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 1. 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

New study on Thylacosmilus atrox: “not a marsupial saber-tooth predator”??

Janis et al. 2020
bring us some heretical views regarding Thylacosmilus, the famous saber-toothed marsupial (Fig. 1).

Figure 2. Thylacosmilus skull. Note the deep maxillae in dorsal contact containing giant canine roots. These are not present in Patagosmilus.

Figure 1. Thylacosmilus skull. Note the deep maxillae in dorsal contact containing giant canine roots. These are not present in Patagosmilus.

For those in a hurry:
The Janis et al. study includes a phylogenetic analysis of placental sabertooth cats that nests the saber-toothed marsupial, Thylacosmilus (Fig. 1), at the base of the clade (Fig. 2). In a way, that is true, but this is missing so many transitional taxa that we’re left with apples and oranges. So, that’s not going to work because Janis et al. are testing analogy and convergence, rather than homology. It’s better to test apples and apples, even when dealing with stress tests, etc.

Figure 1. Cladogram from Janis et al. 2020. Note the lack of marsupial taxa, other than Thylacosmilus at the base.

Figure 2. Cladogram from Janis et al. 2020. Note the lack of marsupial taxa, other than Thylacosmilus at the base. Be wary whenever the taxon under review nests at the base of the cladogram.

Lacking here
is a phylogenetic analysis that includes the closest marsupial relatives of Thylacosmilus: 1. Schowalteria (Fig. 3), 2. Vincelestes + Conorytes, 3. Huerfanodon 4. Monodelphis + Chironectes. That’s how they line up in the large reptile tree (LRT, 1698+ taxa). You need related taxa to decipher the phylogenetic bracketing of Thylacosmilus based on homology, not analogy. The last two taxa are extant. One is an omnivore, the other an aquatic carnivore. Among the extinct taxa, Schowalteria, Vincelestes, Conoryctes and Huefanodon all appear to have been marsupial saber-toothed predators, contra the Janis et al. headline. Vincelestes goes back to the Early Cretaceous.

Figure 1. Showalteria. Not much there. Adding more rounds out the skull, a likely marsupial relative of Vincelestes.

Figure 3. Showalteria. Not much there, but enough to nest it with Thylacosmilus. Is this a predator? According to Wikipeia,.. no.

Janis et all. 2020 wrote:
“While the superficial appearance of Thylacosmilus atrox resembles that of placental saber- tooths, its detailed anatomy makes this animal an ecomorphological puzzle, and the analyses performed here show it to be unlike other carnivores, saber-toothed or otherwise”

That’s because they omitted related taxa…where are the comparable marsupials?

“While we can demonstrate that T. atrox could not have been a predator in the mode proposed for the saber-toothed feliform carnivorans, it is challenging to propose an alternative mode of life.”

That’s because they omitted related taxa…where are the comparable marsupials?

“We note that, while there is often the temptation to shoehorn an extinct animal into the ecomorphological role of an extant one (see Figueirido, Martín-Serra & Janis, 2016)—or even, as in this case, the proposed ecomorphological role another extinct animal—T. atrox may well have had no analogs in the extant or extinct fauna.

That’s because they omitted related taxa…where are the comparable marsupials?

“We extend this discussion of extinct animals without living analogs in the conclusions. Here we present some ecomorphological hypotheses for T. atrox that align with the peculiarities of its anatomy.”

  1. We note that the unusual subtriangular shape of these canines makes them appear more like a claw than a blade; and, like a claw, they appear well-adapted for pulling back.
  2. Our biomechanical study shows that both the skull and the canines of T. atrox are better in resisting pull back stresses than those of S. fatalis.
  3. The small infraorbital foramen of T. atrox supports the hypothesis that its canines were not used for killing prey, as it would not require such careful and precise positioning of the canines.
  4. The postcanine teeth of T. atrox exhibit blunted tip wear, unlike the shearing wear on the teeth of carnivores that specialize on flesh
  5. T. atrox was clearly not a bone-crusher: this type of diet is contraindicated by the DMTA analysis and the lack of cranial specializations (including evidence for powerful jaw adductors) seen in extant bone-crushers
  6. T. atrox had less powerful jaw adductor and head depressor muscles than placental saber-tooths. The cervical and caudal cranial anatomy are not indicative of the ability for extreme head elevation and forceful head depression, as observed in the anatomy of placental saber-tooths, implicated in those carnivores for a predatory head strike.
  7. The virtual absence of incisors (certainly the absence of a stout incisor battery) in T. atrox is challenging for the hypothesis of a cat-like mode of feeding, as it would have been unable to strip flesh from a carcass or transport its prey.

Janis et al. wonder:
“Could [soft internal organs] have been the preferred diet of the marsupial saber-tooth?”

Janis et al. propose:
“Incisor loss or reduction in mammals is correlated with the use of a protrusible tongue in feeding, as seen in myrmecophageous mammals.”

