Tiny Santanichthys is a bonefish

Updated April 28, 2021
with a closer look at Santanichthys now nesting with bonefish like Albula and Opisthoproctus.

This was a paragraph from the earlier post:
Two very closely related taxa, one 20x times larger,
enter the LRT today. Santanichthys (Silva Santos 1995; Figs. 1, 5) is only 3m in length. Notelops (Woodward 1901; Figs. 2–4) reaches 60cm in length. Both are from the Santana Formation, Early Cretaceous.

Figure 1. Tiny Santanichthys is a phylogenetically miniaturized taxon at the base of the Ostariophysi clade.
Figure 1. Tiny Santanichthys is a phylogenetically miniaturized taxon at the base of the Ostariophysi clade.

Santanichthys diasii 
(Silva Santos 1958; Filleleul and Maisey 2004; Early Cretaceous; 3cm; DGM-DNPM 647P) was a tiny Santana Formation fish considered the oldest characiform and otophysan. Here Santanichthys nests with Albula the extant bonefish (Fig. 2). According to Filleleul and Maisey, this is the earliest appearance of a Weberian apparatus, a sound amplifier that connects the swim bladder to the auditory system.

Figure 2. The skull and diagram of tiny Santanichthys from Filleul and Maisey 2004, colors added.

These taxa are considered members of the Characiformes,
a clade that traditionally includes piranha. Likewise the large reptile tree (LRT, 1801 taxa then, 1839 taxa now; subset Fig. x) nests them together, derived from the piranha clade. Traditionally Characiformes also includes knife fish and catfish. These clades are not related to piranha in the LRT, nor are they related to bonefish.

Figure 1. Albula vulpes skull with highly derived facial bones reidentified here. Note the lateral premaxillary processes and 'floating' cheek bones. Green vertebrae are caudals.
Figure 3. Albula vulpes skull with highly derived facial bones reidentified here. Note the lateral premaxillary processes and ‘floating’ cheek bones. Green vertebrae are caudals.

Santanichthys and Albula
share a long list of traits. Tiny Opisthoproctus (Fig 4) has fewer vertebrae, like Santanitchthys.

Figure 3. Opisthoproctus nests with Santanichthys in the LRT.
Figure 4. Opisthoproctus nests with Santanichthys in the LRT.
Figure x. Rayfin fish cladogram

If your studies dive deep into fish science  
you’ll come across the traditional clade Ostariophysi, in which member taxa all have a Weberian apparatus (see above). The LRT indicates that some fish with this trait evolved it independently, while others later lost it by convergence. Be careful. Lumping taxa together using one trait or a dozen is called “Pulling a Larry Martin.” Try to always determine clades with a phylogenetic analysis that tests hundreds of traits and then determine your clades based on a last common ancestor and all of its descendants. Convergence is rampant.

Membership within the clade
Ostariophysi (Lord 1922) includes

Gonorynchiformes — milkfish, untested in the LRT

Cypriniformes — perch, a clade distally derived from Santanichthys in the LRT.

Characiformes — piranha, a clade that proximally precedes Santanicthys in the LRT

Siluriformes — catfish, a clade unrelated to Santanichthys in the LRT

Gymnotiformes — knife fish, a clade that distally precedes Santanichthys in the LRT


References
Filleul A and Maisey JG 2004. Redescription of Santanichthys diasii (Otophysi,
Characiformes) from the Albian of the Santana Formation and Comments on Its Implications for Otophysan Relationships. American Museum Novitates 3455:21pp.
Forey PL 1977. The osteology of Notelops Woodward, Rhacolepis Agassiz Pachyrhizodus Dixon (Pisces: Teleostei). Bulletin of the British Museum (Natural History) 28(2):123–204.
Silva Santos R 1995. Santanichthys, novo epiteto generico para Leptolepis diasii Silva Santos, 1958 (Pisces, Teleostei) da Formacao Santana (Aptiano), Bacia do Araripe, NE do Brasil. Anais da Academia Brasileira de Ciencias 67:249–258.
Woodward AS 1901. Catalogue of the Fossil Fishes in the British Museum (Natural History), 4. xxxviii + 636 pp., 19 pis, 22 figs. Brit. Mus. (Nat. Hist.), London.

wiki/Santanichthys
wiki/Notelops
wiki/Characiformes
wiki/Ostariophysi

Prohalecites: the new tiny ancestor of all bony ray fin fish

Updated February 10 2021
with a some details modified in the morphology resulting in a shifting of this taxon to the base of the ray-fin fish clade (not the spiney/lobe-fin fish clade).

