Happy Thought for 17 May 2024

Have a Happy Thought:

 

Most of us have heard (and are secretly tickled by) the fact that the ancestors of whales walked on land, then decided to “go back” into the oceans to swim and live their best lives.

 

What I hadn’t heard until recently was that apparently the ancestors of turtles made a similar decision. A few times.

 

It starts with the long-ago ancestors of all four-legged creatures on Earth, which lived mostly in the water.

Then the turtles split off, and their ancestors adapted to living on land.

Then some of the turtles started living in the water again, leading to most of the sea- and pond-turtles we know today.

But then some of those decided to come back onto land, re-joining their distant cousins that we now call tortoises.

         So yes: all tortoises are turtles, but not all turtles are tortoises.

 

Which means that the ancestors of box turtles have gone through ocean -> land -> water -> land. Who knows what they’ll decide next?

 

By the way, how can we tell whether a long-extinct turtle (or turtle ancestor) lived on land?

Some scientists looked at all the living species of turtles today, whether they live on land or in water. They wondered if you could tell ‘environment’ from the shell shape – since most land-based turtles have rounded, dome-like shells; and most water-living turtles have flatter shells, giving them a more stream-lined swimming profile.

Image: Sketchplanations

 

But notice the use of the word “most” – not all turtles follow this rule. Take for example the very flat-shelled, and very appropriately named, pancake tortoise:

Image: Pancake tortoises (Malacochersus tornieri) at the Buffalo Zoo by Dave Pape.

 

Back to the drawing board, where the scientists drew a relationship between the length of the humerus (upper arm bone) and the length of the hand. Basically, they found that short hands are good for walking on land, while long hands are good for swimming. And this is a relationship that does hold up amongst all living turtles.

Image: figure from Joyce WG, Gauthier JA. Palaeoecology of triassic stem turtles sheds new light on turtle origins. Proc Biol Sci. 2004 Jan 7;271(1534):1-5. doi: 10.1098/rspb.2003.2523. PMID: 15002764; PMCID: PMC1691562.

 

Which means that if you’re ever trying to figure out whether a turtle is a tortoise or not, just get in there and measure* its upper arm and hand. Surely it won’t mind.

 

* turtles and tortoises may not have teeth, but they still bite. So don’t do this.