Janis et al. propose:
“T. atrox has been ‘‘shoehorned’’ (see Figueirido, Martín-Serra & Janis, 2016) into the saber-tooth ecological role: much less attention has been paid to the way in which this animal differs in its morphology from placental saber-toothed predators, making a similar type of predatory behavior unlikely.

“We advance the suggestion that it was not an active predator, but rather relied on the use of existing carcasses, deploying its large canines for carcass opening rather than for killing, a hypothesis supported by our biomechanical analyses that show superior performance in a ‘‘pull-back’’ scenario.”

“Thylcosmilus atrox was a very different type of carnivorous mammal than the placental saber-tooths: the oft-cited convergence with placentals such as Smilodon fatalis deserves a rethink, and the ‘‘marsupial saber-tooth’’ may have had an ecology unlike any other known carnivorous mammal.”

Figure 3. Maximum gape of Thylacosmilus.

Figure 4. Maximum gape of Thylacosmilus. At upper left is the placental, Smilodon, for comparison.

Geology = Environment
Thylacosmilus was found in a vast Miocene tidal flat environment with a wide variety of terrestrial and shalllow aquatic taxa. So that doesn’t help much.

Saving the best for last: getting back to Late Cretaceaous Schowalteria
Wikipeda report, “It is notable for its large size, being among the largest of Mesozoic mammals,[ as well as its specialization towards herbivory, which in some respects exceeds that of its later relatives.” This is based on tooth wear, with nearly all crowns gone (Fig. 3) in the only specimen known. Traditionally Schowalteria has been allied with styinodonts (Carnivora close to seals, but not to sea lions) and to taeinodonts (which the LRT found to be poylphyletic). Certainly, what little is known of Schowalteria is similar to these taxa. Others have omitted these LRT sisters in their analyses. Janis et al. omit the word ‘sister’ from their text.

Figure 1. Patagosmilus to scale alongside Hadrocodium. These sabetooth taxa are not directly related to Thylacosmilus in the LRT.

Figure 5. Patagosmilus to scale alongside Hadrocodium. These sabetooth taxa are not directly related to Thylacosmilus in the LRT.

Patagosmilus has been called a sister to Thylacosmilus,
but the LRT nested Patagosmilus (Fig. 5) with the tiny basal therian, Hadrocodium (Fig. 5) as we learned earlier here. Taxon inclusion produces surprises like this.

At times like this it’s good to test.
Deletion of Thylacosmilus changes nothing in the LRT. Deletion of Schowalteria changes nothing in the LRT.

In my opinion
it was great that Janis et al. 2020 tested Thylacosmilus with its placental analogs, but they should also have tested Thylacosmilus with its marsupial homologs.


References
Janis CM, Figueirido B, DeSantis L, Lautenschlager S. 2020. An eye for a tooth: Thylacosmilus was not a marsupial ‘‘saber-tooth predator’’. PeerJ 8:e9346 http://doi.org/10.7717/peerj.9346

https://en.wikipedia.org/wiki/Ituzaing%C3%B3_Formation

https://pterosaurheresies.wordpress.com/2018/12/20/marsupial-sabertooth-taxa-are-polyphyletic/

Reversals produce whale teeth, part 2

Short one today.
You might remember over a year ago a presentation on the evolution of mammal molars from simple to complex and back to simple again in toothed whales (Odontoceti, Fig. 1).

Figure 3. Mammal tooth evolution alongside odontocete tooth evolution, reversing the earlier addition of cusps.

Figure 1. Mammal tooth evolution alongside odontocete tooth evolution, reversing the earlier addition of cusps.

On the same note,
here’s a presentation of three skulls, Pachygenelus (pre-mammal cynodont), Megazostrodon (last common ancestor of all mammals in the large reptile tree (LRT, 1698+ taxa), and Maiacetus, a toothed pre-whale with limbs (Fig. 2).

Figure 1. The pre-mammal, Pachygenelus, the first mammal, Megazostrodon, and a transitional toothed whale, Maiacetus, with teeth highlighted to show the reversal in odontocete molars.

Figure 2. The pre-mammal, Pachygenelus, the first mammal, Megazostrodon, and a transitional toothed whale, Maiacetus, with teeth highlighted to show the reversal in odontocete molars. This may be the first time Megazostrodon was compared to a pre-whale.

Just concentrate on the teeth today,
and note how the simple cones (Fig. 3) of basal therapsids, then the canine led simple triangles of basal cynodonts (Fig. 2) then multicusped teeth of basal mammals (Fig. 2), slowly reversed over time to become, once again, triangles, then simple cones in odontocete whales (Fig. 4).

Figure 3. Basal therapsids, including Cutleria, with simple cones for teeth, as in odontocete whales.

Figure 3. Basal therapsids, including Cutleria, with simple cones for teeth, as in odontocete whales.

Figure 4. The killer whale (Orcinus orca) skeleton and skull with parts colorized.

Figure 4. The killer whale (Orcinus orca) skeleton and skull with parts colorized. Simple conical teeth line the jaws as in pre-cynodont synapsids.

