Placoderm jaw de-volution

About an hour ago, 
the question of pelvic girdles before jaws in vertebrate (more specifically, placoderm) evolution was reviewed in light of the LRT.

Now let’s re-examine
another tiny placoderm whose interrelationships were originally misinterpreted due to taxon exclusion.

Hu, Lu and Young 2017
studied jaw structure in a really tiny unnamed Devonian placoderm, ANU V244 (Fig. 1, shown 3x larger here) preserved in 3D. Unfortunately, Hu, Lu and Young followed tradition when they thought placoderms represented the genesis of jaw evolution, preceding the appearance of jaws in sharks and bony fish.

By contrast,
the large reptile tree (LRT, 1697+ taxa, subset Fig. 2) recovers sturgeons and Chondrosteus at the genesis of jaws, immediately preceding sharks + bony fish. In the LRT placoderms nest deep within bony fish, after the great dichotomy. Placoderms like the ANU specimen represent a reduction of jaw elements, not the acquisition. Placoderm precursors, like Cheirodus and Eurynotus, lose or fuse the maxilla. The ANU specimen also loses the premaxilla and reduces the mandible and dentary, which retains teeth.

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

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

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on the catfish + placoderm clade, starting with a spiny shark, Diplacanthus.

The ANU specimen nests with the much larger Romundina (Fig. 2), a bottom feeder with reduced jaw elements and large cheeks.

The ANU specimen
is only one of several placoderms with reduced jaws (Fig. 3).

The LRT has been adding fish taxa over the past year, when the first catfish was nested with the first few placoderms. As it stands now, catfish are still closely related to placoderms in the LRT (subset Fig. 2).

The origin of placoderms would make a great PhD thesis, seen from all angles.

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified

Figure 3. A sample of taxa related to Autroptyctodus with homologous skull bones color identified.

Once again,
a valid phylogenetic analysis that includes a sufficient number of pertinent taxa is key to understanding interrelationships. Don’t get turned around by using the traditional list of too few taxa. Don’t assume your predecessors and professors are correct. Test their hypotheses. Add taxa to provide, determine and validate the proper phylogenetic context in all cases. In can tell you from experience, it will be rewarding.


References
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

Antiarch placoderms: pelvic girdles before jaws? No.

Zhu et al. 2012
report that “An antiarch placoderm (Fig. 1) shows that pelvic girdles arose at the root of jawed vertebrates.” They are wrong according to the the large reptile tree (LRT, 1697+ taxa).

Contra Zhu et al. 2012,
jaws were just disappearing, not just appearing, in this taxon. Pelvic fins and their pelvic anchors are known in many more primitive taxa in the LRT.

Taxon exclusion, once again,
rises to the top of paleontological sins (of omission).

Figure 1. Parayunanolepis, an antiarch placoderm and the subject of the Hu et al. paper.

Figure 1. Tiny Parayunanolepis, an antiarch placoderm and the subject of the Zhu et al.2012 paper, shown more than 2x life size.

From the Zhu et all. 2014 abstract:
“To date, it has generally been believed that antiarch placoderms (extinct armoured jawed fishes from the Silurian–Devonian periods) lacked pelvic fins. Parayunnanolepis xitunensis represents the only example of a primitive antiarch with extensive post-thoracic preservation, and its original description has been cited as confirming the primitive lack of pelvic fins in early antiarchs. Here, we present a revised description of Parayunnanolepis and offer the first unambiguous evidence for the presence of pelvic girdles in antiarchs.”

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

By contrast,
in the LRT tiny  Parayunnanolepis nests with the much larger Bothriolepis, a highly derived placoderm. Several taxa preceding these two have pelvic fins and jaws.

A valid cladogram
is the most important tool in recovering the order of gradually accumulating traits.

Earlier you may remember,
placoderms arose from ordinary fish, not the other way around. The LRT has reordered many tree branches, all due to taxon inclusion. In this fashion the LRT helps recover overlooked hypothetical interrelationships.

References
Zhu M, Yu X-B, Choo B, Wang J-Q and Jia L-T 2012. An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters Palaeontology 8:453–456.

