Ventastega
The paleontologists are going too far. This is getting ridiculous. They keep digging up these collections of bones that illuminate tetrapod origins, and they keep making finer and finer distinctions. On one earlier side we have a bunch of tetrapod-like fish — Tiktaalik and Panderichthys, for instance — and on the later side we have fish-like tetrapods, such as Acanthostega and Ichthyostega. Now they're talking about shades of fishiness or tetrapodiness within those groups! You'd almost think they were documenting a pattern of gradual evolutionary change.
The latest addition is a description of Ventastega curonica, a creature that falls within the domain of the fish-like tetrapods, but is a bit fishier than other forms, so it actually bridges the gap between something like Tiktaalik and Acanthostega. We look forward to the imminent discovery of yet more fossils that bridge the gap between Ventastega and Tiktaalik, and between Ventastega and Acanthostega, and all the intermediates between them.
Here's Ventastega's place in the phyletic universe, and I think you can see what I mean — all those species represent an embarrassment of riches, revealing the flowering of the tetrapod transition.
The skull can be compared to others, and the meat of the description of this animal is largely a description of each of the bones of the skull, categorizing and comparing them, and showing that we really are looking at a beast that is partway between Tiktaalik and Acanthostega.
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Skulls of Tiktaalik, Ventastega, Acanthostega and Ichthyostega in dorsal view, showing the skull roof (grey) used in the morphometric comparison. In Ventastega and Acanthostega the internasal fontanelle is shown darker grey. Not drawn to scale.
I know, you really just want to see what it looks like. Here's a diagram of the bits and pieces of this wonderful fossil.
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a, Whole-body reconstruction showing known skeletal elements on a body outline based on Acanthostega. Scale bar, 10 cm. b, c, Skull reconstruction in lateral and dorsal views, based on material presented here and described previously. d, Reconstructed association of skull and shoulder girdle in lateral view. e, Shoulder girdle in anterior view. Curvature of cleithrum based on LDM G 81/522. Unknown bones are indicated with vertical hatching. Scale bar for b–e, 10 mm. f, g, Life reconstructions of head in lateral and dorsal views (copyright P. Renne, 2007). an, anocleithrum; ang, angular; cla, clavicle; clei, cleithrum; de, dentary; fr, frontal; icl, interclavicle; i.fon, internasal fontanelle; it, intertemporal; ju, jugal; la, lacrimal; mx, maxilla; m.ro, median rostral; na, nasal; pa, parietal; pmx, premaxilla; po, postorbital; pof, postfrontal; pop, preopercular; pospl, postsplenial; pp, postparietal; prf, prefrontal; pter, pterygoid; qj, quadratojugal; sang, surangular; scapcor, scapulocoracoid; spl, splenial; sq, squamosal; ta, tabular.
There's one important fact Ahlberg warns us about, though. When you see a detailed, species-packed cladogram like the one shown above, it is tempting to see the roster of species as a linear series, with one form succeeding another. This is not the case! Many of those species were dead ends, and we're seeing the tips of the branches, not necessarily any of the members of the main trunk. What all these fossils tell us is a combination of fortunate trivia — it's good to live your life along the water's edge if you hope to be fossilized — and amazing success. These early tetrapods were exploring a new niche and were radiating into diverse morphologies at a rapid rate, and so what we're also seeing is a portrait of a spectacularly successful strategy, the exploitation of the boundary between land and water by large animals.
Ahlberg PE, Clack JA, Luksevics E, Blom H, Zupins I (2008) Ventastega curonica and the origin of tetrapod morphology. Nature 453(7199):1199-204.