As long-time readers know, baleen whales had their own evolution
as mysticetes arose from mesonychids, hippos, anthracobunids and desmostylians in turn, according to results recovered from the large reptile tree, which minimizes taxon exclusion by testing a wide gamut of nearly 1700 taxa.

Still waiting for a competing analysis
that tests a similar gamut of taxa. Emails to whale experts have not earned replies.


References

https://pterosaurheresies.wordpress.com/2019/01/02/mammal-tooth-evolution-toward-complexity-and-then-simplicity/

Bolosaurid thecodont tooth implantation

Snyder et al. 2020 report,
“Analysis of the specialized dentition of the bolosaurid parareptiles Bolosaurus from North America and Belebey from Russia, utilizing a combination of histological and tomographic data, reveals unusual patterns of tooth development and replacement. The data confirm that bolosaurid teeth have thecodont implantation with deep roots, the oldest known such example among amniotes,”

Bolosaurus is known from the Early Permian. The oldest amniote, Silvanerpeton, from the Viséan (Early Carboniferous) needs to be examined for tooth implantation before making this statement.

“and independently evolved among much younger archosauromorphs (including dinosaurs and crocodilians) and among synapsids (including mammals).”

In the large reptile tree (LRT, 1690+ taxa) synapsids and dinosaurs are both archosauromorphs. Given that the last common ancestor of all amniotes may also have had thecodont tooth implantation, this is not confirmed as ‘independently evolved.’ 

Co-author Reisz still believes in the invalid clade Parareptilia and is not aware of the basal dichotomy splitting Reptilia (Amniota is a junior synonym) between Archosauromorpha and Lepidosauromorpha.

Figure 2. Eudibamus skull revised here with new data compared to bolosaurids, on the left, and basal diapsids, on the right. Post crania for bolosaurids is very fragmentary. Bolosaurids are related to pareiasaurs and turtles, all derived from millerettids. Can you see why Eudibamus was confused with bolosaurids?

Figure 1. Eudibamus skull revised here with new data compared to bolosaurids, on the left, and basal diapsids, on the right. Post crania for bolosaurids is very fragmentary. Bolosaurids are related to pareiasaurs and turtles, all derived from millerettids. Can you see why Eudibamus was confused with bolosaurids?

Snyder et al. report,
“Finally, the nearly complete Eudibamus cursoris (Berman, 2000) of Germany represents the only other known taxon within the group.”

In the LRT Eudibamus is an unrelated diapsid (Fig. 1). Adding taxa reveals this interrelationships. In the LRT bolosaurids are sisters to diadectids, all derived from Milleretta and more ancestral taxa. None of these are bipedal with a parasagittal gait.

Snyder et al. report,
“Uninterrupted marginal tooth rows in the fossil record of Paleozoic tetrapods are rare, usually associated with unusual attachment and replacement cycles, such as in captorhinids.”

In the LRT bolosaurids and Milleretta are derived from captorhinids. I can’t find any examples in the LRT of interrupted marginal tooth rows in taxa surrounding bolosaurids.

Snyder et al. report,
“The level of heterodonty in bolosaurids is also remarkable, with mesial incisiform teeth transitioning to large, transversely bulbous cheek teeth distally along the tooth row. Tooth cusps slope posteriorly and have a conical apex that is more obvious in the larger, posterior teeth.”

Perhaps not so remarkable. Sister taxa, like Diadectes, have long, flat-tipped ‘incisors’ and short conical ‘molars’. Out-group Limnoscelis likewise has long anterior teeth and short posterior teeth. Other outgroups (captorhinids, Colobomycter, Tetraceratops, etc. are likewise heterodont.

In the discussion section, Snyder et al. report, 
“One comparable taxon are the diadectids. Dentitions in both diadectids and bolosaurids show heterodonty, thecodont implantation, as well as evidence of occlusal wear. The molariform cheek teeth of Diadectes are mediolaterally expanded compared to the more teardrop-shaped of bolosaurids, but like bolosaurids, they also frequently possess numerous replacement pits along the length of their jaws, suggesting a similar origin of tooth buds in both taxa.”

An inadequate taxon list prevented Snyder et al. from realizing the close relationship of diadectids and bolosaurids, as recovered several years ago in the LRT. Phonodus (Early Triassic) is also a bolosaurid in the LRT, unacknowledged by Snyder et al.


References
Snyder AJ, LeBlanc ARH, Jun C, Bevitt JJ and Reisz RR 2020. Thecodont tooth attachment and replacement in bolosaurid parareptiles. PeerJ 8:e9168
doi: Âhttps://doi.org/10.7717/peerj.9168 – https://peerj.com/articles/9168/

The origin of teeth: Donoghue and Rücklin 2014

IF you’re interested in teeth,
Donoghue and Rücklin 2014 report, “Just a couple of decades ago a singular dogma hypothesized teeth to have evolved from the tooth‐like dermal denticles that comprise or ornament fish scales, the topological distribution of which extended, through vertebrate phylogeny, from the external dermis to the oral cavity, adapted to a tooth function at the jaw margin.”