 

Rugarhynchos: Late Triassic archosauriform really close to Doswellia

A former Doswellia sp.
(Heckert et al. 2012) has be reexamined and renamed Rugarhynchos sixmilensis by Wynd et al. 2020.

The resemblance is remarkable
(Fig. 1) and the size is similar. Both are from the Late Triassic of North America (Virginia and New Mexico). Wynd et al. did a good job of tracing the bones, but provided no reconstructions (they pictured the premaxilla on a separate page spread). They also misidentified the surangular (SA) as the quadratojugal.

Is this just another species of Doswellia?
We’ve seen more variation in Rhamphorhynchus, and Pteranodon, but naming a new genus is reserved for full professors and their students. In this case, the resemblance is readily apparent, and so are the various enlargements and reductions. The problem lies, as it often does, in the published cladogram (Fig. 2) suffering from taxon exclusion.

Figure 1. Doswellia skull compared to Rugarhynchos, here reduced to a similar length for rapid comparison.

Figure 1. Doswellia skull compared to Rugarhynchos, here reduced to a similar length for rapid comparison.

From the abstract:
“Stem archosaurs exhibit substantial cranial disparity, especially by taxa either shortening or elongating the skull. This disparity is exemplified in the North American Late Triassic proterochampsians by the âshort-facedâ Vancleavea and the ong-faced doswelliids.”

When more taxa are added, as in the large reptile tree (LRT, 1695+ taxa; subset Fig. 3), Vancleavea nests with Helveticosaurus in the Thalattosauria, as we learned several years ago. Missing from the Wynd et al. taxon list are any choristoderes. Those are close relatives to doswellids in the LRT.

“To critically investigate skull elongation and character evolution in these proterochampsians, we evaluate Doswellia sixmilensis, known from much of a skull, cervical centra, and osteoderms from the Bluewater Creek Member of the Chinle Formation of New Mexico.” (See Fig. 1).

Figure 2. Cladogram from Wynd et al. 2020 with colors added to show where these taxa nest when more taxa are added, as in the LRT.

Figure 2. Cladogram from Wynd et al. 2020 with colors added to show where these taxa nest when more taxa are added, as in the LRT. Avemetatarsalia is invalid because it includes both pterosaurs and dinosaurs, neither of which is related to Vancleavea or Phytosauria in the LRT. Remember to check your results for mismatches like this.

From the abstract:
“Rugarhynchos sixmilensis, gen. et comb. nov., exhibits an elongate snout with characteristics known in stem and crown archosaurs, including a downturned premaxilla and fluted teeth.”

In the LRT, archosaurs include only crocs + dinos (including birds). Due to taxon exclusion (chiefly bipedal basal crocodylomorphs) Wynd et al. expand that list to include many other taxa.

“We included R. sixmilensis in a phylogenetic analysis of archosauromorphs consisting of 677 characters and 109 taxa under both parsimony and Bayesian models.”

Now do you see why increasing the number of taxa is MUCH more important than increasing the number of characters? How one taxon relates to other taxa requires a lot of other taxa… and a sufficient number to traits (150+). The LRT includes 238 multi-state taxa and it nests everything from fish to humans with high resolution.

“We recover R. sixmilensis as a doswelliid, sister to Doswellia kaltenbachi. Our parsimony and Bayesian models differ in the placement of Doswelliidae, either as sister to or within Proterochampsidae, respectively.”

Wynd et al. excluded too many pertinent taxa. Here’s where the LRT (Fig.3) nests Doswellia and the pararchosauriformes.

“We use archosauromorph relationships from the Bayesian model to estimate cranial disparity between stem and crown archosaurs and find a narrow breadth of morphological disparity in the stem. Our results suggest that crown archosaurs evolved disparate crania from a low-disparate archosauriform condition.”

Without a valid phylogenetic context (Fig. 3), the results of Wynd et al. cannot be validated. They need more taxa.

Figure 3. Subset of the large reptile tree focusing on the pararchosauriformes and the Choristodera.