…and it still looks like Hybodus,
(Fig. 2) its 50x larger proximal ancestor.

A nice surprise today
as a phylogenetically miniaturized hybodontid shark with gill covers and ray fins, Prohalecites porroi (Figs. 1, 4, 5, Belloti 1857, Deecke 1889, Tintori 1990, MCSNIO P 348, Middle Triassic; 4cm), enters the large reptile tree (LRT, 1780+ taxa) as THE basalmost bony ray-fin fish. 

Figure 1. Prohalecites porroi in situ from Arratia 2015, colors added. No dorsal spines here.

Figure 1. Prohalecites porroi in situ from Arratia 2015, colors added. No dorsal spines here.

That makes Prohalecites close to a late-surviving
human, mammal, reptile, tetrapod and bony fish ancestor. Prohalecites needs to be in every paleo textbook from here on out.

No trace of scales is preserved
in any specimen. No neurocranial material is preserved. Hemichordacentra are present. The preoperular is so slender it is twig-like.

Figure 2. Hybodus fraasi fossil in situ is 50x larger than an adult Prohalecites, the basalmost bony fish.

Figure 2. Hybodus fraasi fossil in situ is 50x larger than an adult Prohalecites, the basalmost bony fish.

Surprisingly little has been written
about Prohalecites. While Arratia 2015 considered it “the oldest of the Teleosti”, she did not mention Hybodus, its proximal ancestor in the LRT.

Tintori 1990 left Prohalecites as a Neopterygian incertae sedis,
“because its characters do not perfectly fit in any of these cited groups.” 

Arratia and Tintori 1998 wrote, 
“Prohalecites possesses an interesting mosaic of primitive and advanced chalacters, some of which have been previously interpreted as synapomorphies of Teleostei.” 

“The election of the outgloup plays a significant role in the phylogenic position of Prohalecites and other neopterygians. Unquestionably, Prohalecites is not a Teleostei.”

Their cladograms nested Prohalecites between Amia and all higher bony fish. Neither sharks nor Hybodus are mentioned in the text. So taxon exclusion hampers an otherwise highly focused study.

Figure 1. Subset of the LRT focusing on ray-fin fish, their speed, niches and extant.

Figure 1. Subset of the LRT focusing on ray-fin fish, their speed, niches and extant.

Once again,
too much focus, not enough of a wide-angle view hampered prior workers. Whenever taxa are tested together that have never been tested together before, new relationships can be recovered. That’s why the LRT was created 10 years ago. You should have so many taxa in your cladogram that it tells you which taxa to include in your more focused study. Cherry-picking taxa has become outdated. That traditional practice leads to false positives and enigmas.

Figure 4. Prohalecites skull from Arratia 2015, colors added.

Figure 4. Prohalecites skull from Arratia 2015, colors added.

Arratia 2015,
wrote on the history and current status of the fish clade Teleostei (Müller 1845).

Figure 3. Prohalecites diagram from Tintori 1990, colors added.

Figure 5. Prohalecites diagram from Tintori 1990, colors added.

According to Arratia,
Müller defined the clade based on soft tissue traits not visible in fossils. Thus, the taxon content of the clade has changed several times over the last century.

From the Arratia abstract:
“The monophyly of the total group Teleostei, which now includes Triassic pholidophorids, is supported by numerous synapomorphies.” This, of course, would be “Pulling a Larry Martin“, which happens frequently out there. Remember, it is better to define a clade by establishing two taxa that recover a last common ancestor on a wide gamut, comprehensive cladogram. Don’t rely on a few or a few dozen traits. Convergence must be allowed in your hypothetical model, because convergence and reversal did happen.

Arratia also wrote,
“Prohalecites from the Ladinian/Carnian (Triassic; c. 240 Ma) boundary represents the oldest stem teleost.” That piqued my interest in this tiny fish only half as long as a human finger.

According to Arratia, 
“during most of the last 170 years there has been a dichotomy in the treatment of teleosts, where fossil and living groups have been studied separately, including distinct classifications.”

