Disc-head placoderms with tiny lateral eyes are anapsid mimics

Phyllolepid placoderms
like Cowralepis (Figs. 1, 2) and Minjinia (Fig. 3) have simple disc-like skulls and lack jaws. In this way they mimic older ‘jawless fish’ (= anapsida), like Drepanaspis (Fig. 4).

Figure 2. Cowralepis was first described as a growth series.

Figure 1. Cowralepis was first described as a growth series.

Anapsids are actually derived armored lancelets,
filtering food with oral cavities prior to the genesis of jaws.

Figure 1. Cowralepis plate and counter plate showing the medial view of the ventral and dorsal halves of this disc-like placoderm.

Figure 2. Cowralepis plate and counter plate showing the medial view of the ventral and dorsal halves of this disc-like placoderm. Red dots are resorted eyeballs.

In counterpoint,
Cowralepis and its Early to Middle Devonian allies had ancestors with jaws according to the large reptile tree (LRT, 1759+ taxa; subset Fig. x). Minjinia (Fig. 3) was originally considered a placoderm close to the shark/bony fish split. In the LRT Minjinia nests with the nearly blind phyllolepid placoderm bony fish, far from sharks.

Figure 1. Minjina in 4 views, mirror-image and colors added.

Figure 3. Minjina in 4 views, mirror-image added.

Phyllolepid placoderms also reduce
their diphycercal tail, pelvic fins and lose  their dorsal fins as they adapt to bottom feeding again, going through a process of reversal and convergence that could be misinterpreted without a wide gamut cladogram like the LRT.

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Figure 4. The large gill chamber (cyan) of Early Devonian Drepanaspis.

Phyllolepid placoderms had their origins
with a Silurian placoderm, Entelognathus (Fig. 5), which has tiny eyes, but not yet a disc-like morphology. Entelognathus was originally misinterpreted (Zhu et al. 2013, 2016) as a placoderm at the genesis of jaws. By contrast, in the LRT Entelognathus was losing its jaws, a process that terminates with disc-like phyllolepid placoderms.

Figure 5. Entelognathus in dorsal and lateral views. This taxon also has tiny lateral eyes and is basal to the phyllolepid placoderms.

Figure 5. Entelognathus in dorsal and lateral views. This taxon also has tiny lateral eyes and is basal to the phyllolepid placoderms.

Entelognathus primordialis (Zhu et al. 2013, 2016; Late Silurian, 419 mya)

Drepanapis gemuendenensis (Schlüter 1887; Gross 1963; Early Devonian 405mya)

Minjinia turgenensis (Brazeau et al. 2020; Early Devonian)

Cowralepis mclachlani (Ritchie 2005; Carr et al. 2009; Middle Devonian)

Figure 1. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure x. Subset of the LRT focusing on the branch of the Osteichthys that includes placoderms and their relatives.

Figure 1. Wuttagoonaspis from Fletcher 1973. Colors added here.

Figure 1. Wuttagoonaspis from Fletcher 1973. Colors added here.

Catfish also produced a similar morphology.
Wuttagoonaspis (Fig. 6) “is a genus of primitive arthrodire placoderms from Middle Devonian Australia” according to the fish workers posting in Wikipedia. In the LRT (subset Fig. x) it nests with the walking catfish, Clarias. Expand the taxon list, let catfish in, and see for yourself where “What-a-goon-aspis” nests.


References
Brazeau et al. (7 co-authors) 2020. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01290-2
Broili F 1929. S. B. Bayer. Akad. Wiss., 1
Carr RK, Joahnson Z and Ritchie A 2009. The phyllolepid placoderm Cowralepis mclachani: Insights into the evolution of feeding mechanisms in jawed vertebrates. Journal of Morphology 270(7):775–804.
Ritche A2005. Cowralepis, a new genus of phyllolepid fish (Pisces, Placodermi) from the Middle Devonian of New South Wales. Proceedings of the Linnean Society of New South Wales 126:215–259.
Schlüter EF 1887. Panserfische, etc. Niederrhein. Ges., Bonn, 1887, 120.
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.
Zhu M et al. 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354.6310 (2016): 334-336.

wiki/Cowralepis
wiki/Entelognathus
wiki/Drepanaspis

 

 

 

Pucapampella: sheds no light on the origin of jaws

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.

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 7. 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.

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.

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 x. Subset of the LRT focusing on fish.

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”

Did shark skeletons evolve from bony ancestors?

Did bone precede cartilage in sharks? 
Or did shark-like cartilage precede bone in bony fish?

