Tentaculites vs orthoconic cephalopod

[Darbi]

How do you distinguish between tentaculites and orthoconic cephalopod fossils? 

[Fossildude19]

Tentaculites only range in size from 5 mm to 20 mm or so. 

Orthocones can be as long as 17 feet or as small as about an inch. 

 

Tentaculites usually have uneven spacing in their ribbing, where as orthocones are pretty symmetrical and evenly spaced. 

 

Tentaculites: 

From HERE                                                                                 From HERE:

                            

[Darbi]

@Fossildude19, thank you! I suspected that the ribbing had to do with differentiating between both. I searched for topics on here on Tentaculites before I asked that question and there were several members being confused with a large tentaculite for a small orthocone.

For large Tentaculites that are tricky and easily mistaken for small orthocones to the untrained eyes, what's the best ways to identify them correctly?

[Fossildude19]

Well, it helps to know what is found at certain sites. Some sites don't have records of orthocone cephalopods. So that leaves Tentaculites. 

Generally though, I find that Tentaculites seem to be much thinner than orthocones. Other than that, I've got nothing.  

[Tidgy's Dad]

Also, of course, tentaculites are pretty much hollow, without any internal structures to speak of, whilst straight shelled cephalopods have septa and a siphuncle. 

[TqB]

1 minute ago, Tidgy's Dad said:

Also, of course, tentaculites are pretty much hollow, without any internal structures to speak of, whilst straight shelled cephalopods have septa and a siphuncle. 

Although tentaculitids do have septa in the juvenile part of the shell...

Hard to find a good image though and I think they're very small at that stage. This is x50 on the original page - roughly 10 cm tall image, so specimen about 2mm long.

 

[Tidgy's Dad]

1 hour ago, TqB said:

Although tentaculitids do have septa in the juvenile part of the shell...

Hard to find a good image though and I think they're very small at that stage. This is x50 on the original page - roughly 10 cm tall image, so specimen about 2mm long.

Indeed. Good info but I've never seen this personally. 

Tis why I said 'pretty much hollow', I should have explained further. 

Thanks. 

[TqB]

Just now, Tidgy's Dad said:

Indeed. Good info but I've never seen this personally. 

Tis why I said 'pretty much hollow', I should have explained further. 

Thanks. 

You were accurate. I'm just being pedantic - I've never seen it either.

[Fossildude19]

@Tidgy's Dad  @TqB

 

Good information, gents!

Thanks for that. 

Cheers!

[Darbi]

@Fossildude19, @Tidgy's Dad, @TqB, that was informative. Thank you!