When one layers established time periods
over the fish portion of the large reptile tree (LRT, 1673+ taxa; Fig. 1) the surprising length of certain ghost lineages and the ability of several clades to survive several hundred million years becomes apparent.
Figure 1. Subset of the LRT focusing on basal vertebrates (= fish). Colors indicate time periods. This chart documents the lack of fossils for several clades and genera in the Silurian and Devonian.
The antiquity of Silurian members in the highly derived lungfish clade
(Guiyu and Psarolepis) helps one understand the coeval Silurian appearances of so-called primitive fish, like acanthdians and placoderms (Entelognathus). Traditional cladograms assumed early taxa must be more primitive, not realizing that phylogenetic analysis indicates a vast undiscovered radiation of taxa in the Silurian (Fig. 1). Most of these are still waiting to be discovered.
What do Silurian and Early Devonian fossil fish in the LRT have in common?
Many were flat bottom dwellers with small eyes.
By contrast, coeval spiny sharks had large eyes and were free-swimmers. Even so they lost their flexible fin rays, they lost large teeth, they kept a large mouth, and they had vestigial skeletons. Such traits are associated today with slow-moving deep sea fish.
So known Silurian fish were not open sea visual predators with great swimming skills. Their ecological absence must have a reason. I wonder if such taxa were gobbled up before they could drift to muddy or silty anoxic regions of the sea floor where they could wait undisturbed to be buried for fossilization? Even a few exceptions are lacking. Very puzzling…
According to Google:
“In North America geologic activity over the last 417 million years has removed or covered up most Silurian rocks. Well-preserved fossils from Silurian reefs can be found in the Great Lake States of Minnesota, Wisconsin, Michigan, and Illinois.” So Silurian exposures are comparatively rare.
How do left column fish differ from right column (Fig. 1) fish?
As a general rule (allowing for many exceptions) left column fish do not appear to be the fast, open water swimmers seen in the majority of primitive right column fish in the Silurian and Devonian. It is noteworthy that not one taxon in the right column has a Silurian through Permian representative. I will add them as they come to my attention. It is also noteworthy that the left column has very few living representatives. I count nine.
Traditional cladograms
put more emphasis on time and exclude extant taxa. That’s why traditional cladograms often nest spiny sharks and placoderms near the base of the basal vertebrates, prior to sharks and bony fish. And they attempt to add tube-feeding sturgeons somewhere in the middle of bony fish. In the LRT taxon exclusion is minimized and more natural evolutionary patterns are recovered based on phenomics (traits).
Some previously unrecognized relationships recovered by the LRT include:
- The wide radiation of clades in the Silurian.
- Devonian taxa take us rapidly to tetrapods, documented by Middle Devonian tracks
- Note the proximity of Silurian lobefins to Viséan (Early Carboniferous) tetrapods, including reptiles.
- Note the unbalanced fossil record with regard to the major dichotomy splitting bony fish
- Proamia is known from the Devonian while a sister taxon, Amia, is known from extant taxa, separated by 360 million years. This is the closest we get to a right column fish fossil in the Silurian or Devonian.
- The time span between tiny Silurian Loganiella and giant extant sisters Rhincodon + Manta is about 430 million years.
- A similar time span splits Hemicyclaspis from living sturgeons.
- A longer time span (~500 my) splits Branchiostoma from its Cambrian precursors.
- When comparing the LRT to traditional cladograms, check to make sure they have similar outgroup taxa. Too often taxon exclusion is an unaddressed issue in those papers, which make them fitting subjects for the next few blogposts.
Cautionary note:
The choosing of fish taxa for the LRT has not been random, but was made on the basis of availability and possible importance. At present the fossil record is skewed toward left column fish prior to the Permian. As more taxa are discovered and added, the subjective second reason will hopefully pale to become less of a factor.
The Pterosaur Heresies 