Good question.
A good answer will come from a cladogram that accurately mirrors evolutionary events.

Brazeau et al. 2020 bring us
a new, partial placoderm skull, Minjinia turgenensis (Fig. 1), that preserves a great deal of internal bone, and not a lot of dermal bone. Brazeau et al. think their specimen answers the above questions because they think placoderms phylogenetically precede sharks + bony fish.

Figure 1. Minjina in 4 views, mirror-image and colors added.

Figure 1. Minjina in 4 views, mirror-image, tail, pectoral fins and colors added for clarity.

From the Brazeau et al. 2020 text:
“Chondrichthyans (sharks and their kin) are the living sister group of osteichthyans and have primarily cartilaginous endoskeletons, long considered the ancestral condition for all jawed vertebrates (gnathostomes). Phylogenetic analyses place this new taxon [Minjinia turgenensis] as a proximate sister group of the gnathostome crown. These results provide direct support for theories of generalized bone loss in chondrichthyans. Furthermore, they revive theories of a phylogenetically deeper origin of endochondral bone and its absence in chondrichthyans as a secondary condition.”

What came first? The large reptile tree (LRT, 1733+ taxa; subset Fig. x) supports the hypothesis that the absence of endochondral bone in sharks and ratfish is a primitive trait retained from more primitive sturgeons (Pseudoscaphorhynchus) and paddlefish (Polyodon).

According to Wikipedia, sturgeons “are unique among bony fishes because their skeletons are almost entirely cartilaginous.”

According to the Caddo Lake Institute, “The only bone in the [paddle] fish’s body is the jawbone.”

What about placoderms? The LRT nests placoderms deep within one branch of osteichtheys close to catfish. The internal and external placoderm skeleton is made of strong bone. Not sure why this major item of evidence has been traditionally overlooked.

Brazeau et al. continue:
“The absence of bone in modern jawless fishes and the absence of endochondral ossification in early fossil gnathostomes appear to lend support to this conclusion.”

Not really. Sturgeons are pre-gnathostomes in the LRT (subset Fig. x). They are at the genesis of jaws, rather than derived taxa losing their jaws, as commonly thought.

Unfortunately,
extensive taxon exclusion ruins the basics of Brazeau et al. 2020.

Instead
the LRT nests Minjinia with the small, unnamed and better preserved bottom-feeding placoderm ANU  V244 specimen (Fig. 2), a more complete taxon not mentioned by Brazeau et al. 2020. Both nest between the more famous bottom-dwelling placoderms Entelognathus and Bothriolepis.

Figure 1. The tiny ANU V244 specimen in various views. Note the scale bars.

Figure 2. The tiny ANU V244 specimen in various views. Note the scale bars.

Considering the fact that sturgeons and paddlefish have so little bone,
sharks and ratfish don’t have that much bone to lose. We just have to remember to take sturgeons and paddlefish out of the clade of bony fish and put them where the LRT (Fig. x) indicates they nest.

Like other fish workers,
Brazeau et al. 2020 used an out-dated traditional cladogram missing so many pertinent taxa that placoderms nested basal to jawed fish. In the LRT (Fig. x) placoderms nest alongside catfish deep within one branch of the Osteichthyes.

Figure x. Subset of the LRT focusing on fish.

Figure x. Subset of the LRT focusing on fish.

The publicity for Minjinia has been extraordinary.
Sci-News.com reported,
“This discovery suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.”

While true, as shown by the LRT (Fig. x), the phylogenetic context of this placoderm fossil was greatly in need of additional taxa.

From cosmosmagazine.com:
“This 410-million-year-old fossil with a bony skull uncovered in Mongolia may force a rethink of how sharks evolved. Minjinia turgenensis, a new species, is an ancient cousin of both sharks and animals with bony skeletons, the researchers say – and that suggests the lighter skeletons of sharks may have evolved from bony ancestors, rather than the other way around.”

Too few taxa mar this study. In the LRT Minjinia does nest with placoderms, but placoderms nest far from sharks, closer to catfish.

Co-author Martin Brazeau was reported as saying,
“Conventional wisdom says that a bony inner skeleton was a unique innovation of the lineage that split from the ancestor of sharks more than 400 million years ago, but here is clear evidence of bony inner skeleton in a cousin of both sharks and, ultimately, us.”

Not related to sharks. Add taxa and placoderms move close to catfish.

“M. turgenensis belongs to a broad group of fish called placoderms, out of which sharks and all other jawed vertebrates – animals with backbones and mobile jaws – evolved.”

