DGS revisions to Paleozoic fish

In order to know taxa better
it’s worthwhile to revisit earlier interpretations once a bit of experience working with clade patterns is gained. The is the next step beyond freshman naiveté. In this case, several taxa (Figs 1–4) were revisited after earlier interpretations beset with first-timer mistakes. The resulting new interpretations update and correct prior errors and improve analytical scores.

Figure 1. Tiny roadkill Shenacanthus now has a new skull with a gracile mandible and other changes. Ozarcus has a palatine strip (cyan) rather than the former maxilla misinterpretation. No taxa in the shark line develop a maxilla. The long hyomandibular (dark green) was broken in two during taphonomy. It is reassembled here with the crack line preserved.

Early Silurian Shenacanthus
(Fig 1, Zhu et al 2022) has a 5mm skull crushed and somewhat scattered. Here, after a bit of experience and sweat equity, Shenacanthus is re-reconstructed. Now it is even more similar to the much larger, much later big-eyed basal shark, Ozarcus.

Early Carboniferous Ozarcus
(Fig 1, Pradel et al 2014) was originally µCT scanned, so it is perfectly presented. Earlier I mistook the strip-like palatine for a maxilla. This simple correction was made by changing its DGS color to cyan. Now no chondriichthyans have a maxilla.

Figure 2. Thelodus has new DGS colors here and these move thelodus closer to sturgeons in the LRT. Several pieces of debris are identified. Click to enlarge.

Silurian Thelodus
(Fig 2) is a dark on dark 2D fossil that was difficult to sort out and interpret. Here, after some experience and several stumbling attempts DGS colors now indicate this specimen was basal to sturgeons with small fins and ventral nares. This linking Thelodus to heterostracans, and indicates their progress in their migration to their eventual position anterior to the orbits in extant sturgeons. A tiny hyomandibular disc (dark green) makes an appearance near the spiracle, prior to its enlargement as yet another type of operculum. FInally, if not yet another piece of debris, the interclavicle makes an appearance at the flat ventral midline of Thelodus. This bone is retained in later placoderms, which underlap it with lateral elements.

Figure 3. The skull of Devonian Euphanerops, a relative of Bikenia in the LRT. And a relative of the conodont, Promissum. All three are early gnathostomes in the tetrapod line. Here the maxillae arc (sans premaxilla, which appears later) and the straight dentary elements are pulled out to reveal their identities.

Devonian Euphanerops,
(Fig 3) is a lamprey-like, lancelet-like (Fig 4) relative of Bikenia and the conodont, Promissum in the LRT. Like conodonts, Euphanerops had ‘jaws’ made up of several separate slender elements which linked more tightly together in more derived gnathostomes basal to tetrapods. Here the maxillae form an arc without a premaxilla. By contrast, the early dentary elements were straight. Conodonts can have a dozen of these, but Euphanerops has fewer. Here all the jaw elements lack teeth or conodont-like sharp processes.

Figure 4. Not two unusual anal fins, but two typical pelvic fins appear in Euphanerops as shown here. The atrium was misidentified as gut and anus by Sansom, Gabbott and Purnell 2013.

The pelvic fins of Devonian Euphanerops,
(Fig 4, Woodard 1900; Janvier and Arsenault 2007) were identified as unusual paired anal fins by Sansom, Gabbott and Purnell 2013 because the atrium excurrent valve was miisidentified as the anus (= the cloaca). Here corrections are offered in DGS colors. So far I have not seen any paired anal fins.

Figure 5. Traced here using DGS methods reveals overlooked vestigial fins in Carboniferous Paratarrasius.

All the fins on the Carboniferous eel-like Paratarrasius
How would you score this taxon? Did it have a straight tail? Or a hetercercal tail? Comparie Paratarrasius (Fig 5) to its LRT sister, Bluefieldius (Fig 6), which has a heterocercal tail. That makes the dorsal fin of Paratarrsius extend to the tail tip (not beyond as the diagram (Fig 5, lower right) shows. In addition, pelvic and anal fins were originally overlooked based on the finless diagram. The pectoral fins were imagined with dashed lines to look like ping-pong paddles. Here all laferal fins are vestigial, but still present. They are given DGS colors for identification (Fig 5) and appropriate scores in the LRT.