A second hypothesis
considered by Donoghue and Rücklin stated, “teeth and tooth‐like structures inside and outside the mouth evolved independently of one another.

A third hypothesis
considered by Donoghue and Rücklin stated,  “Furthermore, teeth are inferred to have evolved multiple times, once in each of the principal lineages of living and extinct jawed vertebrates, from edentate jawed ancestors.”

 

Figure 1. Traditional fish cladogram from Donoghue and Rucklin with color overlays to show where this cladogram differs from the LRT.

Figure 1. Traditional fish cladogram from Donoghue and Rucklin with color overlays to show where this cladogram differs from the LRT.

As noted earlier,
according to the large reptile tree (LRT; 1680+ taxa; subset Fig. 2) the origin of tooth carpets occurs with Early Silurian Loganellia. The origin of marginal teeth takes place in Ozarcus (stem bony fish) and Helodus (sharks, ratfish and rays). A valid phylogenetic context is paramount in studies like this. The traditional cladogram by Donoghue and Rücklin (Fig. 1) is lacking in pertinent taxa (Fig. 2).

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure 2. Updated subset of the LRT, focusing on basal vertebrates = fish.

We looked at the origin of teeth earlier here.
In Rhincodon (the whale shark; Figs. 3, 4) marginal teeth are absent, but palatal teeth create a carpet ideal for capturing the planktonic prey this pre-shark has dined upon since the Silurian.

Figure 1. Whale shark (Rhincodon) tooth pads, not that much different from catfish tooth pads (Fig. 2).

Figure 3. Whale shark (Rhincodon) tooth pads, not that much different from catfish tooth pads (Fig. 2).

Figure 2. The whale shark, Rhincodon, has an enormous gill chamber for capturing planktonic prey.

Figure 4. The whale shark, Rhincodon, has an enormous gill chamber for capturing planktonic prey.

References
Donoghue PCJ and Rücklin M 2014/2016. The ins and outs of the evolutionary origin of teeth. Evolution and Development 18(1):1–12. https://doi.org/10.1111/ede.12099

 

Squamate tooth complexity: Lafuma et al. 2020

Updated July 7, 2020
the LRT moves Meyasaurus, Indrasaurus and Hoyalacerta to the base of the Yabeinosaurus + Sakurasaurus clade within the Scleroglossa and Squamata.

This blogpost builds slowly. 
If you are short of time, drag down to the final paragraphs.

Lafuma et al. 2020 report,
“Complexity increase through cusp addition has dominated the diversification of many mammal groups.”

Be careful with blanket statements like that. What they wrote may be true of pre-mammal cynodonts (adding cusps), but teeth decrease in complexity in the lineage of pangolins, edentates, odontocetes and mysticetes. Carnivores have fewer teeth. So do elephants and manatees.

“However, studies of Mammalia alone don’t allow identification of patterns of tooth complexity conserved throughout vertebrate evolution.”

That sentence needs a re-write. It does not make sense.

“Here, we use morphometric and phylogenetic comparative methods across fossil and extant squamates (“lizards” and snakes) to show they also repeatedly evolved
increasingly complex teeth, but with more flexibility than mammals.”

Starting to sound iffy here knowing that Iguana (Fig. 1) is a basal squamate in the large reptile tree (LRT, 1669+ taxa, subset Fig. 2) and it has complex multi-cusp teeth. In the LRT varanids, sea-going mosasaurs and all legless lizards (including snakes) are all highly derived — and they have simple cones for teeth.

Pet Peeve: The authors don’t discuss lepidosaur pterosaurs that likewise had multi-cusp teeth in the Triassic, and only one cusp or no teeth in derived taxa.

Figure 2. The basalmost tested iguanid, Iguana. Note the resemblance to basalmost scleroglossans.

Figure 2. The basalmost tested iguanid, Iguana, one of the basalmost squamates in the LRT, contra Lafuma et al. who omitted so many outgroup taxa that their cladogram was upside-down.

Lafuma et al. 2020 continue,
“Since the Late Jurassic, six major squamate groups independently evolved multiple-cusped teeth from a single-cusped common ancestor.”

And those six in their phylogenetic order are:

  1. Scincoidea
  2. Polyglyphanodontia
  3. Lacertoidea
  4. Mosasauria
  5. Anguimorpha
  6. Iguania

Sophineta and three members of the Rhynchocephalia are outgroups to Squamata in the Lafuma et al. cladogram. That’s reasonable, but far from complete, and with disastrous consequences (see below).

“Unlike mammals reversals to lower cusp numbers were frequent in squamates, with varied multiple-cusped morphologies in several groups resulting in heterogenous evolutionary rates.”

See above.

The Lafuma et al. 2020 cladogram
lists the following clades of Squamates in this order (LRT order in parentheses).