False. The loss of the mandible in one branch of the placoderms should not be confused with the genesis of the mandible in the clade Gnathostomata following sturgeons, a clade at the genesis of jaws in the LRT.

Again, from cosmosmagazine.com:
“The new find suggests the ancestors of sharks first evolved bone and then lost it again, rather than keeping their initial cartilaginous state for more than 400 million years, the researchers say.”

Not exactly true.  Sturgeons and paddlefish are more primitive and have very little bone. Placoderms, like Minjinia (Fig. 1) have lots of bone and nest deep within bony fish.

Sometimes scientists rush off to get publicity
BEFORE waiting a suitable amount of time for feedback (confirmation or refutation). In this case the peer-review process apparently failed because everyone was working from an old playbook. So did the publicity process.


References
Brazeau et al. (7 co-authors) 2020. Endochondral bone in an Early Devonian ‘placoderm’ from Mongolia. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01290-2
Hu Y, Lu J and Young GC 2017. New findings in a 400 million-year-old Devonian placoderm shed light on jaw structure and function in basal gnathostomes. Nature Scientific Reports 7: 7813 DOI:10.1038/s41598-017-07674-y

https://cosmosmagazine.com/nature/evolution/new-thoughts-on-how-sharks-evolved/
http://www.sci-news.com/paleontology/minjinia-turgenensis-08823.html

 

Vaškaninová et al. 2020 test placoderms to describe the origin of marginal teeth

Vaškaninová et al. 2020 
employ several partial placoderms from Czechoslovakia to demonstrate the antiquity of lingual tooth growth (= from the inside out as in modern fishes; Fig. 1).

Unfortunately taxon exclusion mars this study.
Following tradition, the team thought derived placoderms (in the process of losing their teeth) were primitive taxa just gaining teeth (Fig. 1). Like other workers before them, they omitted too many taxa.

By contrast and using a wider gamut of taxa,
we looked at the origin of marginal teeth earlier here. Marginal teeth first appeared in the late-surviving basal paddlefish, Tanyrhinichthys (Fig. 2). The outgroup taxon, late-shriving Chondrosteus, (Fig. 3) lacked teeth and tooth-bearing bones (the premaxilla, maxilla and dentary).

From the Vaškaninová et al. 2020 abstract:
“The dentitions of extant fishes and land vertebrates vary in both pattern and type of tooth replacement. It has been argued that the common ancestral condition likely resembles the nonmarginal, radially arranged tooth files of arthrodires, an early group of armoured fishes. We used synchrotron microtomography to describe the fossil dentitions of so-called acanthothoracids, the most phylogenetically basal jawed vertebrates with teeth, belonging to the genera Radotina, Kosoraspis, and Tlamaspis (from the Early Devonian of the Czech Republic).

Note: In the LRT these taxa are placoderms in the process of losing their teeth. Teeth developed much earlier in the family tree (Fig. 4).

“Their dentitions differ fundamentally from those of arthrodires; they are marginal, carried by a cheekbone or a series of short dermal bones along the jaw edges, and teeth are added lingually as is the case in many chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods). We propose these characteristics as ancestral for all jawed vertebrates.”

Figure 3. Omitting many pertinent taxa, Vaskaninova et al. constructed this cladogram of tooth evolution. The LRT uses a wider gamut of taxa and recovers a different tree topology.

Figure 1. Omitting many pertinent taxa, Vaskaninova et al. constructed this cladogram of tooth evolution. The LRT uses a wider gamut of taxa and recovers a different tree topology. See figure 4.

In the Vaškaninová et al. 2020 study
basal fish, both jawless and not, are all armored.

Here
in the large reptile tree (LRT, 1707+ taxa) the origin of jaws lacking teeth is close to Chondrosteus (Fig. 3), a derived sturgeon (Fig. 10). In Chondrosteus the upper jaw is the lacrimal. The premaxilla and maxilla have not appeared yet. The lower jaw likewise lacks a dentary and is composed of the surangular and angular.

Figure 2. Skull of Tanyrhinichthys (above) with two bones relabeled. The other fish, Saurichthys, is clearly unrelated.

Figure 2. Skull of Tanyrhinichthys (above) with two bones relabeled. The other fish, Saurichthys, is clearly unrelated. The origin of tiny marginal teeth is close to Tanyrhinnichthys, a basal paddlefish (Fig. 2), the next moreb derived clade in the LRT. The tooth bearing bones (premaxillla, maxilla and dentary) originate as slender dermal layers on the lacrimal and surangular carrying tiny teeth, not much larger than skin denticles.