Figure 6. Bluefieldius is a sister to Paratarrasius in the LRT. Note the tiny lateral fins and heterocercal tail. A dorsal fin is not preserved in this specimen. The twisted torso not revealing the skull dorsal surface indicates the possibility that the dorsal fin, if present, is still buried in the matrix.

These corrections
are improving the scoring of the fish subset of the LRT, which is still ‘in progress’. Perhaps it will always be ‘in progress.’

References
Janvier P and Arsenault M 2007. The anatomy of Euphanerops longaevus Woodward, 1900, an anaspid-like jawless vertebrate from the Upper Devonian of Miguasha, Quebec, Canada. Geodiversitas 29 (1) : 143-216.
Pradel A, Maisey JG, Tafforeau P, Mapes RH and Mallant J 2014. A Palaeozoic shark with osteichthyan-like branchial arches. Nature 13185. doi:10.1038/nature13195e
Sansom RS, Gabbott SE and Purnell MA 2013. Unusual anal fin in a Devonian jawless vertebrate reveals complex origins of paired appendages. Biol Lett. 2013 Jun 23; 9(3): 20130002. doi: 10.1098/rsbl.2013.0002
Zhu Y-A et al (10 co-authors) 2022. The oldest complete jawed vertebrates from the early Silurian of China. Nature 609:954–958. online
Woodward AS 1900. On a new ostracoderm fish (Euphanerops longaevus) from the Upper Devonian of Scaumenac Bay, Quebec, Canada. Magazine of Natural History ser. 7, 5: 416-419.

wiki/Tarrasiiformes
wiki/Thelodus
wiki/Euphaneropidae
wiki/Shenacanthus – not yet posted

A placoderm with dentary teeth

I overlooked this earlier.
The mid-sized placoderm Amazichthys (Fig 1) had a full set of dentary teeth, distinct from the placoderm tradition (but see below) and unrelated to similar teeth found in other fish, according to the LRT.

Described in 2022 by Jobbins et al,
Amazichthys amazingly preserves impressions (and bone) of the entire fish, but unfortunately the skull is exposed in only a few places in both known specimens. So, maxillary data is currently not available.

Perhaps no maxilla was present and the dentary teeth worked against the palatine teeth, as in the tiny related unnamed specimen, ANU V244 (Fig 2).

FIgure 1. Amazichthys data. Colors added here. At bottom is the dentary, articular and quadrate. Note the teeth at various angles and shapes.

The authors reported on the mandible,
“The bone is rather slender at its biting division and presents a row of at least four, less than 1 mm high, preserved teeth. The jaw is higher in the posterior half, i.e., the bony shaft.”

No other attention was paid to the teeth by Jobbins et al.

Figure 2. The unnamed tiny Late Silurian transitional placoderm, ANU V244 was preserved in perfect 3d. Colors added here. The middle right graphic shows the light blue dentary teeth occluding with the rich blue palatine teeth.

Smith and Johanson reported in 2003,
“It is generally thought that teeth are absent in placoderms and that the phylogenetic origin of teeth occurred after the evolution of jaws. However, we now report the presence of tooth rows in more derived placoderms, the arthrodires. New teeth are composed of gnathostome-type dentine and develop at specific locations. Hence, it appears that these placoderm teeth develop and are regulated as in other jawed vertebrates. Because tooth development occurs only in derived forms of placoderms, we suggest that teeth evolved at least twice, through a mechanism of convergent evolution.”

Taking a closer look at a tiny related placoderm,
Millerosteus (Fig 1), a carpet of teeth is preserved on the inner dentary. With similar overlapping cheekbones as the ANU V244 specimen (Fig 2) it is likely these lower teeth occluded only with the carpet of palatine teeth. The maxilla was probably absent, as in ANU V244.

Amazichthys trinajsticae
(Jobbins et al 2022; Late Devonian) was preserved with a body outline indicating a deep caudal and dorsal fin. Otherwise this placoderm was a larger Millerosteus in most respects, but with a shorter thorax shield. The cheek bones remain unknown and are restored here based on Millerosteus.