  1. Gekkota (4th in the LRT and they share an ancestry with Serpentes in the LRT)
  2. Dibamia (last in the LRT, within skinks)
  3. Scincoidea (last in the LRT)
  4. Polyglyphanodontia (third in the LRT)
  5. Lacertoidea (second in the LRT)
  6. Mosasauria (fourth to last in the LRT)
  7. Serpentes (4th and they share an ancestry with Gekkota in the LRT)
  8. Anguimorpha (second to last in the LRT)
  9. Iguania (first in the LRT)

Due to taxon exclusion
the Lafuma et al. 2020 cladogram is inverted (upside-down) compared to the LRT (Fig. 2). As a result, so is their conclusion.

But let’s dig deeper trying to figure out how
this inversion happened. The authors report, “For topology we followed the total evidence phylogeny of Simões et al. – the first work to find agreement between morphological and molecular evidence regarding early squamate evolution.” Take a second look, dear readers. Borrowing a cladogram, taxon exclusion and genomics has given these workers an upside-down topology.

Figure 1.  Subset of the LRT focusing on lepidosaurs and snakes are among the squamates.

Figure 2.  Subset of the LRT from 2019 focusing on lepidosaurs including squamates.

Lafuma et al. 2020 list several hundred more squamate taxa
than the LRT includes, but this is where outgroups become important. Here is a list of missing Protosquamata taxa from the Lafuma et al. taxon list. Adding these taxa would bring much needed polarity to the Lafuma et al. cladogram:

  1. Lacertulus
  2. Schoenesmahl
  3. Fraxinisaura
  4. Hoyalacerta
  5. Indrasaurus
  6. Homoeosaurus
  7. Dalinghosaurus
  8. MFSN 19235
  9. Scandensia
  10. Calanguban
  11. Liushusaurus
  12. Purbicella
  13. Hongshanxi
  14. Euposaurus

But that’s not all… add to that list:
Tetraphodophis, Jucaraseps and Ardeosaurus. These three taxa link Norellius, Eichstaettisaurus and geckos to basal snakes. In the Lafuma cladogram Norellius, Eichstaettisaurus and geckos nest apart from Pontosaurus + Adriosaurus. For some reason, the basalmost gekko in the LRT, Tchingisaurus, nests with the basal amphisbaenan, Sineoamphisbaena in the Lafuma et al. tree. A sister, Sineoscincus, is omitted from the Lafuma et al. tree. Bahndwivici and Yabeinosaurus, nest basal to varanids and mosasaurs in the LRT, but are not listed by Lafuma et al.

If you’re going to report on the order of acquisition of traits,
you have to have your phylogenetic order established correctly. To do that you have to include more outgroup taxa, something that was not done in the Lafuma et al. study. By contrast, the LRT includes outgroup taxa back to Cambrian headless chordates, just to be sure all the bases are covered.


References
Lafuma F, Corfe IJ, Clavel J and Di-Pol N 2020. Multiple evolutionary origins and losses of tooth complexity in squamates. biRxiv preprint: https://doi.org/10.1101/2020.04.15.042796

How primitive are megapodes?

Earlier the large reptile tree (LRT, 1663+ taxa) nested megapodes (like Megapodius) at a more primitive node than any other living bird, except the kiwi (Apteryx) and ratites, like (like Struthio). You might remember, a toothed bird clade restricted to the Early and Late Cretaceous was derived from toothless Crypturus (Fig. 1) in the LRT.

Figure 1. Megapodius is the extant bird nesting at the base of all neognathae (all living birds except ratites).

Figure 1. Megapodius is the extant bird nesting at the base of all neognathae (all living birds except ratites). And it looks like a basal bird, not too this… not too that.

With that in mind
and hoping to understand the reemergence of previously lost teeth in Early Cretaceous birds, I checked out Clark 1960, who reported on megapode embryology.

To set the stage, Clark wrote,
Young birds are exceedingly precocious, being able to fly on the day of hatching and feeding actively only a few days after hatching.” He then referenced Portmann (1938, 1951, 1955) who listed several reptile-like characters of megapodes:

  1. no egg tooth (megapodes hatch by kicking their way out of the shell. The ‘egg tooth’ of chickens temporarily appears on the top of the beak, not the rim);
  2. lack of down feathers in embryos or nestlings;
  3. lack of parental care;
  4. primitive method of incubation (by solar heat, fermentation, vulcanism);
  5. long incubation period (8 weeks for Leipoa);
  6. large number of eggs laid;
  7. slow growth to adult size (especially for Alectura);
  8. primitive structure of the brain;
  9. eggs usually not turned and yet hatch relatively successfully;

Clark added to Portmann’s #9
a general lack of movement of the embryo until just before hatching. This may be related to the use of fermentation as a heat source for incubation. Clark notes,
the presence of aerobic bacteria should presumably greatly deplete the available oxygen supply.” Moving embryos might have suffocated for lack of oxygen. Clark also noted: relatively large yolks, as in reptiles.

I never found a tooth thread
connecting Late Jurassic teeth in stem birds to the reemergence of teeth in Early Cretaceous crown birds (Fig. 2) following Apteryx, ratites and megapodes. Even so, every other trait indicated a transition. The above authors further support the extreme primitive nature of megapodes. Ratites no longer bury their eggs. Kiwis dig burrows.

