Summary for those in a hurry:
Fish experts thought they had a basalmost shark and a basalmost gnathostome (vertebrate with jaws) in an Early Devonian specimen from Bolivia, Pucapampellla (Fig. 1; Janvier & Suarez-Riglo 1986).
Figure 2. A rare early Devonian pre-shark, Pucapampella rodrigae, from Janvier & Suarez-Riglo 1986, colors added. Dorsal surface imagined from clues provided. Note the palatoquadrate and Meckelian cartilage are composed of several bony cartilage precursors fused together.
From the American Museum of Natural History online
“Shedding new light on the evolutionary origins of the jaw, John Maisey, a curator in the Department of Vertebrate Paleontology at the American Museum of Natural History, disclosed the first detailed description of a 400-million-year-old primitive shark relative from Bolivia named Pucapampella.
This new fossil discovery contradicts the belief that chondrichthyans, or sharks and their relatives, are primitive due to their jaw characteristics, and points to an advanced specialization in shark evolution. It also provides a missing link in the understanding of how jawed vertebrates evolved from the jawless state — a crucial initial step toward human evolution.”
By contrast, in the large reptile tree (LRT, 1745+ taxa; subset Figs. 3, 4) the origin of jaws does not involve Pucapampella, but does involve late-surviving, but early radiating sturgeons and Chondrosteus (Fig. 2). The latter had jaws, but no teeth, which appeared at the next node, the one that included the paddlefish Polyodon.
Figure 2. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Euphaneropsps, a late survivor of an Ordovician radiation basal to sturgeons. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor. Chondrosteus, a fish with jaws, but no marginal teeth.
Janvier and Racheboeuf 2018 reported,
“Recent phylogenetic analyses of jawed vertebrates generally yield Pucapampella as the sister-group to all living and fossil chondrichthyans, though sometimes interchangeably with the other stem chondrichthyan Doliodus problematicus, from the Emsian of Canada.”
By contrast, the LRT (Fig. 4) nests Doliodus among the basal bony fish.
Figure 6. Adding Debeerius to the LRT helped revise the shark-subset. Note the shifting of the basking shark, Cetorhnus within the paddlefish clade.
Pucapampella rodrigae (Janvier & Suarez-Riglo 1986; Early Devonian) is one of the earliest known taxa from the shark clade, but not the most primitive and not basal to other sharks in the LRT (subset Fig. 3).
In the LRT a close relative of Pucapampella is Falcatus and a paddlefish mimic, the PF8442 specimen of Bandringa.
From the AMNH continued:
“Little is known about the jaw’s origins,”
The LRT solved that problem.
“…however, due to a poor fossil record of the critical time when the first jaws evolved, sometime before the Devonian period (412-354 million years ago). Until now, a 370-million-year-old shark called Cladoselache provided the paradigm of jaw evolution because good fossils of it have been available to study for more than a century.”
As we have seen before, fish experts tend to exclude living fish from their cladograms that include fossil fish. This sort of taxon exclusion blinds them to the actual origin of jaws, which is well documented in the LRT (Fig. 2) employing traditionally omitted taxa recovered by the software, not cherry-picked.
From the AMNH continued:
“Maisey’s paper on Pucapampella, presented today at a conference on early vertebrate evolution hosted by the Natural History Museum of London, reveals evidence of jaw evolution that pre-dates Cladesolache by roughly 30 million years. “This is the earliest shark braincase that we can actually study in any detail,” said Maisey. “The way we view the early evolution of the jaw now has to change.”
Unfortunately, according to the LRT, Pucapampella does not nest near the origin of jaws. That occurred with toothless Chondrosteus, a taxon known since 1843 as an Early Jurassic late survivor of an earlier Early Silurian radiation.
From the AMNH continued:
“Pucapampella’s phylogenetic position lies at the base of the chondrichthyan lineage. Through detailed morphological analysis, Maisey found that Pucapampella’s upper jaw was attached to the braincase in a way that was atypical for a chondrichthyan, and more like that of an osteichthyan, or bony fish. In evolutionary terms, bony fish have been considered to have a more advanced jaw structure than sharks. However, Pucapampella suggests that the converse it true. The fact that a shark as primitive as Pucapampella had a bony fish-like jaw attachment suggests that modern shark jaws are an more advanced characteristic than the jaws of bony fish. This closer evolutionary relationship between sharks and bony fish, in turn, influences how science may now view the relationship between jawed and jawless vertebrates. “This discovery removes one of the problems of deriving a jawed vertebrate from a jawless one by saying the jaw has a corresponding structure in a lamprey, which is jawless,” explained Maisey.
Adding taxa means the jawless lamprey shifts several nodes away from the origin of jaws in the LRT (Fig. 4). Sturgeons are traditionally ignored… unless you just toss them into the taxon list and let the software decide where they should nest, casting aside bias and tradition.
From the AMNH continued:
“Fossils of Pucapampella have only been found in Bolivia and South Africa, which were geographically closer during the Devonian period than they are today. This part of the Southern Hemisphere was covered by a cold, shallow ocean that dramatically contrasts the warm, tropical waters modern sharks prefer.”
Pucampella is a great find,
but not as great as Maisey and his PR team suggest. Minimizing taxon exclusion gets to the root of any paleo problem, not faster, but better than any other technique. All less parsimonious contenders nest elsewhere and no branch is overlooked.
Figure 4. Subset of the LRT focusing on fish.
References
Janvier P and Suárez-Riglos M 1986. The Silurian and Devonian vertebrates of Bolivia. Bulletin de l’Institut Français d’Etudes Andines, Lima, 15: 73-114.
Janvier P and Racheboeuf 2018. The Palaeozoic vertebrates of Bolivia, with comments on the faunal and environmental context of the “Malvinokaffric Realm’. In Fósilies y Facies d Bolivia. Riglos MS, Farjat AD and Leyton MAP Eds. Santa Cruz de la Sierra, 2018
Publicity
AMNH Public Release: 9-Apr-1999
“New Findings On Primitive Shark Contradicts Current View Of Jaw Evolution”
The Pterosaur Heresies 