References
Jobbins M, Rücklin M, Ferrón HG and Klug C 2022. A new selenosteid placoderm from the Late Devonian of the eastern Anti-Atlas (Morocco) with preserved body outline and its
ecomorphology. Front. Ecol. Evol. 10:969158. doi: 10.3389/fevo.2022.969158.
Smith MM and Johanson Z 2003. Separate evolutionary origins of teeth from evidence in fossil jawed vertebrates. Science 299(5620):1235–1236.

wiki/Placodermi
wiki/Millerosteus
wiki/Holonema
wiki/Amazichthys

The origin of jaws in the placoderm clade

If you ever wondered
how the under-slung parasagittal oral elements of jawless fish with a transverse hinge (Fig 1) evolved into the more familiar arc with paired lateral hinges (Figs 2–4), here’s a series of graphics that show a step-by-step evolution. This series applies only to the placoderm clade and all taxa that descended from placoderms.

As recovered earlier, stem tetrapod jaws developed in a different clade of fish. Thus the traditioinal clade, ‘Gnathostomata‘ is no longer monophyletic. See Birkenia for details.

Figure 1. Images from Lanzetti et al 2023, animated here. This is the oral apparatus of Rhinopteraspis after µCT scanning. These are not jaws.

Rhinopteraspis
(Fig 1) demonstrates the jawless condition in pre-placoderms. The mobile oral elements opened like a castle draw bridge. Note the transverse hinge of the multiple elements and the parasagittal orientation of the medial elements, unlike the jaws in extant vertebrates.

Figure 2. Qilinyu skull in ventral view with original identifications and revised here. The barely conjoined elements of the mandible that once were the tips of the ancestral oral elements shown in figure 1, now gone. The motion is the same. The axis of rotation has migrated to the lateral skull.

The tentative genesis of jaws in Late Silurian Qilinyu
(Fig 2) demonstrates the transition from parasagittal elements in jawless pre-gnathostomes to lateral elements in basal gnathostomes. Note the disappearance of all but the anterior tips of the former mobile oral elements (Fig 1), now tentatively conjoined, but still moving in the same limited arc. This is a very weak jaw.

Figure 3. In Bianchengichthys the jaws are still weak and fragile, but they arc across the margin of the mandible here as the nares rotate from ventral to anterior.

In phylogenetically miniaturized Bianchengichthys
(Fig 3) the jaw evolved into a greater arc that matched the rostral margin enabled by reduction of the nasal elements and migration of the once ventral nares. Whether the mandible elements were strongly conjoined or not cannot be determined from this tiny fractured fossil and its gracile mandible. Either way, the mandible was still fragile.

Figure 4 Skull of the small arthrodire placoderm, Coccosteus, demonstrating the evolution of robust mandibles without matching the upper jaw margins. Compare this ventral view with that of Qilinyu in figure 2.

Not all primitive placoderm jaws were gracile and weak.
Coccosteus (Fig 4) also had an under-slung jaw, like Qilinyu (Fig 2), but it was sharp and robust. Note the similar ventral narial openings in Coccosteus and Qilinyu. Are these excurrent nares? And why do they not appear in the anterior view of Coccosteus where the ventral nasal plate lacks the perforations seen in ventral view? Here we also see the origin of palatal teeth (blue arc in ventral view) recapitulating in arc and chain of gracile elements the earlier origin of the mandible.

Evolution is a fascinating subject. I’m glad you’re along for the ride.

Figure 5. Bothriolepis oral cavity animated. Here the mandible remains immobile and the nasal becomes mobile, scraping bacterial mat perhaps, or ingesting sand, or scraping coral.

PS
Bothriolepis (FIg 5), another placoderm, did not move its mandibles. Instead it moved its nasals, rimmed by scraping premaxillae. The number of mandible elements in Bothriolepis corresponds with those of Rhinopteraspis (Fig 1), including those extra-wide medial elements with a narrow ‘neck’. Based on this example, the premaxilla also evolved several times. And of course it disappeared several times more. That is why phylogenetic analysis can be so vexing to score.

Variation builds upon variation.

References
Zhu et al 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354.6310:334-336.
wiki/Gnathostomata
wiki/Qilinyu
wiki/Poraspis

Overlooked interrelationships document yet another origin of jaws

The placoderm > shark transition

Apparenly the shark ‘palatoquadrate’ = the placoderm preoperculum + jugal.
These two taxa, the tiny unnamed Early Devonian placoderm ANU V244 (Fig 1) and the Middle Devonian chondrichthyan, Gladbachius (Fig 1) apparently have something to teach us about shark and placoderm skull homology.

Figure 1. The unnamed tiny Late Silurian transitional placoderm, ANU V244 was preserved in perfect 3d. Colors added here. Note the jugal (cyan) is curling over the preoperculum (light yellow) as in Gladbachus in figure 2.