Adding taxa in the LRT
separates armored Devonian placoderms from armored Silurian jawless fish.

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Chronology is not as helpful as phylogeny
in figuring out these transitions, so much so that extant taxa need to be added to fill out the tree topology (Fig. 4).

Figure x. Subset of the LRT, focusing on fish for July 2020.

Figure x. Subset of the LRT, focusing on fish for July 2020.

Members of the Placodermi
like their relatives the catfish, are relatively derived taxa in the LRT (Fig. 4). Marginal teeth are missing in catfish and placoderms because they both have lost the maxilla along with their last common ancestor, taxa near late-surviving Diplacanthus.

Figure 5. Radotina is a basal taxon in the Vaskaninova et al. cladogram (Fig. 1).

Figure 5. Radotina is a basal taxon in the Vaskaninova et al. cladogram (Fig. 1). Compare to Romundina (Fig. 6) another basal taxon in Vaskaninova et al.

Basal taxa in the Vaskaninova et al. cladogram,
Romundina (Fig. 6) and Radotina (Fig. 5) are rather specialized terminal taxa in the LRT, leaving no descendants. Chondrosteus and Tanyrhinichthys are more generalized and primitive. All living fish, other than sturgeons (Fig. 10), whale sharks and mantas, are derived from Silurian sisters to these two taxa in the LRT.

Figure 10. What little we know of Radotina and where the same bone appears on the more complete Romundina, a terminal taxon in the Placodermi.

Figure 6. What little we know of Radotina and where the same bone appears on the more complete Romundina, a terminal taxon in the Placodermi.

Vaškaninová et al. provide the parts for Kosoraspis
(Fig. 7), a basal taxon without resolution in figure 1. Here (Fig. 8) I provide a possible restoration in which the large curved green bone identified as the ‘preopercular’ is re-identified as a postfrontal (orange in Fig. 8) based on similarities to Clarias, the walking catfish (Fig. 9).

Figure 8. From Vaškaninová et al. 2020, the parts for Kosoraspis. See figure 9 for a reconstruction where the largest bone here (green preopercular) is relabeled a postfrontal.

Figure 7. From Vaškaninová et al. 2020, the parts for Kosoraspis. See figure 9 for a reconstruction where the largest bone here (green preopercular) is relabeled a postfrontal.

Figure 9. Kosoraspis restored as a Devonian catfish like Clarias (Fig. 10).

Figure 8. Kosoraspis restored as a Devonian catfish like Clarias (Fig. 10). Those tooth plates are similar to those in catfish.

FIgure 1. Clarias, the walking catfish is a living placoderm with skull bones colorized as homologs of those in Entelognathus (Fig. 2). Here the mandible shifts forward and the opercular shifts backwards relative to Entelongnathus in the Silurian.

FIgure 9. Clarias, the walking catfish is a living placoderm with skull bones colorized as homologs of those in Entelognathus (Fig. 2). Here the mandible shifts forward and the opercular shifts backwards relative to Entelongnathus in the Silurian.

Determining when teeth and jaws first appeared
in basal vertebrates has been a contentious issue largely because pertinent taxa have been left out of the solution. Apparently Vaškaninová et al. left out several taxa key to understanding this transition from toothless jaws to toothy jaws. They considered taxa in the process of losing teeth, but placed them at the genesis of developing teeth.

Once again,
more taxa resolve problems like this better than more characters do.

Figure 1. 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. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor.

Figure 10. 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. Acipenser, a sturgeon with a protrusible tube for a mouth and reduced armor.

If this helps,
here again (Fig. 10) are three taxa preceding the origin of jaws with marginal teeth. These interrelationships have gone unnoticed by fish workers who continue to nest sturgeons with jawed fishes. The next taxon following these three had large jaws: Chondrosteus (Fig. 3).

Figure 11.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish. Note the lack of teeth. 

Figure 11.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish. Note the lack of teeth.

Here again are whale sharks and mantas
(Fig. 11) on their own branch derived from Silurian sisters to Thelodus and LoganelliaThese taxa have jaws, but lack marginal teeth, similar to Chondrosteus (Fig. 3).

As mentioned above,
it is so important to include a wide gamut of taxa, including extant taxa.


