCD said:

Even the different sides seem to match, but it can't be right?

Yup. Nature can be quite a trickster leaving suggestively curved rocks lying around like that.

 

Part of being a successful fossil hunter is being able to spot an interesting find when it is partially buried/hidden from just the hint of a suggestive portion of the fossil. Developing a well honed search image of the types of fossils that occur in an area is key to productive fossil hunting. Shape is one big clue to spotting fossilized bone but texture is possibly more important. Most fossil bones tend to be fragmented and so quite often shape can be obscured (or in this case mimicked). Some aspect of bone texture generally remains long after the bone has turned to stone. The outer smooth cortex surface of the bone generally has a distinctive texture and in longer bones a direction to the "grain". The inner portion of the bone has a spongy texture (cancellous bone). This is usually quite evident at the ends of broken fragments of bone.

 

The other thing to consider is if the rock in question shares a texture with the other rocks found nearby. This rock likely had a grainy sandstone texture similar to the other rocks in the stream. Sandy texture to a "bone" is usually a strong contraindication for it being a fossil. Similarity to other rocks in the area should suggest to you that this rock only differs by its novel shape and nothing more.

 

We all pick up suggestive rocks while out hunting. It's our way of learning and improving our search image. You were correct in being hesitant and seeking opinions here on the forum. Your gamble in not lugging this rock home paid off this time.

 

I know little (practically zero) about the potential fossils in the Bay Area. You might try to search in the internet for geologic maps for your area. Knowing the age of the rocks exposed where you are fossil hunting can let you know what types of fossils are possible to find.

 

 

Cheers.

 

-Ken

[RCD]

9 hours ago, DPS Ammonite said:

It has no bone texture; it is a well worn piece of silt to sandstone. BTW, nice piece of red Franciscan jasper underneath.

 

Thanks. And thanks for pointing out the jasper, always wondered what that is and now I know. 

[RCD]

3 hours ago, digit said:

Yup. Nature can be quite a trickster leaving suggestively curved rocks lying around like that.

 

Part of being a successful fossil hunter is being able to spot an interesting find when it is partially buried/hidden from just the hint of a suggestive portion of the fossil. Developing a well honed search image of the types of fossils that occur in an area is key to productive fossil hunting. Shape is one big clue to spotting fossilized bone but texture is possibly more important. Most fossil bones tend to be fragmented and so quite often shape can be obscured (or in this case mimicked). Some aspect of bone texture generally remains long after the bone has turned to stone. The outer smooth cortex surface of the bone generally has a distinctive texture and in longer bones a direction to the "grain". The inner portion of the bone has a spongy texture (cancellous bone). This is usually quite evident at the ends of broken fragments of bone.

 

The other thing to consider is if the rock in question shares a texture with the other rocks found nearby. This rock likely had a grainy sandstone texture similar to the other rocks in the stream. Sandy texture to a "bone" is usually a strong contraindication for it being a fossil. Similarity to other rocks in the area should suggest to you that this rock only differs by its novel shape and nothing more.

 

We all pick up suggestive rocks while out hunting. It's our way of learning and improving our search image. You were correct in being hesitant and seeking opinions here on the forum. Your gamble in not lugging this rock home paid off this time.

 

I know little (practically zero) about the potential fossils in the Bay Area. You might try to search in the internet for geologic maps for your area. Knowing the age of the rocks exposed where you are fossil hunting can let you know what types of fossils are possible to find.

 

 

Cheers.

 

-Ken

Yup, sandy/non-bone-like texture is why I didn't bring it home ;-) Thanks for the thoughtful and helpful reply. Much appreciated. RCD

[Brandy Cole]

@RCD I've been hunting for around 2 years now and a piece similar to that may still fool me now and then until I get it home and wash it off.