New Tiktaalik paper

[SteveF]

From the latest Nature:

Downs, J.P. et al. (2008) The cranial endoskeleton of Tiktaalik roseae. Nature, 455, 925-929.

Among the morphological changes that occurred during the 'fish-to-tetrapod' transition was a marked reorganization of the cranial endoskeleton. Details of this transition, including the sequence of character acquisition, have not been evident from the fossil record. Here we describe the braincase, palatoquadrate and branchial skeleton of Tiktaalik roseae, the Late Devonian sarcopterygian fish most closely related to tetrapods. Although retaining a primitive configuration in many respects, the cranial endoskeleton of T. roseae shares derived features with tetrapods such as a large basal articulation and a flat, horizontally oriented entopterygoid. Other features in T. roseae, like the short, straight hyomandibula, show morphology intermediate between the condition observed in more primitive fish and that observed in tetrapods. The combination of characters in T. roseae helps to resolve the relative timing of modifications in the cranial endoskeleton. The sequence of modifications suggests changes in head mobility and intracranial kinesis that have ramifications for the origin of vertebrate terrestriality.

http://www.nature.com/nature/journal...ture07189.html

[GenesisNemesis]

I love the words.

[SteveF]

Carl Zimmer has his usual excellent discussion:

http://blogs.discovermagazine.com/lo...-the-ear-bone/

[Ray Moscow]

This even made this morning's International Herald Tribune.

[Per Ahlberg]

It's an interesting and satisfying paper, though not exactly revolutionary. In essence, we knew already that two important changes occur in the skull during the fish-tetrapod transition. One is that a joint that had previously divided the skull into two separate blocks (running vertically, between the eyes and the inner ears) disappears; the other is that the hyomandibula, a bone that supports the gill cover in fish, transforms into a middle ear bone called the stapes.

Previous work had shown the beginning of these transformations in Panderichthys, a transitional form that is slightly earlier and more primitive than Tiktaalik (see: Ahlberg, P. E., Clack, J. A. & Luksevics, E. 1996. Rapid braincase evolution between Panderichthys and the earliest tetrapods. Nature 381, 61-64, and Brazeau, M. D. & Ahlberg, P. E. 2006. Tetrapod-like middle ear architecture in a Devonian fish. Nature 439, 318-321). Primitive versions of the tetrapod condition have been documented in Acanthostega , Ichthyostega and, lately, Ventastega (Clack, J. A. 1998. The neurocranium of Acanthostega gunnari Jarvik and the evolution of the otic region in tetrapods. Zool. J. Linn. Soc. 122, 61-97. Clack, J. A., Ahlberg, P. E., Finney, S. M., Dominguez Alonso, P., Robinson, J. & Ketcham, R. A. 2003. A uniquely specialized ear in a very early tetrapod. Nature 425, 66-69. Ahlberg, P. E., Clack, J. A., Luksevics, E., Blom, H. & Zupins, I. 2008. Ventastega curonica and the origin of tetrapod morphology. Nature 453, 1199-1204).

Tiktaalik proves to slot in very neatly and intermediately between these previously known forms. The material is also better preserved than that of Panderichthys showing us details of the braincase and hyomandibula that cannot be seen in the latter, and revealing a very nicely preserved gill skeleton that is still essentially fish-like.

[Ray Moscow]

[creo mode]Oh, sure. Just how do you evilutionists explain the missing link between Panderichthys and Tiktaalik?!!!

It seems like the more species we find, the more missing links there are that evilution fails to explain.

[/creo mode]

[Per Ahlberg]

Minus points for correct spelling and grammar undermining the verisimilitude of your pastiche.

[Jet Black]

Originally Posted by Per Ahlberg

It's an interesting and satisfying paper, though not exactly revolutionary. In essence, we knew already that two important changes occur in the skull during the fish-tetrapod transition. One is that a joint that had previously divided the skull into two separate blocks (running vertically, between the eyes and the inner ears) disappears; the other is that the hyomandibula, a bone that supports the gill cover in fish, transforms into a middle ear bone called the stapes.

