Are Ichthyosauri Dolphins? I Asked Benjamin Moon

Hans-Georg Lundahl to Benjamin Moon

22/11/13 à 12h48

Ichthyosaurus - Dolphin?

There has been found a fossil with an ichthyosaurus female giving birth to one while having another still in womb.

Any objections more specific than "too old, before dolphins developed"?

I know it has a more pointed snout than most dolphins, but that could be a variable within a kind./HGL

Benjamin Moon to Hans-Georg Lundahl

22/11/13 à 15h10

Re: Ichthyosaurus - Dolphin?

Hi,

Ichthyosaurs are not from the same group as dolphins, they are a more ancient group of reptiles that evolved 250 million years ago, becoming extinct about 90 million years ago. Dolphins are mammals that evolved around 50 million years ago; they would never have met ichthyosaurs. Because of this great difference in their origins, we see many differences, particularly in the makeup of their skull, paddles and their body: most obvious is that dolphins and whales have a horizontal tail fin, whereas ichthyosaur fossils show they had a vertical tail fin (more similar to a tuna’s or shark’s). These differences stem from their different origins and led to different biologies and lifestyles, despite the similarity in their shape.

Ichthyosaurs giving birth to live young (rather than laying eggs) has been known about for about 150 years. Specimens found in the Lower Jurassic (180 million years ago) Posidonia Shale of southwestern Germany have preserved them apparently in the process of giving birth. See this image for a well-known example:

Ichthyosaur giving birth
http://www.lucasbrouwers.nl/blog/wp-content/uploads/2009/10/ichthyosaur-giving-birth.jpg.

Yours,

Benjamin Moon
[email will be disclosed if he wants to]

Ph.D. Student
Palaeobiology and Biodiversity Research Group
School of Earth Sciences
University of Bristol
Wills Memorial building
Queens Road
Bristol
BS8 1RJ

Check out my blog: http://ichthyosaurs.wordpress.com/

Hans-Georg Lundahl to Benjamin Moon

22/11/13 à 17h18

Re: Ichthyosaurus - Dolphin?

So, anatomically speaking, most telling difference is tail fin?

The image seems to indicate the ichthyosaur is pretty squeezed. Are any non-squeezed ichtyosaur fossils repsonsible for the knowledge?

Yours,

Hans-Georg Lundahl
http://creavsevolu.blogspot.com

Benjamin Moon to Hans-Georg Lundahl

26/11/13 à 13h00

Re: Ichthyosaurus - Dolphin?

Far from it. The tail tail fin is just the most obvious thing you would see in a complete fossil, speaking to the more general audience. A few other differences include:

• dolphins have a greater degree of fusion of the skull bones, ichthyosaurs have none but do have bones not present in dolphins
• the jaw joint in ichthyosaurs is made by the articular and surangular whereas in dolphins it is made by the surangular alone
• ichthyosaurs do not have a melon organ, or any indication of one in the skull's form
• ichthyosaur pectoral girdles are not the same as dolphins
• later ichthyosaur paddles are made of more closely packed and simply more bones than dolphins
• dolphins have reduced and lost their pelvic girdles and hind paddles, whereas ichthyosaurs did not lose them
• the tail fin in ichthyosaurs is supported by bones in the ventral part but in dolphins the support remains in the middle only

There are many other differences which require more detailed observation to find, and a better knowledge of both groups. These reflect the vastly different origins of the two groups: the last common ancestor of dolphins and ichthyosaurs would have been around some 330–360 million years ago; this is when we see the separation of the groups that later became what are known as ‘mammals’ and ‘reptiles’.

Yours,

Benjamin Moon
[email will be disclosed if he wants to]

et c. as above

Hans-Georg Lundahl to Benjamin Moon

26/11/13 à 16h02

Re: Ichthyosaurus - Dolphin?

I was actually most conscious of number of bones. In paddle.

As to tail fin in complete fossil, how many of them are there?

Not waiting for your answer I look it up on palaeocritti (I am btw with Nobu Tamura's blessing saving it on a blog, since it is not paid for after 2016) and see that the answer is quite a few:

Ichthyosaurus communis

Holotype lost Neotype: BMNH R1162 Almost complete skeleton at the BMNH London. Numerous complete and partial skeletons

Ichthyosaurus 'intermedius'

Partial skeletons, of which some include skulls.

Ichthyosaurus breviceps

Holotype: BMNH 43006 A complete specimen. Several referable specimens including skull and post cranial material are assignable to this species.

Ichthyosaurus conybeari

Holotype: BGS GSM 956 A beautifully preserved, complete skeleton comprising a skull

= one of the best attested palaeogenera, in my view. I am in South Africa and many Biarmuschians or Gorgonopsids are attested by half a skull of a fragmentary skull plus postcranial elements ...

Thank you for info!

Hans-Georg Lundahl

Since then

I am actually in Belgium on my salvage blog, and the two genera are Iguanadons and Ichthyosaurus:

Palaeocritti Blog : Ichthyosaurus
http://palaeocritti.blogspot.com/2013/12/ichthyosaurus.html

Each name links back to the page on original site:

Palaeocritti - a guide to prehistoric animals
Ichthyosaurus
http://www.palaeocritti.com/by-group/ichthyopterygia-1/ichthyosaurus