Deviant Echinoid with only 4 ambs

[Bill Thompson]

I found this deviant echinoid.

Only 4 ambs.

Goniopygus whitneyi

Glen rose Formation, Texas

Lower Cretaceous

I thought I would share it.

Edited December 20, 2015 by Bill Thompson

[Coco]

I love it. I have a recent Arbacia punctulata from South Carolina like that but I don't find my pics. I think I will have to do them again...

Coco

[TqB]

Fascinating, I've never seen one do that before.

[sdsnl]

Wow! Is that a special species or is it pathological?

[Foshunter]

Nice, thanks for sharing

Tom

[erose]

That's really cool. i know there are Cincinnatian edrioasteroids that sometimes only have four ambs. And I think I've seen pics of ones with six.

Mutants for a new world!

[Auspex]

Pretty amazing what a little loose wiring in the DNA can do!

[FossilDAWG]

Amazing specimen.

I suspect this is a nice illustration of deterministic development. Obviously, a fertilized egg (= a single cell) will give rise to a whole adult organism through a process in which cells divide and also become specialized to form specific tissues and organs. In many animals, at some fairly early point in the blastoderm stage each cell becomes developmentally fixed so from that point forward it can only give rise to a very specific set of tissues. If a cell happens to die after that point, all the cells/tissues that would have developed from that cell will be missing. Generally that means the animal will die because it is missing its gut or nervous system, but sometimes (depending on the exact structures that end up missing) you can have a viable but incomplete individual. In this case, I suspect a cell that should have generated 1 of the 5 ambulacra and adjacent interambulacral plates died (or it's parent failed to divide).

Don

Edited December 20, 2015 by FossilDAWG

[Coco]

Here is my recent Arbacia punctulata from South Carolina. On the left side is a "normal" sea urchin test with five ambulacra, on the middle is a sea urchin with only four ambulacra, and on the right side you can see one with a not complete ambulacra ! The 2 on the right side are pathological.

Coco

[TqB]

Great specimens, Coco!

I like Don's explanation for a completely missing ambulacrum. What might happen to produce a partial one?

[PaleoWilliam]

Nice!

[Coco]

Great specimens, Coco!

I like Don's explanation for a completely missing ambulacrum. What might happen to produce a partial one?

I don't know, but on this specimen we can see an additional plate directly above the ambulacra which stopped its growth, or at least this plate is modified. is it in touch with the stop of growth of the ambulacra ? It is a mystery for me, but besides this anomaly, I love very much this sea urchin because it still has its central plates.

I specify that sea urchins grow up from the apex (the top).

Coco