Unnamed genus
(ANU V244, Young, Lelièvre and Goujet 2001; Hu, Lu and Young 2017; Early Devonian) nests at the base of several clades in the LRT. That’s why it keeps cropping up. It is a phylogenetically miniaturized descendant of placoderms like Coccosteus. Here the palatine (blue) has a carpet of teeth. The dentary is a slender strip.The maxilla is not yet developed. The jugal (cyan) is curling over the preoperculum (light yellow) as in Gladbachus (Fig 2).

Figure 2. The large Middle Devonian basal chondrichthyan, Gladbachus here compared to the tiny unnamed Early Devonian ANU V244. Note the large palatoquadrate of Gladbachus is built upon the similar cheek elements in ANU V244, only wider and larger. The gill bars have migrated posteriorly.

Gladbachus adentatus
(Heidtke & Krätschmer 2001; Burrow and Turner 2013; Coates et al. 2018; Middle Devonian, est. 54cm) was originally considered an ‘unfamiliar’ basal chondrichthyan close to acanthodians (spiny sharks). Here it nests near the base of the shark clade. This toothless or tiny-toothed taxon with a wide gape and flat skull was apparently a filter feeder with enormous gill bars. The unique holotype preserves the pelvic area. Compare Gladbachus to the earlier, smaller, narrower ANU V244 specimen (Fig 1).

Hopefully this bit of insight will help complete the fish subset of the LRT. These are the sort of problems that affect scores and resolution.

References
Coates MI, et al (7 co-authors) 2018. An early chondrichthyan and the evolutionary assembly of a shark body plan. Proceedings of the Royal Society B 285(1870):20172418.
Heidtke UHJ and Krätschmer K 2001. Gladbachus adentatus nov. gen. et sp., ein primitiver Hai aus dem Oberen Givetium (Oberes Mitteldevon) der Bergisch Gladbach – Paffrath-Mulde (Rheinisches Schiefergebirge). Mainzer geowiss. Mitt. 30, 105–122.
Hu Y, Lu J and Young 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
Young G C, Lelièvre H and Goujet D 2001. Primitive jaw structure in an articulated brachythoracid arthrodire (placoderm fish; Early Devonian) from southeastern Australia. J. Vertebr. Paleontol. 21, 670–678.

wiki/ANU V244 – not yet posted
wki/Gladbachus

Xenorophus: closer to giant Livyatan than to dolphins in the LRT

Boessenecker and Geisler 2023
described new specimens of the South Carolina Late Oligocene odontocete, Xenorophus, (Figs 1, 3) otherwise known for over 150 years. The authors considered this taxon “an ancient dolphin” in their headline, but called it a “dolphin-size cetacean” in their abstract, then “an early diverging dolphin (stem Odontoceti)” in their text.

Figure 1. Xenorophus skull from Boessenecker and Geisler 2023. Colors added here.

Xenorophus sloanii
(Leidy 1869, Boessenecker and Geisler 2023, Late Oligocene) nests with the much larger Livyatan in the LRT, not with dolphins and not prior to Simocetus, despite its dolphin-like size.

Taxon exclusion: Livyatan was not in the authors’ taxon list, but was mentioned in the text when they described worn down teeth and other trivia.

Figure 2. Subset of the LRT focusing on Odontoceti and their ancestors. Here Xenorophus nests with Livyatan.

Phylogenetically
Xenorophus fits neatly into a gradual evolutionary sequence transitional between the archaeocete, Aetiocetus, and sperm whales (Physeter, Fig 3). The dolphin clade nests as the sister clade to the sperm whale clade in the LRT (Fig 2).

Figure 3. Xenorophus and kin to a common skull length and to scale.

Boessencker and Geisler reported.
“the early toothed mysticete Coronodon appears to only have a single ‘hanging’ rib.”

By contrast, Coronodon is basal to Odontoceti in the LRT (Fig 2). Mysticetes arose from a completely separate list of ancestors that include oreodonts, mesonychids, hippos, anthracobunids and desmostylians in the LRT. The myth of cetacean monophyly is traditional. It needs to stop.

Figure 4. Cladogram from Boessenecker and Geisler 2023, slightly modified to fit this narrow column. The red circle icon is added because mysticetes are not related to odontocetes. Yellow boxes are taxa also found in the LRT (Fig 2). Note the pig (Sus) and cow (Bos) used as cherry-picked outgroup taxa here.