Figure 1. Click to enlarge. Toothed birds of the Cretaceous to scale.

Figure 1. Click to enlarge. Toothed birds of the Cretaceous to scale.

In the post-cladistic era
Dekker and Brom 1992 wrote, “Among megapodes, four different incubation-strategies may be distinguished:

  1. mound-building,
  2. burrow-nesting between decaying roots of trees,
  3. burrow-nesting at volcanically heated soils, and
  4. burrow-nesting at sun-exposed beaches.”

Dekker and Brom employed a cladogram
originally published by Cracraft and Mindell (1989), which mistakenly nested megapodes with galliforms (chickens and kin) due to taxon exclusion. Dekker and Brom wrote, We conclude that similarities shared with reptiles and kiwis are due to convergence.” That traditional nesting is not confirmed by the LRT due to taxon exclusion. Burying and burrowing are primitive, but give no clue as to how Early Cretaceous birds redeveloped small teeth at first, large teeth later. Neither does megapode embryology. Perhaps that’s why this novel hypothesis of interrelationships has never appeared elsewhere. 


References
Clark GA Jr. 1960. Notes on the embryology and evolution of the megapodes (Aves: Galliformes). Postilla 45:1–7.
Cracraft J and Mindell DP 1989. The early history of modern birds: a comparison of molecular and morphological evidence.— In: B. Fernholm, K. Bremer & H . Jörnvall, eds. The Hierarchy of Life: Molecules and Morphology in Phylogenetic Analysis: 389-403. Amsterdam, New York, Oxford.
Dekker RWRJ and Brom TG 1992. Megapode phylogeny and the interpretation of the incubation strategies. xxx 19–31.  Zoologische Verhandelingen  278(2): 19–31.
Portmann A 1938.
Beitrage zur Kenntnis der postembryonalen Entwick- lung der Vogel. Rev. Suisse Zool., 45: 273-348.
Portmann A 1951. Ontogenesetypus und Cerebralisation in der Evolution der Vogel und Sauger. Rev. Suisse Zool., 58: 427-434.
Portmann A 1955. Die postembryonale Entwicklung der Vogel als Evolu- tionsproblem. Acta XI Congr. Int. Orn., 1954. Pp. 138-151.

Cretaceous toothed birds evolved from toothless megapodes in the LRT

Today’s heretical dive
into the origin of Cretaceous toothed birds (Fig. 1) brings new insight to a clade that has been traditionally misrepresented as a stem clade, often represented by just two highly derived toothed taxa, Ichthyornis and Hesperornis (Fig. 1). In the large reptile tree (LRT, 1659+ taxa; subset Fig. 3) Cretaceous toothed birds arise from extant toothless Megapodius (Figs. 1, 2; Gaimard 1823). How is this possible?

Toothy jaws from toothless jaws? 
That seems to break some rules. And if the LRT (Fig. 3) is valid, that makes toothed Cretaceous birds crown bird taxa.

Figure 1. Click to enlarge. Toothed birds of the Cretaceous to scale.

Figure 1. Click to enlarge. Toothed birds of the Cretaceous to scale. They are derived from toothless taxa.

Earlier Field et al. 2020
claimed to discover the ‘oldest crown bird‘ fossil when they described Asteriornis (66 mya), a screamer (genus: Chauna) relative. Unfortunately, due to taxon exclusion, Field et al. 2020 did not consider the ostrich sister, Patagopteryx (80 mya), nor did they understand that Juehuaornis (Wang et al. 2015; Early Cretaceous, Aptian, 122mya; Figs. 1, 4) was also a crown bird taxon, the oldest crown bird, derived from Megapodius, the extant mound builder.

One look
(Fig. 1) at the similarity of Megapodius to basal Cretaceous toothed and toothless birds, like Juehuaornis (Figs. 1, 4), makes the relationship obvious. The LRT recovered that relationship based on hundreds of traits and minimized convergence by testing relationships among 1659 taxa.

So, where did those Cretaceous teeth come from?
Megapodius and Juehuaornis both lack teeth. Basalmost toothed taxa had tiny teeth (Fig. 1) Derived toothed taxa had larger teeth. Try to let that sink in. Teeth re-appeared in these Cretaceous birds.

How is that possible? Consider this:
Juehuaornis is smaller than Megapodius. The sternum and keel of Juehuaornis are smaller than in Megapodius. Why is this important? As we learned earlier, at the genesis of many major and minor clades phylogenetic miniaturization (the Lilliput Effect) is present. That’s how gulls become hummingbirds and rauisuchians become dinosaurs. When adults are smaller they mature more quickly and they retain juvenile traits into adulthood. They also develop new traits, in this case, perhaps ontogeny recapitulated phylogeny.

The tooth genes got turned on again,
at first in a minor way… later in a major way.

Figure 2. Click to enlarge. Origin of birds from Archaeopteryx to Megapodius.