Looking at the simple cladogram in figure 5 of Arratia 2015,
it looks like Teleostei includes Pholidophorus, Leptolepis, their last common ancestor and all descendants. Two other cladograms are shown in Arratia figure 8 based on earlier analyses. A full page cladogram is shown in Arratia figure 9 placing Amia and Lepisosteus as two of five outgroup taxa. Basal ingroup taxa include Pachycormus and Aspidorhynchus. Synapomorphies were listed for each node followed by a report on the pertinent traits. All this was for nought because the phylogenetic context was incomplete and invalid.

Arratia concludes, 
“The results demonstrate the importance of including fossil teleosts in the phylogenetic analysis, especially because some of their characters and combination of characters introduce a new perspective in understanding the origin and early radiation of the group, and indirectly provide a new scenario to interpret homologous characters.”

It goes without saying that Arratia 2015 did not include
placoderms, or lobefins in a teleost clade defined by Pachycormus and Aspidorhynchus in the LRT.

Prohalecites demonstrates, once again,
phylogenetic miniaturization at the genesis of a major clade, despite its late (Middle Triassic) appearance in the fossil record.

Was Prohalecites larger in the Silurian?
Maybe. It’s worth looking for. Or maybe bony fish began by neotony.

Earlier we looked at the origin of bone ‘islands’
on a cartilage substrate in the hatchlings and juveniles of the extant taxon, Amia. Prohalecites documents the origin of bone in the tiny adult bony fish descendants of hybodont sharks.


References
Arratia G and Tintori A 1999. The caudal skeleton of the Triassic actinopterygian †Prohalecites and its phylogenetic position, p. 121–142. In: Mesozoic Fishes 2—Systematics and Fossil Record. G. Arratia and H.-P. Schultze (eds.). Verlag Dr. F. Pfeil, München.
Arratia G 2015. Complexities of early Teleostei and the evolution of particular morphological structures through time. Copeia 103(4):999–1025.
Bellotti C 1857. Descizione di alcune nuove specie di pesci fossili di Perledo e di altre localtta lombarde. 419–432. In Sopani A (ed) Studi geologici sulla Lomabardia. Editore Turati, Milano.
Deecke W 1889. Über Fischea ùs verschiedenen Horizonten der Trias. Palaeontogaphica 45:97–138.
Müller J 1845. Über den Bau und die Grenzen der Ganoiden, und über das natürliche System der Fische. Physikalisch-Mathematische Abhandlungen der ko¨niglichen Akademie der Wissenschaften zu Berlin 1845:117–216.
Tintori A 1990. The actinopterygian fish Prohalecites from the Triassic of northern Italy. Palaeontology 33:155–174.

 

wiki/Prohalecites
wiki/Teleostei

Adding taxa updates the origin of placoderms

A year ago
when fish (= basal vertebrates) were first added to the large reptile tree (LRT, now with 1757+ taxa; subset Fig. 1), the extant walking catfish, Clarias, nested with the Silurian placoderm, Entelognathus, rather than any other extant bony fish when there were very few other bony fish to nest with. Since then, adding taxa has separated these two, but they still nest as charter members of the unnamed catfish-placoderm clade. That was a heretical hypothesis then, and it remains so today.

Traditional fish paleontologists
consider placoderms basal to sharks and ratfish + bony fish. Stensioella was considered the most basal placoderm by Carr et al. 2009, who did not list outgroup taxa. These hypotheses are not supported by the LRT (subset Fig. 1) where placoderms arise from coelacanths among the bony fish, far from sharks and ratfish.

The LRT divides placoderms into four clades;

  1. Arthrodira (open ocean predators like Dunkleosteus, Coccosteus, Fig. 3)
  2. Antiarchi (armored jawless bottom dwellers like DicksonosteusBothriolepis, Fig. 2)
  3. Ptyctodontida (chimaera-like taxa like Australoptyctodus, Fig. 2)
  4. Phyllolepida (tiny-eye taxa like Entelognathus, Cowralepis)

Several traditional placoderms nest elsewhere in the LRT.

  1. Rhenanida – nests with catfish in the LRT
  2. Wuttagoonaspis – nests with catfish in the LRT
  3. Stensioellida – nests with Guiyu-like lobefins in the LRT
  4. Brindabellspida – nests with the tetrapodomorph Elpistostege

Several traditional placoderms have not yet been tested in the LRT.

  1. Petalichthyida (includes Diandongpetalichthys)
  2. Acanththoraci (closely related to rhenanids, nesting with catfish)
  3. Pseudopetalichthyida (similar to rhenanids, nesting with catfish)

After testing
in the LRT (subset Fig. 1) placoderms are still bony fish close to catfish and this clade still arises from coelocanths.