Sadly,
whale experts Boessenekcer and Geisler still follow the myth that pigs, cows and mysticetes (baleen whales) are related to the clade Odotoceti (Fig 4). This is what they are still teaching at the university level. Build your own LRT to confirm, refute or modify this hypothesis of toothed whale interrelationships.

I would hate to be a whale expert nowadays. Due to peer group (= academic pressure) they don’t have the freedom to test new taxa and develop new hypotheses. They are stuck with pigs and cows for outgroup taxa. It’s good to be independent of this influence.

Figure 5. Odontocete evolution according to the LRT from several years ago. Pakicetus is a giant tenrec, not a relative of pigs and cows. a

In the LRT,
adding taxa recovered anagalids, sengis and tenrecs as odontocete ancestors (Figs 2, 5). Andrewsiphius (Fig 5) is considered a basal whale by Wikipedia, and it nests basal to extant tenrecs in the LRT. More evidence that adding taxa, as in the LRT, resolves phylo issues.

Taxon exclusion remains the number one problem facing paleontology.

References
Boessenecker RW and Geisler JH 2023. New Skeletons of the Ancient Dolphin Xenorophus sloanii and Xenorophus simplicidens sp. nov. (Mammalia, Cetacea) from the Oligocene of South Carolina and the Ontogeny, Functional Anatomy, Asymmetry, Pathology, and Evolution of the Earliest Odontoceti. Diversity 2023, 15, 1154.
https:// doi.org/10.3390/d15111154
Leidy J. 1869. The extinct mammalian fauna of Dakota and Nebraska, including an account of some allied forms from other localities, together with a synopsis of the mammalian remains of North America. J. Acad. Nat. Sci. Phila. Second Ser. 1869, 7, 8–472.

wiki/Simocetus
wiki/Dwarf_sperm_whale
wiki/Physeter
wiki/Livyatan (Leviathan)
wiki/Xenorophus

Two new Jurassic Morrison Formation ?gekkos: Helioscopus and Limnoscansor

Meyer et al 2023 described
“the first North American stem gekkotan based on a three-dimensionally preserved skull.” Another more complete, but crushed specimen (Limnoscansor Fig 2) was also included in this study. The Gauthier-led team also reported, “They are among the oldest divergences in the lizard crown, so understanding the origin of geckoes (Gekkota) is essential to understanding the origin of Squamata, the most species-rich extant tetrapod clade.”

According to the large reptile tree (LRT, 2037 taxa) gekkos are not essential to understanding the origin of Squamata, which is led by iguanids with immediate outgroups going back to the Permian (see Lacertulus and Saurosternon). By contrast gekkos ARE essential to understanding the origin of snakes (Fig 1), a result not recovered by Meyer et al, apparently due to taxon exclusion, discussed earlier when we looked at Gauthier er al 2012 in a 3-part series. The authors relied on that decade-old cladogram in this 2023 study.

Figure 1. Subset of the LRT focusing on snakes and their ancestors including tested gekkos.

Meyer et al wrote,
“To assess the phylogenetic affinity of Helioscopus dickersonae within Squamata, we used a modified version of the dataset of Gauthier et al. 2012, one of the largest and most comprehensive morphological analyses of squamates.”

In 2012 we looked critically at Gauthier et al 2012 in a three part series (links below).

Figure 2. From Meyer et al 2023 Limnoscansor in situ. Colors and comparable taxa added here.

In the cladogram of Meyer et al 2023
their two new taxa, Limnoscansor and Helioscopus, both nest with Ardeosaurus, while Schoenesmahl, Eichstaettisaurus and Norellius nest nearby at the base of their Pan-Gekkota.

Meanwhile, in the LRT
(Fig 1) other than the pre-squamate Schoenesmahl (formerly Bavarisaurus), all these taxa are not gekkos, but gekko-like pre-snakes ~ close to gekkos.

Another outgroup issue
These authors included Huehuecuetzpalli as a squamate outgroup, and it is one in the LRT. However this was a cherry-picked taxon (along with Sphenodon) – not a natural proximal outgroup recovered by a wide gamut analysis, like the LRT. The authors are unaware that Huehuecuetzpalli nests with Macrocnemus in the LRT (Peters 2007), not close to the origin of Squamata, but closer to Tanystropheus and pterosaurs.

