I just bet myself that everyone on the forum can name more than 20 types of headless animals.
A clue. At least one species of headless animal can weigh more than 200 kg.
This came to me when I realised that headless animals probably evolved from animals with heads – on at least two occasions. Either that or similar looking heads evolved independently multiple times.
mollwollfumble said:
I just bet myself that everyone on the forum can name more than 20 types of headless animals.A clue. At least one species of headless animal can weigh more than 200 kg.
This came to me when I realised that headless animals probably evolved from animals with heads – on at least two occasions. Either that or similar looking heads evolved independently multiple times.
Cnidarians and Echinoderms .. = a few.
roughbarked said:
mollwollfumble said:
I just bet myself that everyone on the forum can name more than 20 types of headless animals.A clue. At least one species of headless animal can weigh more than 200 kg.
This came to me when I realised that headless animals probably evolved from animals with heads – on at least two occasions. Either that or similar looking heads evolved independently multiple times.
Cnidarians and Echinoderms .. = a few.
Keep going.
mollwollfumble said:
roughbarked said:
mollwollfumble said:
I just bet myself that everyone on the forum can name more than 20 types of headless animals.A clue. At least one species of headless animal can weigh more than 200 kg.
This came to me when I realised that headless animals probably evolved from animals with heads – on at least two occasions. Either that or similar looking heads evolved independently multiple times.
Cnidarians and Echinoderms .. = a few.
Keep going.
I’m really more busy with other things to use my grey cells on this at this moment.
What’s your definition of head?
Witty Rejoinder said:
What’s your definition of head?
Different to foot when put in mouth?
Thanks Moll.
Mike.
roughbarked said:
Witty Rejoinder said:
What’s your definition of head?
Different to foot when put in mouth?
can’t they be a bit feetish
Witty Rejoinder said:
What’s your definition of head?
Now THAT is a ticklish question.
But first, let’s go back to the Cambrian and beyond and see which animals do and do not have heads.
Have I got this right?
Sponges – headless
Placozoa – headless
Siphonophore (eg. portuguese man o war) – headless
Bryozoa – headless
Flatworm – head
Roundworm (nemotode) – head
Other types of worms (annelid, peanut worm, etc.) – head
Echinoderm (starfish, urchin, crinoid, etc) – headless
Cnidarian (coral, anemome, jellyfish) – headless
Ctenophore (comb jelly) – headless
Arthropod – head
Tunicate – head in larval stage but lost as an adult
Chordate – head
Gastropod and cephalopod – head
Brachiopod and bivalve – headless.
The 200 kg headless animal I referred to is the giant clam, a bivalve.
Another contender for the heaviest headless animal would be a sponge, sponges have been found with sizes up to that of a minivan. https://www.noaa.gov/news/scientists-discover-largest-sponge-known-during-deep-sea-exploration
So, how does the presence of a head or headlessness tie in with the evolutionary tree of life?
We can take sponge, placozoa and siphonophore as ancestral – headless.
The head first developed in the flatworms, and continued on to the roundworms and other types of worms.
But then the head was lost as these evolved into echinoderm, cnidaria, ctenophore.
The other headed creatures evolved from the worms as arthropod, tunicate, chordate.
The head was lost again in the adult tunicate.
So far, the head has evolved once and been lost twice.
But what of the molluscs? Being bilateria, they would have avoided the loss of head of the echinoderms and their relatives.
Wikipedia has a picture of the hypothetical ancestral mollusc, shown below. This mollusc has a “distinct head”. A head that was later lost in bivalves and brachiopods, but not in gastropod and cephalopod.The head is also retained in chitons.
So, to summarise, the ‘head’ as a feature of animals, only needed to evolve once in the tree of life, in flatworms. But was later lost by three separate groups of animals. First by the group that includes echinoderm, cnidaria and ctenophore. Second by the group that includes brachiopod and bivalve. Also by adult tunicates. Is that correct?
Bubblecar said:
LOL
Bubblecar said:
ah, the Australian government, a headless dinosaur
Mysterious headless animal on a tree branch.
https://www.timesnownews.com/the-buzz/article/mysterious-headless-animal-on-a-tree-branch-turns-out-to-be-a-croissant/745745
fsm said:
Mysterious headless animal on a tree branch.https://www.timesnownews.com/the-buzz/article/mysterious-headless-animal-on-a-tree-branch-turns-out-to-be-a-croissant/745745
Looks like I was wrong about ctenophora and cnidaria. They didn’t lose their head because they never had one.
.
Perhaps also wrong about the hemichordata. Although they have something that looks like a head, it contains no mouth, eyes or sensory organs. Instead, what looks like a head contains the kidney and heart. As the echinoderms and hemichordata form a taxonomic group, they could be relics of a headless ancestral bilateria rather than an inbstance of an animal losing its head. Reducing the number of losses of head in the ancestral tree to two.