Figure 3. Subset of the large reptile tree focusing on the pararchosauriformes and the Choristodera, still similar since 2015. Euparkeria is basal to the Euarchosauriformes, including Archosauria.

The skull of Rugarhynchos was added to a graphic
(Fig. 4) that included Doswellia and its relatives to scale. Many of these taxa were omitted from Wynd et al. 2020.

Figure 3. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa.

Figure 4. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa.

Wynd et al. considered Rugarhynchos a proterochampsid.
With more taxa added (Figs. 3, 4) that’s not confirmed by the LRT. Doswellia is slightly closer to choristoderes, a clade not shown in the Wynd et al. cladogram (Fig. 2). It would have been better if Wynd et al also added a variety of proterosuchids, as in the LRT. They are all as different and distinct as Rugarhynchos is from Doswellia.


References
Wynd BM, Nesbitt SJ, Stocker MR and Heckert AB 2020. A detailed description of Rugarhynchos sixmilensis, gen. et comb. nov. (Archosauriformes, Proterochampsia), and cranial convergence in snout elongation across stem and crown archosaurs. Journal of Vertebrate Paleontology Article: e1748042
doi: https://doi.org/10.1080/02724634.2019.1748042
https://www.tandfonline.com/doi/full/10.1080/02724634.2019.1748042

The sarcastic fringehead (Neoclinus blanchardi) enters the LRT with a GASP!

They fight for territory by ‘kissing’ each other with giant multi-colored lips.
And yet, the sarcastic fringe head (Neoclinus blanchardi; Girard 1858) is a sister to the far less flamboyant cave dweller, the wolfish, Anarhichas (Fig. 3) in the large reptile tree (LRT, 1696+ taxa).

Figure 1. The sarcastic fringehead becomes bizarre when opening its mouth in a threat gesture.

Figure 1. The sarcastic fringehead becomes bizarre when opening its mouth in a threat gesture.

When two related taxa are placed in the same view
it becomes easier to see at a glance the relationship and evolutionary changes. Color helps decipher the bones here (Fig. 2).

Figure 2. Neoclinus blanchardi, the sarcastic fringehead, skull, here colored with tetrapod homologs.

Figure 2. Neoclinus blanchardi, the sarcastic fringehead, skull, here colored with tetrapod homologs.

The mahi-mahi
Coryphaena) is the fast, free-swimming outgroup to this clade.

Figure 1. Skull of the wolffish, Anarhichas. Compare to the pupfish in figure 3.

Figure 1. Skull of the wolffish, Anarhichas. Compare to the pupfish in figure 3.

Here’s a David Attenborough video from the BBC.
All the action happens at the end of this segment.

References
Girard CF 1858. Fishes. In: General report upon zoology of the several Pacific railroad routes, 1857 (, ed.), Beverley Tucker, Washington, D.C. No. U.S. Senate Document No. 78: i-xiv, 1-400, pls. 1-21.

 

Campbellodus diagram reconstructed

Long 1997
presented his version of the Late Devonian ptyctodontid placoderm, Campbellodus (Fig. 1, Miles and Young 1977).

Figure 1. Campbellodus diagram from Long 1997. Compare to reconstruction in figure 2.

Figure 1. Campbellodus diagram from Long 1997. Compare to reconstruction in figure 2. Colors added here.

Based on phylogenetic bracketing,
using Cheirodus (Fig. 2) as a guide, I moved the originally disarticulated elements of Campbellodus from the Long 1997 diagram into a reconstruction (Fig. 2) not far from its phylogenetic sister (Fig. 1). It helps to have a blueprint when working with roadkill.

Figure 2. Cheirodus and Campbellodus to scale. These two nest together in there LRT.

Figure 2. Cheirodus and Campbellodus to scale. These two nest together in there LRT. Compare to original diagram in figure 1.

Campbellodus decipiens (Miles and Young 1977; Late Devonian) The posterior mandible parts in the diagram are re-identified here as internal jaw elements + a hyomandibular. Other elements are colorized using tetrapod homologs. Currently (Fig. 3) traditional placoderms are diphyletic, with ptyctodontids apart from the rest of the placoderms.