Adding taxa will help the Gauthier team, but don’t hold your breath waiting for that to happen. It’s been a decade since Gauthier et al 2012 made their earlier mistakes without rectification.

I won’t be adding Limnoscansor and Helioscopus to the LRT.
The former appears to be very close to Ardeosaurus (Fig 2) and the latter is not represented by enough distinct material, despite being preserved in 3D and µCT scanned.

Gosh it feels good to get away from fish systematics for at least one post every so often.

References
Gauthier JA, Kearney M, Maisano JA, Rieppel O and Behlke ADB 2012. Assembling the squamate tree of life: perspectives from the phenotype and the fossil record. Bull. Peabody Mus. Nat. Hist. 53, 3–308.(doi:10.3374/014.053.0101)
Meyer D, Brownstein CD, Jenkins KM, Gauthier JA 2023. A Morrison stem gekkotan reveals gecko evolution and Jurassic biogeography. Proc. R. Soc. B 290: 20232284.
https://doi.org/10.1098/rspb.2023.2284
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.

Assembling the Squamate Tree of Life – part 1

Assembling the Squamate Tree of Life – part 2 – Tchingisaurus

Assembling the Squamate Tree of Life – part 3 – the Tritosauria and the Krypteia

Salmon and moray eels now nest together in the LRT

Once again,
when finally brought together these taxa appear to be especially good matches for each other… except post-cranially. Now Salmo, the salmon (Fig 1), nests basal to moray eels (Gymnothorax = Lycodontis) and deep sea gulper eels (Eurypharnyx) in the large reptile tree (LRT, 2306 taxa, subset Fig 2).

Figure 1. The skull of the extant salmon (Salmo) compared to the skull of Gymnothorax funebris (formerly Lycodontis). Diagrams from Gregory 1933. Colors added here.

Billfishes and bowfins are also part of this clade
in the LRT (Fig 2), which was never designed to lump and split fish, but keeps doing a great job scoring basic traits due to the application of tetrapod homologs to skull bones. Correctly dentifying those tetrapod homologs has been the issue until now.

Figure 2. Subset of the LRT focusing on Salmo and its closest relatives, the moray eels (Gymnothorax) and the deep sea gulper eel, (Eurypharynx).

More and more taxa
are joining the bowfin, Amia, in the LRT. This appears to be a novel hypothesis of fish phylogeny. The work is still not finished. This is what life-long learning is all about.

References
wiki/Salmon
wiki/Eurypharynx
wiki/Gymnothorax

Cui et al document the heretofore missing half of the small-eyed ‘placoderm’, Entelognathus

Cui et al wrote,
“The 425-million-year-old fish Entelognathus (Fig 1) combines an unusual mosaic of characters typically associated with jawed stem gnathostomes or crown gnathostomes. Strikingly, its scales are large and some are rhomboid, bearing distinctive peg-and-socket articulations; this combination was previously only known in osteichthyans and considered a synapomorphy of that group.”

Don’t rely on traits to define a clade. We call this “Pulling a Larry Martin.” Instead use the last common ancestor method in phylogenetic analysis (Fig 2) and then describe the traits, making allowances for possible (or probable) convergence elsewhere.

Taxon exclusion appears to be responsible
for the phylogenetic issues that vexed these authors. By testing more taxa in the large reptile tree (LRT, 2306 taxa, Fig 2) Entelognathus nested with other very similar taxa with robust scales, like Guiyu, Miguashaia and Dialipina. All four nested between traditional placoderms and traditional catfish, including Hoplosternum, the extant armored catfish. Among the three closest relatives in the LRT only Guiyu is mentioned in the Cui et al text. Catfish are not usually included in placoderm analyses.

Figure 1. Enteolgnathus model in two views from Cui et al 2023.. LRT related taxa added here include Dialipina, Miguashaia and Guiyu. Note all four share a similar size, overall shape and differ only in the subtle details. All four are heavily scaled.

Cui et al wrote,
“The presence in Entelognathus of an anal fin spine, previously only found in some stem chondrichthyans, further illustrates that many characters previously thought to be restricted to specific lineages within the gnathostome crown likely arose before the common ancestor of living jawed vertebrates.”

Anal fins, rather than spines, are preserved in two LRT related taxa (Fig 1). The anal portion of Guiyu is not exposed. A tiny primitive taxon, Shenacanthus, has an anal spine seen here.

