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Author Topic:   Fossils are all dead – the myth of living fossils
caffeine
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Message 1 of 4 (866388)
11-10-2019 1:47 PM


There are no living fossils.

Some may find this statement a little hyperbolic, arguing that morphological stasis does indeed exist. But I would argue that the concept is much better abandoned; for two very simple reasons. One, that stasis in certain features does not mean stasis in everything, and the focus on what is unchanged causes people to totally miss all that has changed. Two, and linked to this, is that the concept leads people to fundamental misunderstandings of the fossil record and the history of life.

I was inspired to write this topic precisely because the sort of misunderstanding that I'm talking about is evident both in Faith's recent topic about the presence of different fossil groups in the record and in mike the wiz's topic proposal.

Describing something as a 'living fossil' is often simply a declaration of ignorance of the diversity of a group of organisms. Organism x is the only living representative of group y, and group y have been around for 200 million years! Therefore x is a living fossil.

One of the first Google results I got for living fossil, a list on the website Mother Nature Network, exemplifies this misunderstanding in its choice of living fossils; going so far as to include the platypus. The platypus is often described as primitive, but this is nonsense – it's not bizarre because it's primitive. It's bizarre because it's weird and extremely derived. MNN informs of 'platypus-like fossils' date back 167 million years; but this is falsely equating 'platypus' with 'monotreme'. The platypus fossil record is much more recent and only consists of teeth. We don't know when the various unique features of the platypus appeared; but there's no evidence they're especially old.

You may protest that this is a just a particular case of an amateur journalist's ignorance; but such misunderstandings are rife throughout descriptions of living fossils – even in the professional literature. Consider a classic case – the coelacanth.

Note the below image of reconstructed body shapes of living coelacanths and their fossil relatives (taken from Casane, D., & Laurenti, P. (2013). Why coelacanths are not “living fossils.” BioEssays, 35(4), 332–338).

Aside from the size (modern coelacanths are large compared to most, but not all, fossil species), modern coelacanths (that's Latimeria, at the bottom) have completely different fin configurations to most; presumably related to their bottom-dwelling lifestyle, and a very different skull shape. Latimeria's fin bones are almost symmetrical, unlike the configuration of even known fossil crossopterygian. Fossil coelacanths all have ossified swim bladders – Latimeria does not.

Coelacanths are not an example of morphological stasis. They're more similar to fossil coelacanth's than anything else alive because they are coelacanths; but they're not fossil coelacanths. They're a new, modern, derived form of an ancient and diverse group. Like pretty much everything else on earth.

I could write more, but this probably far too long for an OP already, so I'll leave you with this lovely collage from Darren Naish, showing artists impressions of various members of another group known, incorrectly, to have remained unchanged for many millions of years.


  
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Message 2 of 4 (866390)
11-10-2019 3:08 PM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Fossils are all dead – the myth of living fossils thread in the Proposed New Topics forum.

  
1.61803
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Message 3 of 4 (866445)
11-11-2019 1:10 PM


***The Nautilus has just entered the chat***

"You were not there for the beginning. You will not be there for the end. Your knowledge of what is going on can only be superficial and relative" William S. Burroughs

Replies to this message:
 Message 4 by caffeine, posted 11-12-2019 3:19 PM 1.61803 has acknowledged this reply

  
caffeine
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Message 4 of 4 (866559)
11-12-2019 3:19 PM
Reply to: Message 3 by 1.61803
11-11-2019 1:10 PM


***The Nautilus has just entered the chat***

The nautilus? Which nautilus do we have the honour of addressing? There are between 3 and 6 species of nautilus alive today; depending on your opinion. Now, you may protest that living nautilus are all very similar to each other - and you would be right, they are. But this is not because they're preserved some ancient body plan over aeons. It's because modern nautilus are an extremely recent radiation - they're very closely related to one another. What we're doing with nautiluses when we call them a classic fossil is the same thing we're doing with platypuses - we're taking an isolated survivor of an ancient group and declaring it unchanged simply because nothing else has survived - which makes no sense.

Now, we do have fossil nautilus, and you're probably thinking that they look exactly the same as modern nautilus do, so the argument doesn't apply. But were fossil nautilids the same as the living ones? What about this one?

Or perhaps this?

The variation of shell shape is much larger than that; I'm restricting myself to those classified in the order Nautilida. If we're including everything more closely related to nautilus than anything else living, we can go wild:

Now, you might think I'm being a bit disingenuous. Sure, there was great variety among extinct nautiloids, but there were also types of nautilus shell in the late Cretaceous that have very similar designs to living ones.

There is more to a nautilus than its shell though; and we don't have evidence that what was inside the shell looked the same. We have little to no evidence of the soft body parts of extinct nautilids. That means we don't have evidence they were different, but assuming they were the same - the way 'living fossil' thinking leads us - causes us to make all sorts of unjustified assumptions about evolution. It leads us to think thoughts like this:

quote:
Contrary to other cephalopods having no more than eight (octopuses) or ten (squids, cuttlefish) tentacles, nautilus own a many – about 90 – of them.

That's from the website of the German Malacazoological Society, under their list of things which make Nautilus living fossils. Note the unspoken assumption that the things which make nautilus different from other cephalopods are, by definition, the ancestral condition. After, nautilus are living fossils, as everyone knows.

But is that so? In this case, probably not. If you watch the early embryological stages of development of a nautilus, it begins by developing ten arm buds, like a squid. Only later in development do the buds separate into the many tentacles of the adult. The squidlike condition is much more plausible as the ancestral one.

The idea that nautilus are living fossils rests on only two things:

They have no close living relatives.
Shells like those of modern nautilus evolved long ago.

We have no basis whatsoever to assume stasis in the evolution of these creatures life histories, biochemistry or soft body anatomy.

Edited by caffeine, : No reason given.


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 Message 3 by 1.61803, posted 11-11-2019 1:10 PM 1.61803 has acknowledged this reply

  
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