As a reminder,
fish experts do not yet recognize the similarities in traits shared by Campbellodus and Cheirodus. Hopefully, that will change someday, because the two are quite similar.

Figure x. Newly revised fish subset of the LRT

Figure 3. Newly revised fish subset of the LRT

We’re going to have to homologize fish skull bones
with tetrapod skull bones someday. Sooner would be better than later. Many bones, like the frontals, quadrate and premaxilla are already homologized. Just a few remain.


References
Long JA 1997. Ptyctodontid fishes from the Late Devonian Gogo Formation, Western Australia, with a revision of the German genus Ctenurella Orvig 1960. Geodiversitas 19: 515-555.
Miles RS and Young GC 1977. Placoderm interrelationships reconsidered in the light of new ptyctodontids from Gogo Western Australia. Linn. Soc. Symp. Series 4: 123-198.

wiki/Campbellodus

The Berlin Naturkundemuseum Pterodactylus reconstructed

The MBR 3655 specimen of Pterodactylus in situ
looks like roadkill. Here (Fig. 1) a second sort of DGS (Digital Graphic Segregation) is used to reassemble the jumble. This sort does not rely on someone tracing each bone with transparent color. This goes faster and further minimizes freehand bias and error. More of the pertinent pixels in the original are used in the reconstruction.

Figure 1. The MBR3655 specimen of Pterodactylus reconstructed using DGS methods from the in situ photo.

Figure 1. The MBR3655 specimen of Pterodactylus reconstructed using DGS methods from the in situ photo. The foot proportion pattern is unique and the sternum rccalls that of Scaphoganthus.

When added
to the Large Pterosaur Tree (LPT, 289 taxa) this taxon nests at the base of one of the Pterodactylus clades that include the Vienna specimen (NHMW 1975/1756) and the n21 specimen (BSP1937 I 18). Still have not found two identical (conspecific) taxa from the Solnhofen Formation except the only known juvenile Rhamphorhynchus, a mid-sized juvenile of one of the largest species discussed earlier here.


References
Broili F 1938. Beobachtungen an Pterodactylus. Sitz-Bayerischen Akademie der Wissenschaten, zu München, Mathematischen-naturalischenAbteilung: 139–154.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

wiki/Pterodactylus

Placoderms evolve from ordinary fish

Everyone thought placederms were exotic primitive fish
that evolved long before regular fish evolved. That turns out to be not true when more taxa are added.

Eurynotus crenatus (Traquair 1880)
is a largely overlooked bony fish that links Cheirodus and catfish to placoderms (Fig. 1) in the large reptile tree (LRT 1696+ taxa; subset Figs. 2,4).

Figure 1. Eurynotus crenatus nests between Cheirodus and basal placoderms like Coccosteus and Autroptycodus in the LRT. Friedman et al. provided the jugal and mandible of Eurynotus, somewhat distinct from the Watson 1928? engraving at right. Note how the large jugal of Eurynotus splits to form a posterior squamosal.

Figure 1. Eurynotus crenatus nests between Cheirodus and basal placoderms like Coccosteus and Autroptycodus in the LRT. Friedman et al. provided the jugal and mandible of Eurynotus, somewhat distinct from the Watson 1928? engraving at right. Note how the large jugal of Eurynotus splits to form a posterior squamosal.

Eurynotus data is hard to find.
Recently Friedman et al. 2018 discussed the durophagus (hard-shelled prey) feeding strategies of Eurynotus. Otherwise only the Traquair 1880 (Fig. 1) provides Eurynotus data for the LRT. (Thanks to Dr. Friedman for providing a PDF!)

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Figure 2. Subset of the LRT focusing on catfish + placoderm clade.

Friedman et al. reported, “Several lineages of durophagous placoderms appeared during the Devonian. The most diverse of these, the ratfish-like ptyctodonts. Isolated dental plates represent the most common remains of ptyctodonts, which have greatly reduced cranial and thoracic armour relative to other placoderms.” 