The authors made the traditional mistake of assuming a single genesis of jaws in living vertebrates. The LRT falsified that in October 2023.

Figure 2. Subset of the LRT focusing on placoderms and kin surrounding Entelognathus. Here it nests with similar taxa with small eyes and large scales including Dialipina, Miguashaia and Guiyu. Only the latter is mentioned in the text.

Cui et al wrote,
“The phylogenetic analysis placed Entelognathus and Qilinyu (Fig 3) in a clade as the immediate sister lineage of crown gnathostomes, confirming both the pivotal position and themonophyly ofmaxillate placoderms.”

By contrast, in the LRT Qiilinyu (Fig 3) nests with a more similar jawless taxon, Poraspis (Fig 3, not mentioned in the text), marking the genesis of placoderms prior to the invention of an anteriorly curved moveable mandible in this clade of gnathostomes.

Remember, our ancestors developed jaws independent of placoderms, sharks and catfish, as documented in October 2023 here. This very recent news post-dates submission of the Cui et al manuscript and figures to the editors of Nature. So no one in academia knew this then.

Figure 3. Jawless Poraspis to scale with the related basal placoderm, Qilinyu. New tetrapod homology colors are applied here. At first glance, Qilinyu does resemble Entelognathus, but other taxa in the LRT are more similar to each one, separating them from each other phylogenetically.

Cui et al reported,
“Our analysis generated 100,000 trees,” which was their ‘maximum trees in memory’ limit. When this happens, something (usually scoring), is wrong, based on 25 years of experience. Go back and review your scores and get rid of taxa based on scraps.

Entelognathus primordialis
(Zhu et al. 2013; Zhu et al 2016, Cui et al 2023; Late Ludlow, Late Silurian, 419 mya; IVPP V18620) is a genus of placoderm fish with tiny eyes. Here (Fig 4) skull bones are re-identified with their tetrapod homologies. Pre-teeth are tiny pustules and wrinkles on the bone. Only a few days ago we looked at the scooping mouth of Entelognathus.

Documenting the back half of Entelognathus is welcome news.
The heavy scalation comes as no surprise to the LRT which was expecting that based on the similar traits found in related taxa (= phylogenetic bracketing, Fig 1 ). It is also important to keep working on your cladogram if it keeps recovering the maximum limit of trees (see above). Add taxa and re-check your scores. Ideally a single, fully-resolved tree should be your goal (if you can avoid using taxa based on scraps).

References
Cui X-D, Friedman M, Yu Y, Zhu Y-A and Zhu M 2023. Bony-fish-like scales in a Silurian maxillate placoderm. Nature Communications doi.org/10.1038/s41467-023-43557-9
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/Entelognathus

A new scooping mouth tip for the small-eyed placoderm, Entelognathus

A postfrontal looking and acting exactly like a hyomandibular

I was walking around with a cartoon question mark over my head
for the longest time whenever I studied Jagorina (Fig 2) and Gemuendina (Fig 3), two ray-like traditional placoderms. I wondered, ‘what were the transitional taxa that connected them with more traditional placoderms?’ It seemed they tended to stand apart from all the others.

In evolution no taxon should ever stand alone or apart.

Once again,
it took a plesiomorphic Bauplan to understand an otherwise cryptic derived taxon. This time the largely traditional, but notably ultrawide-skull placoderms, Stenosteus (Fig 2) and Titanicthys (Fig 1) provided that transitional Bauplan. Both seem to have been weak-jawed bottom feeders sifting and filtering quantities of sand for randomly buried prey items.

Figure 1. Late Devonian Titanichthys compared to scale with Late Silurian Entelognathus. See figure 2 for more closely related taxa. Note the large red quadrate, the jaw joint, and its relationship to the jaw joint anchor, the hyomandibular (dark green), a bone that started externally, but soon moved internally to do its job.

Starting with Titanichthys
(Fig 1) note the proximity of the postfrontal (orange) to the fused postorbital-preoperuclar-lacrimal (amber-light yellow-tan) compared to the more posterior hyomandibular (dark green in Fig 1). Also note the medial connection of the postfrontal to the two small skull bones, the intertemporal and supratemporal (yellow-green and green).

In almost all gnathostomes the hyomandibular connects to the jaw joint (the red quadrate) with an internal process (not shown here).

