The fin to finger transition: 1996, 2004 and 2019.

It is now early Fall, 2019
and everyone knows and agrees that certain lobe fin fish developed traits like choanae (internal nares), fingers and toes in their transition to the clade Tetrapoda (Fig. 3). Let’s see how far we’ve come.

Way back in 1996
Cloutier and Ahlberg presented cladograms that illustrated the fish families that contributed to tetrapod characters (Fig. 1). In short: ray-fin fish give rise to 1) coelacanths 2) onychodontids, and 3) porolepiformes. These gave rise to 4) lungfish, 5) rhizodontids, 6) osteolepiformes and 7) pre-tetrapods.

Figure 1. From Cloutier and Ahlberg 1996, colors added. Here ray fin fish give rise to onychodontids, the porolepiformes, lungfish, and pre-tetrapods.

Figure 1. From Cloutier and Ahlberg 1996, colors added. Here ray fin fish give rise to onychodontids, the porolepiformes, lungfish, and pre-tetrapods.

In 2004 Long and Gordon
presented two diagrams (here combined into one) that graphically show basically the same transition from Eusthenopteron at the bottom to Pederpes at the top (Fig. 2). Unfortunately Long and Gordon substituted a Tulerepton pes when otherwise they showed a series of hands (manus) for other taxa. Such mistakes rarely happen after referees and editors examine submissions, but should be pointed out whenever they appear.

Figure 2. Combined figures from Long and Gordon 2004 showing the traditional evolution of traits at the fin-to-finger transition.

Figure 2. Combined figures from Long and Gordon 2004 showing the traditional evolution of traits at the fin-to-finger transition.

 

In 2019
in the large reptile tree (LRT, 1578 taxa; subset Figs. 3, 4) many more taxa are listed and many more taxa nest between the primitive and derived members listed above.

FIgure 1. Subset of the LRT focusing on lobefin fish. Overlays indicate key traits in the origin of tetrapods.

Figure 3. Subset of the LRT focusing on lobefin fish. Overlays indicate key traits in the origin of tetrapods. Figure 4 follows.

Also shown are the derived traits
in the order of their appearance in this lineage.

Figure 4. Subset of the LRT focusing on basal tetrapods. The light green line shows the most direct route to Tulerpeton.

Figure 4. Subset of the LRT focusing on basal tetrapods immediately following figure 3. The light green line shows the most direct route to Tulerpeton, found in much the same stratum as pre-tetrapods and ichthyostegids.

What you might glean
from the above subset of the LRT (Fig. 4) is the rapid radiation of amphibian/basal tetrapods during the last of the Devonian, 365 mya. The stratigraphic dates (in green) remind us of the paucity of fossils known from this phylogenetic sequence in which some primitive taxa post-date several derived taxa. In other words, we should not be surprised to find representatives from nearly all of these clades in late Devonian strata someday.

Not immediately apparent
is the hypothesis that both Acanthostega and Ichthyostega returned to a more aquatic (more tadpole-like) existence. These are only two of several phylogenetic trends reversing the overall trend toward a terrestrial niche (e.g. Gephyrostegus and Tulerpeton) and amniote reproduction as recovered in Silvanerpeton, the last common ancestor of all amniotes (=reptiles) in the LRT.


References
Cloutier R and Ahlberg PE 1996. Morphology, characters, and the interrelationships of basal sarcopterygians. Ch17 in Interrelationships of Fishes. Ed. Staissny MLJ, Parenti LR and Johnson GD. Academic Press NY PDF
Long JA and Gordon MS 2004. The greatest step in vertebrate history: a paleobiological review of the fish-tetrapod transition. Physiological and Biochemical Zoology 77(5):700–719. 

 

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