Figure 2. Click to enlarge. Origin of birds from Archaeopteryx to Megapodius. Pseudocrypturus is the sister taxon to the kiwi (Apteryx, Fig. 3), the most basal crown birds, but Juehuaornis is known from much older fossils despite being more derived than Megapodius.

How close were Cretaceous toothless taxa,
like Juehuaornis, to toothed Jurassic ancestors, like Archaeopteryx? Depends on how you look at it.

Chronologically
Juehuaornis is from the Aptian, Early Cretaceous, 122 mya. Archaeopteryx is from the Tithonian, Late Jurassic, 150 myaA transitional taxon, Archaeornithura (Fig. 2) is from the Hauterivian, Early Cretaceous, 131 mya, splitting the time difference. Archaeornithura had teeth and lacked a pygostyle, but had a shorter tail than the most derived Archaeopteryx (Fig. 2).

Morphologically
toothless Juehuaornis follows toothless Megapodius (Figs. 1, 3) and is separated from toothy Archaeornithura by at least three taxa (Figs 2, 3). The question I ask is: did the Cretaceous sisters to these toothless taxa have teeth subsequently lost in later generations over the past 140 million years? Or were teeth lost in  Early Cretaceous transitional taxa (represented by late-survivors (Fig. 2)) only to be regained in the toothy extinct clade (Fig. 3)? For now, let’s leave all options open, but toothlessness followed by toothy jaws is the only option currently supported by phylogenetic evidence (Fig. 3).

Figure 2. Subset of the LRT focusing on bird origins. Crown birds and toothed birds are highlighted.

Figure 3. Subset of the LRT focusing on bird origins. Crown birds and toothed birds are highlighted.

This is what happens when you let the cladogram tell you what happened,
rather than gerrymandering the taxa inclusion list and scores to get the results your professors and colleagues will approve and permit publication.

Figure 2. Juehuaornis reconstructed. Note the scale bars. This is a tiny bird.

Figure 4. Juehuaornis reconstructed. Note the scale bars. This is a tiny bird and the oldest known crown bird.

I should have reported
that Juehuaornis (122 mya) was the oldest known crown bird earlier. I just had to see the toothed birds in phylogenetic order (Fig. 1), making sure they made sense after seeing them listed in the cladogram (Fig. 3).

Add taxa
and your cladograms will be better than most. Create reconstructions to scale and see if your cladograms make sense. When it’s right, it all works out with a gradual accumulation of traits between every node, echoing evolutionary events from deep time. Let me know if this novel hypothesis of interrelationships was published previously anywhere so that citation can be promoted.


References
Gaimard JP 1823. Mémoire sur un nouveau genre de Gallinacés, establi sous le nom de Mégapode. Bulletin General et Universel des Annonces et de Nouvelles Scientifiques 2: 450-451.
Wang R-F, Wang Y and Hu Dong-yu 2015. Discovery of a new ornithuromorph genus, Juehuaornis gen. nov. from Lower Cretaceous of western Liaoning, China. Global Geology 34(1):7-11.

wiki/Megapodius
wiki/Megapode

From jawless fish to toothless jaws: Hemicyclaspis to Chondrosteus

Updated December 16, 17, 2019
with Thelodus moving to the most basal position in this phylogenetic sequence of jawless fish. Chondrosteus is removed, replaced with Pachycormus due to a reinterpretation of bones and the resulting tree topology shift.

Adding jawless fish
to the large reptile (LRT, 1611+ taxa) sheds new light on the origin of jaws and the basic topology at the base of the LRT.

Figure x. Chondrosteus was revised and no longer fits here. Pachycormus is inserted in its place.

Figure x. Chondrosteus was revised and no longer fits here phylogenetically.  Pachycormus is inserted in its place.

Thelodus
(Fig. 1) was crushed to a thin film with a ventral exposure. Here the round lacrimal and angular jaw bones are highlighted. The lateral armor (green) is barely ossified.

Osteostraci,
like Hemicyclaspis (Fig. 1), have a ventral opening at the front of ventral surface of the skull, similar to their ancestors, like Birkenia, which retain lancelet-like cilia surrounding the oral opening. Perhaps Hemicyclaspis did, too.

Sturgeons,
like Acipenser (Fig. 1), have a longer rostrum and a posterior tube mouth. The maxilla and dentary are not yet present. Those bones grow teeth. Teeth are not present. Neither are the bones that grow them. So the lacrimal and surangular create the protrusible rim of that tube mouth and neither connects to the quadrate. Nesting sturgeons at the base of fish with teeth is the opposite of traditional cladogram topologies, in which sturgeons are considered ‘aberrant’ or ‘regressive’ (see below).

Figure 1. Old woodcut illustration labeling the upper mouth tube bone the lacrimal. Mn = mandible. h = quadrate. g = hyobranchial. Weave of bones above the lacrimal are palatal bones (pterygoid, ectopterygoid, palatine and vomer, plus a remnant gill bar. This taxon really exaggerates the rostrum, similar to the related spoonbill.