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

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

The pertinent taxa in the first list
(Fig. 2) start with the small, Early Devonian spiny shark Diplacanthus and end with the rather flat nearly jawless placoderm, Dicksonesteus also from the Early Devonian. That tells us that every taxon between them was part of the Early Devonian fauna. That also tells us the radiation of taxa in figure 2 must have occurred much earlier, sometime in the middle of the mysterious Silurian, which preserves very few gnathostome fish fossils.

Figure 2. Taxa from the LRT nesting prior to the clade Placodermi.

Figure 2. Taxa from the LRT nesting prior to the clade Placodermi. See figure 3 for the arthrodire clade within Placodermi. Robustichthys is basal to catfish and lacks a squamosal.

Phylogenetic miniaturization
occurs at the origin of placoderms with the smallest specimen in figure 2, Romundina, half the size of its predecessor, Eurynotus. In like fashion, the smallest placoderm in figure 3is the unnamed ANU V244 specimen, is also half the size of its predecessor, the aforementioned Eurynotus.

Figure 3. Arthrodires and their ancestor, Euryodus. See figure 2 for Euryodus ancestors. Note the phylogenetic miniaturization at the origin of the arthrodires.

Figure 3. Arthrodires and their ancestor, Euryodus. See figure 2 for Euryodus ancestors. Note the phylogenetic miniaturization at the origin of the arthrodires.

Phylogenetically, the lack of marginal teeth
in placoderms goes back to a late-surviving taxon from the Jurassic, the angelfish-mimic,  Cheirodus (Fig. 1). Note the hidden palatine teeth in Cheirodus that in the arthrodires, Coccosteus and Dunkleosteus become visible and act as marginal teeth/plates. The Silurian ancestors of Cheirodus may not have been so uniquely angelfish-like. That shape is apomorphic due to the separation in time.

Ptyctodonts, like Austroptyctodus,
(Fig. 2) do not nest with other traditional placoderms in the LRT, but nest closer to Cheirodus.  These are the sort of results the LRT recovers only because it tests more taxa.


References
Carr RK, Johanson Z and Ritchie A 2009. The phyllolepid placoderm Cowralepis mclachlani: Insights into the evolution of feeding mechanisms in jawed vertebrates. Journal of Morphology. 270 (7): 775–804.
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
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.
Young GC 1980. A new Early Devonian placoderm from New South Wales, Australia, with a discussion of placoderm phylogeny: Palaeontographica 167A pp. 10–76. 2 pl., 27 fig.
Zhu et al. 2012. An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates. Biology Letters Palaeontology 8:453–456.
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
wiki/Bothriolepis
wiki/Dicksonosteus
wiki/Romundina
wiki/Qilinyu
wiki/Parayunnanolepis
wiki/Lunaspis
wiki/Coccosteus
wiki/Mcnamaraspis
wiki/Dunkleosteus

New tiny ‘Ctenochasma’ at the Field: Lauer Foundation Collection

The Lauer Foundation for Paleontology provided
this tiny crushed Ctenochasma? elegans? Fig. 1) to the Field Museum, Chicago, USA. The foundation number is: #LF 2296. It enters the large pterosaur tree (LPT, 250 taxa; Fig. 2) distinct from all other tested pterosaurs.

Figure 1. A tiny Ctenochasama micronyx undergoes DGS here. Every bone is present, but no soft tissue this time. Note the tiny claws on all digits along with the slightly spoon-shaped rostrum and needle-like teeth.

Figure 1. A tiny Ctenochasama micronyx undergoes DGS here. Every bone is present, but no soft tissue this time. Note the tiny claws on all digits along with the slightly spoon-shaped rostrum and needle-like teeth.

Tiny, yes, but not a juvenile.
As we learned earlier, pterosaur hatchlings have adult proportions. Pterodaustro presents the phylogenetically closest example, in this case. Phylogenetic miniaturization is how we get tiny pterosaurs. And tiny pterosaurs are transitional taxa. That’s how we get derived pterosaurs. Note the tiny Ctenochasma? elegans specimens all nest together (Fig. 2). These tiny pterosaurs are adults that would have produced 8x smaller hatchlings, often about the size of house flies. and therefore unable to fly without risking desiccation due to a high surface-to-volume ratio. In other words, hatchlings of tiny pterosaurs could have flown, but needed to keep their wings folded. So they walked, picking up small prey in damp leaf litter. And that’s why so many pterosaur tracks are from pterodactyloid-grade pterosaurs, many of which continued to feet quadrupedally as they grew into phylogenetically larger genera.