References
Vaškaninová V, Chen D, Tafforeau P, Johanson Z, Ekrt B, Blom H and Ahlberg PE 2020. Marginal dentition and multiple dermal jawbones as the ancestral condition of jawed vertebrates. Science 369(6500): 211-216 DOI: 10.1126/science.aaz9431
https://science.sciencemag.org/content/369/6500/211

placoderm jaws

News:
https://phys.org/news/2020-07-advanced-technology-evolution-teeth.html

The LRT vs. The Rise of Fishes (Long 1995)

Figure 1. The Rise of Fishes 1995 book by fish expert John Long.

Figure 1. The Rise of Fishes 1995 book by fish expert John Long.

The majority of data sources
for fish here and in ReptileEvolution.com come from the excellent ink drawings of WK Gregory 1933 and the excellent photos of John Long 1995 in his book, ‘The Rise of Fishes’ (Fig. 1). Newer editions of the book are out there, but I don’t have them. Long’s own updates may erase some of the issues raised here. 

Long 1995 writes:
“The first osteichthyans are very poorly known from fossils, represented by a few scales and a mere fragments of bone. The oldest articulated remains, showing what their bodies and heads were like, are about 400 million years old.”

The LRT demonstrates
restricting phylogenetic fish taxa to just Silurian and Devonian fossils is unnecessary and restrictive. There are plenty of extant, yet still primitive, taxa one can plug into any fish phylogenetic analysis, like the large reptile tree (LRT, 1656+ taxa), for one. Without fossils the LRT can recreate the fish ‘tree of life’ starting with lancelets, then sturgeons, then sharks, and ending with sea horses, anglerfish and tetrapods.

Long includes sturgeons in the clade of bony fish (Osteichthyes)
despite his notes that sturgeon skeletons retain large amount of cartilage and much of the external armor is bone. In the LRT jawless armored Osteostraci are basal to sturgeons prior to the evolution of terminal jaws.

Long “pulls a Larry Martin”
when he states, “Osteichyan fishes are characterized by having a well-ossified internal bony skeleton” then backtracks by noting, “although the earliest fossil forms show the least degree of ossification of the vertebrae and internal bones.”

By contrast,
in the LRT only the last common ancestor of all extant bony fish (sans sturgeons and paddle bills) determines clade membership, despite the presence or absence to certain ‘key’ traits, like a bony internal skeleton.

Figure x. Subset of the LRT focusing on fish.

Figure x. Subset of the LRT focusing on fish.

The LRT confirms Long’s 1995 fish cladogram
with regard to nesting jawless fish (Agnatha) basal to jawed fish (Gnathostomata), naturally.

In Long 1995, Osteostraci
is the proximal outgroup to the Gnathostomata.

In the LRT, a member of the Osteostraci is basal to tube-mouth sturgeons in the LRT. Members of the Thelodonti are basal to Gnathostomata in the LRT.

In Long 1995, the basal dichotomy in Gnathostomata
splits Placodermi from Acanthodii. Those two are not related in the LRT.

In the LRT basal Gnathostomata splits taxa with transverse toothless jaws (LoganelliaRhincodon + Manta) from taxa with U-shaped toothy jaws (Falcatus) in the LRT. No suprageneric taxa are employed (exception: Reptilia, Lepidosauromorpha, Archosauromorpha).

In Long 1995, Silurian Placodermi give rise to 
“protosharks”, then Cladoselache, then Holcephalomorpha (ratfish) and Neoselachii (true sharks)

In the LRT, Placodermi include and give rise to catfish, all derived from Cheirodus/Amphicentrum. In the LRT, the second great dichotomy splits ratfish + sharks apart from xenacanthid ‘sharks‘, hybodontid ‘sharks‘ and Pachycormus.

In Long 1995, Silurian Acanthodii give rise to
Lophosteiformes (known only from ornate epidermal scales) and Actinopterygii (ray-finned fish)

In the LRT, Acanthodii are not primitive, but arise from ray-fin fish like the Cretaceous, bony-finned Bonnerichthys and the extant Osteoglossum.

Cause for concern
This major time gap between known fossils and phylogenic first appearance is cause for concern on the one hand, and a call to action on the other. At present it does not make sense that placoderms and acanthodians (spiny sharks) are present in the Silurian while intervening and more primitive taxa in the LRT are no yet known from the Silurian… and yet, we’ve seen this before with Jurassic multituberculates preceding more basal members of the placental clades, Glires, Carnivora and Primates.

Phenomic phylogenetic analyses, like the LRT, deliver
the gradual accumulation of derived traits that support evolutionary theory. Missing Silurian taxa will have to be discovered whenever someone discovers them.