Figure 1. Old woodcut illustration labeling the upper mouth tube bone the lacrimal. Mn = mandible. h = quadrate. g = hyobranchial. Weave of bones above the lacrimal are palatal bones (pterygoid, ectopterygoid, palatine and vomer, plus a remnant gill bar. This taxon really exaggerates the rostrum, similar to the related spoonbill.

As you can see (Fig. 2), I am not the first worker 
to determine that the traditional ‘maxilla’ on sturgeons is instead the lacrimal.

Sturgeons, continued.
Gill covers (operculum) appear. While feeding on the bottom with the mouth buried in sediment, water cannot enter the mouth. So instead water enters the top of the operculum and exits out the back for respiration.

Note the close correspondence
between the torso ossifications, fin placement, tail shape and skull shape on the sturgeon and its osteostracan ancestor, Hemicyclaspis (Fig. 1).

Figure 1. Chondrosteus skull re-illustrated and compared to the original reconstruction and in situ drawing. Compare to Trachinocephalus in figure 2.

Figure 1. Chondrosteus skull re-illustrated and compared to the original reconstruction and in situ drawing. Compare to Trachinocephalus in figure 2.

Chondrosteus
(Fig. 1) is now deleted from this list, now nesting with lizardfish.

Are sturgeons jawless fish?
In the LRT sturgeons are transitional between jawless fish and traditional gnathostomes.

Jollie 1980 reported in his growth study on sturgeons,
“It is a conclusion that the endocranium has been drastically altered in form and in the reduction of its ossifications but that the dermal head skeleton is basically that of an actinopterygian fish which shows many regressive tendencies such as the variable multiplication of ossified units. The jaws in this group are unique both in terms of suspension and in lacking a premaxilla. The post-temporal of the pectoral girdle has a unique relationship with the endocranium which involves the exclusion of the lateral extrascapular. An interclavicle is present. In spite of such features, the developmental story and adult ossifications of the sturgeon support the idea of a common, and understandable, bone pattern in actinopterygians and osteichthians.”

Jollie did not place Acipenser and Hemicyclaspis
in a phylogenetic context. In the LRT (subset Fig. 4) Pseudoscaphorhynchus is a tested sturgeon.

Figure 6. Subset of the LRT focusing on fish.

Figure 4. Subset of the LRT focusing on fish.

Are sturgeons bony fish?
Not according to the LRT. Much of their skeleton is cartilaginous and they nest basal to cartilaginous taxa. So between cilia and jaws, the transitional trait is a tube. Marginal teeth seem to have appeared three times by convergence in this scenario and once gained, were quickly lost in placoderms + catfish. Add to those palatal tooth carpets found in catfish, mantas and whale sharks.

Apologies for earlier errors.
As I’ve often said, I’m teaching myself vertebrate paleontology one taxon at a time using the LRT as a terrific tool for figuring things out.


References
Jollie M 1980. Development of head and pectoral girdle skeleton in Acipenser. Copeia 1980(2):226–249.

Actinopterygii = ray fin fish
Osteichthyes =  bony fish

wiki/Gnathostomata

Pseudictops: what little we know is unique

There are not many mammals with crenulated/serrated teeth.
Pseudictops lophiodon (Matthews, Granger and Simpson 1929, Sulimski 1968, Late Paleocene, 57 mya; Fig. 1; AMNH 21727) is one such mammal. From the start Pseudictops was compared to anagalids like Leptictis (Fig. 2), a basal elephant shrew and ancestor to tenrecs, pakicetids and odontocete whales.

Figure 1. Pseudictops lophiodon compared to the slightly larger Siamotherium.

Figure 1. Pseudictops lophiodon compared to the slightly larger Siamotherium. The mandible is extremely robust and appears to nearly lack a coronoid process, distinct from most mammals.Note the crenulations and and/or robust serrations on the anterior teeth.

Figure 1a. Pseudictops anterior teeth.

Figure 1a. Pseudictops anterior teeth.

The dentary incisors
are deeply rooted in a deep dentary. Not sure why the two dentaries (Fig. 1) have distinct shapes. Perhaps they are not actually related to one another or perhaps some parts are missing from the smaller one and plasterered over.

Figure 2. Leptictis, an early Oligocene elephant shrew.

Figure 2. Leptictis, an early Oligocene elephant shrew.

Now that you’ve met Pseudictops, a quick look at Ictops
reveals a cranium with a double parasagittal crest, as in sister taxon, Leptictis

Figure 6. Rhynchocyon (above) and Macroscelides (below) compared. Though both are considered elephant shrews, they nest in separate major mammal clades in the LRT.

Figure 3. Rhynchocyon (above) and Macroscelides (below) compared. Though both are considered elephant shrews, they nest in separate major mammal clades in the LRT.


References
Matthew WD, Granger W and Simpson GG 1929. Additiions to the fauna of the Gashato Formatin of Mongolia. American Museum Novitates 376:1–12.
Sulimski A 1968. Paleocene genus Pseudictops Matthew, Granger and Simpson 1929 (Mammalia) and its revision. www.palaeontologia.pan.pl/Archive/1968-19–1011-129–10-14.pdf