Unfortunately,
Friedman et al. had it backwards due to taxon exclusion. They did not understand that ptyctodonts (Fig. 1) were placoderm-mimics, sisters to Cheirodus and closer to catfish in the LRT.

Figure 3. Eurynotus nests between Cheirodus and ptyctodont placoderms in the LRT.

Figure 3. Eurynotus nests between Cheirodus and ptyctodont placoderms in the LRT.

Eurynotus crenatus (Agassiz 1835; Traquair 1880; Friedman et al. 2018; Early Carboniferous, 310 mya) is widely considered a relative of Cheirodus and a member for the Lepidoid family of ganoid scale fish. Here Cheirodus and Eurynotus respectively are transitional taxa leading to the ptycodontid placoderm mimics (Austroptyctodusand other placoderms, like Coccosteus,  Note the absence of marginal teeth and the absence or fusion of the maxilla with the large jugal. These are traits retained by all later placoderms and their extant relatives, the catfish.

Figure x. Newly revised fish subset of the LRT

Figure 4. Newly revised fish subset of the LRT

And now a brief word for
Vertebrate Palaeontology Researcher in Residence Darren Naish.

Quoting from his ResearchGate.net page:  “I am currently working on a giant volume (over 1200 pp) titled THE VERTEBRATE FOSSIL RECORD.” That’s good. I support any wide-gamut look at vertebrate evolution. Naish started TVFR several years ago. He is currently seeking donations through Patreon where he includes sample pages (generously illustrated) along with his goals.

I have several concerns with this large project:

  1. Science textbooks, by their very nature, start to become out-dated the moment they are published. Nowadays its better to let others know of your hypotheses online. Those can be updated constantly. Online publication also guarantees worldwide distribution for free, both for the publisher and the reader.
  2. No indication of a wide gamut phylogenetic analysis appears to be at the core of this  project. In an open letter here, along with several private emails I have encouraged Naish to do this first, so he won’t be at the mercy of the innumerable academically published papers he will cite that suffer from taxon exclusion.
  3. He is sending his chapters out for review, which is standard practice in academic circles. Unfortunately that system perpetuates earlier textbook errors, leaving no room for repairs or novel hypotheses. In order to get a good review or a good grade in academia you have to repeat what the professor professes or your colleagues think is safe. We’ve seen where this goes horribly awry several times, but the go-to example can be found here.
  4. Naish knows how to build a phylogenetic analysis, but if the results someday mirror those in the Large Reptile Tree and its subtrees, that might be hard for him to accept—especially after all the bad press he has given ReptileEvolution.com. So, unless his wide-gamut, comprehensive cladogram goes in another direction, Naish has painted himself into a corner. After all, he has the PhD and I do not. This is more than ironic, because I started as a journalism student and retired as a scientist, while Naish started as a scientist and, if not careful, will end up being a journalist recounting the work of others in his TVFR.
  5. It’s never too late, Darren. Do the right thing and build your wide-gamut phylogenetic analysis. Then you will have the authority to posit your hypothesis—after running the experiment that must be run.

References
Agassiz JLR 1835. Recherches sur les Poissons fossiles, 5 volumes. Imprimerie de Petitpierre et Prince, Neuchaatel, 1420 pp.
Agassiz JLR 1835. On the fossil fishes of Scotland. Report of the British Association for the Advancement of Science, British Association for the Advancement of Science, Edinburgh.
Friedman M, Pierce SE, Coates M and Giles S 2018. Feeding structures in the ray-finned fish Eurynotus crenatus (Actinopterygii: Eurynotiformes) implicationsfor trophic diversification among Carboniferous actiniopterygians. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 109, 33–47.
Traquair RH 1880. X.—On the structure and affinities of the Platysomidae pp. 343 F.R.S.E., Keeper of the Natural History Collections in the Museum of Science and Art, Edinburgh.
Watson DMS 1928. On some points in the structure of palæoniscid and allied fish. Proceedings of the Zoological Society of London 1928, 49–70.

wiki/Eurynotus (not yet written)