Figure 3. Subset of the LRT focusing on ctenochasmatids and kin.

Figure 2. Subset of the LRT focusing on ctenochasmatids and kin.

Earlier we looked at the Fossilienatlas.de specimen
assigned to Ctenochasma elegans #204 (Fig. 4), which provides a similar morphology in reconstruction. No scale bars were provided with the Lauer Collection specimen, but the size can’t be too far off from this.

Figure 3. Click to enlarge. Private pterosaur #2042 together with St/Ei1, which nests at the base of the ctenochasmatidade, close to Angustinaripterus.

Figure 3. Click to enlarge. Private pterosaur #2042 together with St/Ei1, which nests at the base of the ctenochasmatidade, close to Angustinaripterus.

References
https://www.fieldmuseum.org/blog/meet-pterosaur-flock
https://www.lauerfoundationpse.org/about

Looking for the sternal complex in a tiny pterosaur

All pterosaurs have a sternal complex
(sternum + interclavicle + wrap-around clavicles), even the flightless ones. This tiny specimen (Fig. 1) probably had a sternal complex, but where is it? As everyone knows, it should be between the elbows, but it’s not there.

Figure 1. Tiny pterosaur mistakenly named Pterodactylus? pulchellus. I cannot find the sternal complex here. It should be between the elbows. That tiny red triangle under the mid-humerus is the ventral coracoid.

Figure 1. Tiny pterosaur mistakenly named Pterodactylus? pulchellus. I cannot find the sternal complex here. It should be between the elbows. That tiny red triangle under the mid-humerus is the ventral coracoid.

Pterodactylus? pulchellus BM NHM 42735 is the same size as the closely related Gmu-10157 specimen, but has a longer rostrum. The BM NHM specimen is one node closer to the common ancestor of cycnorhamphids + ornithocheirids in the large pterosaur tree (LPT, 242 taxa). The sternal complex appears to be missing or displaced in this otherwise undisturbed tiny specimen. Soft tissue confirms the narrow chord wing membrane and dual uropatagia. Pedal digit 5 remained long.

Figure 2. The GMU 10157 specimen and the P? pulchellus BM NHM 42735 specimens to scale and full size.

Figure 2. The GMU 10157 specimen and the P? pulchellus BM NHM 42735 specimens to scale and full size.

These tiny adults,
(Fig. 2) derived from slightly larger scaphognathids (Fig. 3) are transitional taxa undergoing phylogenetic miniaturization at the genesis of Cycnorhamphidae + Ornithocheiridae. They have not been given novel generic names by established workers because the traditionalists among them consider these to be babies/juveniles of larger, undiscovered taxa. Thus they have remained relatively ignored, despite their pristine preservation and sometimes gravid condition.

Figure 3. Click to enlarge. Taxa in the lineage of Cycnorhamphidae + Ornithocheiridae in the LPT.

Figure 3. Click to enlarge. Taxa in the lineage of Cycnorhamphidae + Ornithocheiridae in the LPT.

The key to finding the missing sternal complex
on this relatively undisturbed specimen is to look to the only area of the skeleton that is slightly disturbed (Fig. 4). The gastralia basket is expanded beyond its natural contours in the BM NHM specimen and that’s where I find (thanks to DGS) a displaced sternal complex, separated from the coracoids and jammed back into the stomach, surrounded by gastralia, almost hidden from view.

Figure 4. Here the sternal complex of the BM NHM 42735 specimen is colored indigo.

Figure 4. Here the sternal complex of the BM NHM 42735 specimen is colored indigo.

Not sure how that happened during taphonomy,
but there you go: mystery solved!

Shenzhoupterus skull in situ with sternum in blue.

Figure 5. Shenzhoupterus skull in situ with sternum in blue.

Earlier a sternal complex was found beneath the skull
of Shenzhoupterus (Figs. 5, 6) using the same techniques, contra traditional reconstructions (Lü, Unwin, Xu and Zhang 2008; see skull diagram insert matching no other pterosaur skull morphology in Fig. 6). Despite its derived state, the newly reconstructed Shenzhoupterus skull (Fig. 6 standing skeleton) bears all the hallmarks of sister taxa.

Shenzhoupterus reconstructed alongside original interpretation of skull.