If you’re looking for teeth, these taxa probable had a premaxilla pasted beneath the tiny nasals, but the maxilla had not yet appeared.

Figure 2. Jagorina (left) and Stenosteus (right). Colors added here. Note the fusion of the parietals, the loss of the frontals and nasals and the connection of the bar-like postfrontal to the fused preorbital-preopercular-lacrimal in Jagornia. That same connection is not made in Stenosteus, which has a more primitive skull. The postfrontal was identified here based on its geographic relationship with other bones, not by what it does in this case, which is different from nearly all other vertebrates.

Compare the Titanichthys setup with the similar, but different elements
in the more derived wide-skull of Jagornia (Fig 2 left). Also compare Titanichthys (Fig 1) to the more primitive Stenosteus (Fig 2 right), which is narrower and closer to Coccosteus Fig 2 lower right corner. In Jagornia we see the fusion of the parietal elements, the loss of the frontals + nasals and the retained connection of the now bar-like postfrontal to the fused preorbital-preopercular-lacrimal, now also reduced to a curved bar forming the palate.

The same postfrontal connection is made in Gemuendina (Fig 3), a ray-like relative of Jagorina. The quadrate (red) is reduced to a vestige in both. The hyomandibular remains a broad surface plate in both alongside the tabulars (light red).

Figure 3. The ray-like placoderm Gemuendina skull in situ. Colors added here. Compare to Jagorina in figure 2. Note the plate-like hyomandibular (dark green) not associated with the mandible, which is anchored here by the postfrontal (orange).

Now that the cartoon question mark over my head has popped,
and the (hypothetical) solution is shared, it’s worthwhile to really appreciate the prevalence of convergence in vertebrates. Problems like this, especially when not recognized, lead to incorrect scoring in phylogenetic analysis. This is why the fish subset of the LRT is taking so long to complete. Hope this helps in your own studies.

If this is already taught at the uiniversity level, let me know.

Evolution and synonyms of the hyomandibular and intertemporal

Five fat, flat basal gnathostomes compared

Sometimes it is worthwhile
to gather together and compare related taxa in one image so we can more clearly see and appreciate both the subtle and obvious traits and proportions that lump and separate them in analysis. These taxa (Fig 1) are all basal gnathostomes close to or within traditional placoderms in the LRT. We looked at each one earlier. Here they are collected together and updated letting one inform on another. None are larger than a human hand. Some are as small as a finger.

This is a difficult bunch. The original authors assigned several to lobe fin clades.

Figure 1. Bianchengichthys, Miguashaia, Dialipina and Guiyu in situ and reconstructed using DGS methods. In the LRT these four nest close to one another. Note the fuzzy-tipped posterior fins in the top three taxa along with the armored pectoral fins sporadically preserved. Also note the lack of any internal vertebrae in the bottom two taxa, both opened up by similar a ‘bite mark’, in contrast to Dialipina. Also note the transformation of the carapace into a dorsal spine in the bottom three taxa, displaced to the tail in Dialpina.

The fifth flat, fat basal gnathostome,
Early Devonian Drepanaspis (Fig 2), is re-presented here with more attention to detail and revised DGS colors based on the Bauplan of Bianchengichthys (top image Fig 1). Drepanaspis lacks a thoracic shield, but has a heavily scaled thorax and tail, as in Middle Devonian Miguashaia, Early Devonian Dialipina and Late Silurian Guiyu (Fig 1). Under closer examination with Bianchengichthys and the other three taxa (Fig 1) as my new guides:

Here in Drepanaspis nares are identified atop the nasals, as in Bianchengichthys.

Here a premaxilla (yellow) with tiny teeth is revealed beneath a broken piece of nasal.

Here a tiny vestige of a quadrate (red) is located anterior to the tiny orbit notch.

Here the former caudal fin is reinterpreted as a dorsal fin appearing prior to what appears to be a broken off caudal fin, creating a sort of double caudal fin.

Figure 2. Drepanaspis, is presented here with more attention to detail and revised DGS colors.

If you know of another published interpretation of these five taxa
with as much attention to detail, let us all know in the comments. Mistakes will be corrected.

Once again
having a good Bauplan (Fig 1) really helps to understand difficult taxa (Fig 2) ~ one more reason to keep adding taxa to your own analysis and studies. Each one informs on the other, especially in the most difficult taxa.