Previous work had shown the beginning of these transformations in Panderichthys, a transitional form that is slightly earlier and more primitive than Tiktaalik (see: Ahlberg, P. E., Clack, J. A. & Luksevics, E. 1996. Rapid braincase evolution between Panderichthys and the earliest tetrapods. Nature 381, 61-64, and Brazeau, M. D. & Ahlberg, P. E. 2006. Tetrapod-like middle ear architecture in a Devonian fish. Nature 439, 318-321). Primitive versions of the tetrapod condition have been documented in Acanthostega , Ichthyostega and, lately, Ventastega (Clack, J. A. 1998. The neurocranium of Acanthostega gunnari Jarvik and the evolution of the otic region in tetrapods. Zool. J. Linn. Soc. 122, 61-97. Clack, J. A., Ahlberg, P. E., Finney, S. M., Dominguez Alonso, P., Robinson, J. & Ketcham, R. A. 2003. A uniquely specialized ear in a very early tetrapod. Nature 425, 66-69. Ahlberg, P. E., Clack, J. A., Luksevics, E., Blom, H. & Zupins, I. 2008. Ventastega curonica and the origin of tetrapod morphology. Nature 453, 1199-1204).

Tiktaalik proves to slot in very neatly and intermediately between these previously known forms. The material is also better preserved than that of Panderichthys showing us details of the braincase and hyomandibula that cannot be seen in the latter, and revealing a very nicely preserved gill skeleton that is still essentially fish-like.

at what sort of stage would it have become important in sound transmission? was it already involved in sound transmission to some degree even in fish?

[damitall]

Don't you fret, Per.

Casey Luskin will set you right!

[SophistiCat]

NYT: Fish Fossil Yields Anatomical Clues on How Animals of the Sea Made It to Land

[Worldtraveller]

Originally Posted by Per Ahlberg

Minus points for correct spelling and grammar undermining the verisimilitude of your pastiche.

And bonus points for the use of pastiche!!

[Per Ahlberg]

Originally Posted by Jet Black

at what sort of stage would it have become important in sound transmission? was it already involved in sound transmission to some degree even in fish?

This is an interesting question. The fish hyomandibula braces against a bony vertical bridge on the side wall of the braincase called the lateral commissure. The "feet" of the lateral commissure attach to the side wall of the otic capsule, which houses the inner ear: this suggests the possibility that vibrations can be transmitted from the hyomandibula to the inner ear, but there is no direct contact so the transmission (if present at all) is probably extremely inefficient. In your standard bony fish, the main job of the hyomandibula is to support the bony gill cover. This is still just about the case in Panderichthys, where there is a point contact between the hyomandibula and the opercular bone (the biggest bone of the gill cover series), but in Tiktaalik the gill cover bones have been lost and the hyomandibula is further shortened compared to Panderichthys. However, in both of these animals the inner end of the hyomandibula still attaches to the lateral commissure.

In the earliest tetrapods such as Acanthostega the lateral commissure has disappeared and the hyomandibula has turned into a stapes. The footplate of the stapes sits in a hole in the wall of the otic capsule and is thus in direct contact with the inner ear. This looks like a much better setup for sound transduction, but the stapes of these earliest tetrapods was big and chunky, and does not seem to have been associated with a tympanum. So how did it work? We don't really know. The "middle ear cavity" of Panderichthys, Tiktaalik and the earliest tetrapods was probably still an open spiracle used for breathing; it may be that the stapes primitively served to stabilise the rear wall of the spiracle, or to serve as the attachment for a valve in the spiracle, and that it became co-opted into a hearing role because it already picked up some sound vibrations in its original function.

[orangeatheist]

Anyone else see here see the word "Tiktaalik" and just speed over it with "Tic-Tac"?

[Steviepinhead]

Tch-tch-tch.

Or is that, "Tut-tut!"?

Say, Per, when you note that--

in Tiktaalik the gill cover bones have been lost and the hyomandibula is further shortened compared to Panderichthys

--does the loss of the gill cover bones, in and of itself (themselves?), tell us anything about the water ==> land transition?

Or, in isolation, not so much, but put into context of the head-neck-fin changes, it fits well into the bigger picture?

[Per Ahlberg]

The latter, I'd say. It fits in with a general, gradual de-emphasising of the gills and creation of a more mobile neck.