On the other hand,
Long 1995 makes no attempt to provide any generic last common ancestors with jaws for Acanthodii + Placoderm, nor does he spell out any gradual accumulations of derived traits at transitional points, not does he employ more than a token number of generic or specific taxa in his cladogram. Suprageneric taxa are often a problem in taxonomy, as noted earlier. They can be too easily cherry-picked.

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

Figure 2. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

So, are placoderms and acanthodians ‘bony fish’?
In the LRT: yes, despite having less than fully ossified internal skeletons. That could be a retained primitive trait or a reversal in both cases. Both are derived from taxa in which the external scales are robust, providing support to the body, allowing the skeleton to degenerate. As for that heterocercal tail in acanthodians (Fig. 2) and placoderms… based on the evidence, that is a primitive trait, not a reversal. Intervening taxa, like extant Trachinocephalus, appear to have independently evolved a diphycercal tail and bony skeleton, which their Silurian direct ancestors probably lacked.

Cherry picking traits,
even ‘important’ traits, is ‘Pulling a Larry Martin‘, something the wise professor taught us not to do. Find your clades using specific or generic taxa tested against 200+ traits to find that last common ancestor, no matter the ‘key’ traits that tickle your fancy.


References
Gregory WK 1933. Fish skulls. A study of the evolution of natural mechanisms. American Philosophical Society 23(2) 1–481.
Long JA 1995.
The Rise of Fishes. Johns Hopkins University Press. Baltimore and London.

 

SVP abstracts – Silurian gnathostome

Zhu et a. 2019 bring us
a new Silurian fish they claim is close to the origin of jawed vertebrates (= Gnathostomata).

From the abstract:
“Modern jawed vertebrates or crown-group gnathostome include the last common ancestor of living bony and cartilaginous fishes and all its descendants. The gross morphology of the earliest modern jawed vertebrates, and how they arose from stem gnathostomes, were previously unknown due to a lack of articulated fossils.”

These taxa are not unknown in the large reptile tree (LRT, 1592 taxa). Put enough taxa in an analysis and one will end up close to the origin of gnathostomes. There will be a last common ancestor. In the LRT Thelodus, a ?jawless (phylogenetic bracketing indicates some sort of transverse jaws are present) Silurian fish is the current proximal outgroup to all tested taxa with jaws. In LRT the extant whale shark (Rhincodon), angel shark (Squatina) and horn shark (Heterodontus) are basal members of the Gnathostomata and the first taxa with primitive tooth carpets.

“The recent discovery of the Xiaoxiang Fauna from the Silurian of South China revolutionarily adds to the diversity of Silurian jawed vertebrates. However, considerable morphological gap is still present between stem- and crown-group gnathostomes.”

Not so, when appropriate taxa are included.

“Here, we report a new bony fish very close to the crown-group gnathostome node, also from the Xiaoxiang Fauna. The attributed specimens include a head, jaws and an articulated postcranial skeleton.”

“The new fish displays a unique suite of characters: the dermal pectoral girdle condition transitional between Entelognathus and osteichthyans, the braincase profile recalling the condition in Janusiscus and early chondrichthyans, and the premaxillae and lower jaw largely showing osteichthyan features. This mosaic character combination suggests the tentative phylogenetic position of this new taxa in the most basal segment of the osteichthyan stem, possibly forming a quintessential component of the evolutionary transition between placoderms and osteichthyans.”

In the LRT taxa between placoderms and osteichthyans are either acanthodians (spiny sharks) on one branch, or catfish (also with spiny fins) on the other branch. Catfish are whales-shark mimics with regard to their jaws and teeth, likely representing some sort of reversal to that basal condition.

“For the first time, we are able to look into a near-complete bony fish close to the last common ancestor of all the living jawed vertebrates, and reconstruct the acquisition sequence of osteichthyan characters based on a series of fossils in morphological proximity. The fact that most of these fossils are from the Silurian Xiaoxiang Fauna, suggests that this fauna is unprecedentedly close to the initial radiation of jawed vertebrates.” 

Figure 2. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies.

Figure 1. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies. Check out the LRT for a slightly updated version of this cladogram.

This is all very interesting, and welcome, but let them look at the structure of Rhincodon as it relates to Thelodus at least once before settling down with the Zhu et al. hypothesis.

Figure 4. Manta compared to Thelodus and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes.

Figure 4. Manta compared to Thelodus and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes.


References
Zhu et al. 2019. A new Silurian bony fish close to the common ancestor of crown gnathostomes.