[Steviepinhead]

Gotcha, thanks! I apprehend vague recollections of Dawkins' groups of genetic changes which are all more beneficial when "traveling" as a collectivity than when occurring in isolation.

Synergy, or some such.

[BWE]

Does limb development really herald the transition to land? I wonder if it is a later transition feature than say, different lensing material for eyes that can withstand air better, more neck motion to help survey terrain where the body isn't buoyant or, as above, the ability to hear out of water? Just flopping up to the beach could have a lot of advantages. Seals for example.

Supersport could probably make that into a beauty.

[Steviepinhead]

Originally Posted by BWE

Does limb development really herald the transition to land? I wonder if it is a later transition feature than say, different lensing material for eyes that can withstand air better, more neck motion to help survey terrain where the body isn't buoyant or, as above, the ability to hear out of water? Just flopping up to the beach could have a lot of advantages. Seals for example.

Supersport could probably make that into a beauty.

So, you're saying that the tetrapoddy fishies flopped up onto the beach in pursuit of the fleeing seals?

What I got from what Per said is that, yeah, it's NOT "just" limb development, but a host of related features: neck, hearing, gill covers (or not), ...

[Per Ahlberg]

There's a bunch of things going on more or less simultaneously, or at least in very rapid succession, during the fish-tetrapod transition. First the dorsal and anal fins are lost, the eyes move on top of the head (where they sit under little raised bony "eyebrows", much as in a crocodile), the gills become proportionately slightly smaller, and the body becomes somewhat flattened top to bottom. The fleshy lobes of the pectoral fins enlarge, and the upper arm bones and shoulder girdle begin to resemble those of primitive tetrapods (presumably indicating that the movement pattern is changing). This is the stage represented by Panderichthys. Likely this involved a shift from normal open-water swimming to a life in very shallow water with the odd short excursion over land. Terrestrial locomotion was probably "tripodal", supported by pectoral fins at the front and the whole tail region at the back. The pelvic fins are small and the pelvis is not attached to the backbone.

The next step, represented by Tiktaalik (but note comments on the uncertainty about its exact phylogenetic position in my long post in the other thread) is little different except that the bony gill cover disappears, the shoulder girdle begins to lose its dorsal contact with the skull, and the head becomes even more tetrapod-like in shape.

The next step after that, represented by early tetrapods such as Acanthostega, sees much more dramatic change, and is the one that would traditionally be identified as the "origin of tetrapods". Here the fin webs disappear from the pectoral and pelvic fins and are replaced by feet with seven or eight digits apiece, the hindlimb and pelvis enlarge, the pelvis becomes attached to the backbone, the hip and shoulder sockets reorient so the limbs come to stick out sideways rather than backwards, the shoulder girdle loses all contact with the skull, the vertebral column strengthens, and the tail lengthens. This must relate to a shift to the kind of quadrupedal locomotion (whether in water or on land is another matter) that has characterised tetrapod locomotion ever since. The dentition also changes substantially, with enlargement of the marginal teeth and reduction of the big fang pairs that sit further inside the mouth in lobe-finned fishes, suggesting changes in the mode of prey capture. The hyomandibula, a bone that supports the gill cover in fish, is reconfigured into a middle ear bone called the stapes. This is now connected to the inner ear and so probably already has a crude sound-transmitting function.

After this key phase of the transition, further steps involve losing the gills and tail fin, and reducing the number of digits to five.

No doubt changes were occurring simultaneously to the eye, lungs and other soft organs, but we have no direct evidence as these structures do not preserve as fossils.

[Dlx2]

Originally Posted by Per Ahlberg

The latter, I'd say. It fits in with a general, gradual de-emphasising of the gills and creation of a more mobile neck.

The loss of the operculum would, I'd say, suggest that the use of the pharyngeal cavity in a buccal-pharyngeal pump has been de-emphasized. I would be really hesitant to think it means that the buccal pump has been de-emphasized as far as feeding mechanisms go, especially with the branchial skeleton as developed as it is. I've heard the "mobile neck" thing suggested quite a lot for a very long time, but I'm gradually less convinced that this is a major issue at this point in the tetrapod lineage.

Just my two cents here.