Figure 6. Shenzhoupterus reconstructed alongside original interpretation of skull.

While we’re on this subject,
Shenzhoupterus does not nest with azhdarchoids, as originally hypothesized, but with tiny Nemicolopterus, between dsungaripterids and tapejarids in the LPT—and neither of these clades are related to azhdarchids in the LPT, contra traditional thinking that excludes tiny taxa and large swathes of congeneric taxa.


References
both of the tiny taxa listed above await description and publication other than in:
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

Shenzhoupterus was first described in:
Lü J, Unwin DM, Xu L and Zhang X 2008. A new azhdarchoid pterosaur from the Lower Cretaceous of China and its implications for pterosaur phylogeny and evolution. Naturwissenschaften 95 (9): online (preprint). doi:10.1007/s00114-008-0397-5. PMID 18509616.

New Champsosaurus paper perpetuates old myths

Whenever taxon exclusion mistakes are made and reviewed here,
I try to write to the lead author of the paper. Below is a recent email directed to Professor Dudgeon et al. 2020 on their recent review of the well-preserved skull of Champsosaurus (Figs. 1, 3), which they re-examined using computed tomography analysis.

Figure 1. Champsosaurus from Dugeon et al. Here the nasal is the ascending process of the premaxilla. The prefrontal is the nasal fused to the prefrontal. The postorbital is the postfrontal and vice versa.

Figure 1. Champsosaurus from Dugeon et al. Here the nasal is the ascending process of the premaxilla. The prefrontal is the nasal fused to the prefrontal. The postorbital (pro) is the postfrontal (pof) and vice versa.

Dear Dr. Dudgeon:

It’s always good to see new studies on old skulls.

Based on phylogenetic bracketing the bone traditionally identified as the ‘nasal’ is the ascending process of the premaxilla. That makes the purported ‘prefrontal’ a fused nasal + prefrontal. The postorbital and postfrontal are mislabeled with the other bone identity based on Tchoria (Fig. 2), a taxon not mentioned in your text. See attached.

Choristoderes are not ‘neodiapsid reptiles.’ Phylogenetically they are archosauriformes arising from Proterosuchus, Elachistosuchus and Tchoria. Phylogenetic miniaturization in that lineage lost the antorbital fenestra. See links below.

https://pterosaurheresies.wordpress.com/2013/08/13/champsosaurus-and-its-snorkel-nose/
http://reptileevolution.com/reptile-tree.htm
http://reptileevolution.com/champsosaurus.htm
http://reptileevolution.com/youngina-bpi2871.htm
http://reptileevolution.com/hyphalosaurus.htm
http://reptileevolution.com/lazarussuchus.htm

Best regards,

Figure 1. Tchoria and phylogenetic bracketing help identify bones in the skull of Champsosaurus (Fig. 2).

Figure 2. Tchoria and phylogenetic bracketing help identify bones in the skull of Champsosaurus (Fig. 2).

So, the Dudgeon et al. paper
is yet another great example of a situation in which phylogenetic analysis and bracketing (= comparing related taxa) sheds more light on a specimen than high-resolution micro-computed tomography scanning and/or adding characters (= looking more deeply into one taxon to the exclusion of others).

Figure 2. Champsosaurus skull with premaxilla in yellow.

Figure 3. Champsosaurus skull with premaxilla in yellow, nasal + prefrontal in pink. Bone identities determined by phylogenetic bracketing with Tchoria. See figure 2.

The greatest benefit 
available from the large reptile tree (LRT, 1631 taxa) is this sort of phylogenetic bracketing based on the validated nesting of sisters that have never been tested together in prior studies. You can look more deeply into one skull, as Dudgeon et al. did. Or you can examine many skulls, as ReptileEvolution.com and the LRT enable workers to do (Figs. 2, 4). In this case, using computed tomography on one skull did not put an end to traditional myths regarding the identity of bones in Champsosaurus.

Note to readers who like to harp on these issues:
More characters were not needed to resolve these problems. More taxa were needed.

Firsthand access + computed tomography did not help Dudgeon et al. Rather, a century-old drawing (Brown 1905, Fig. 3), access to several sister taxa for comparison (Figs. 2, 4) and Adobe Photoshop were the tools needed to resolve this issue.

It helps to know what you are dealing with.
Only a wide-gamut phylogenetic analysis that minimizes taxon exclusion can tell you where a specimen nests in the cladogram. Too often workers like Dudgeon et al. rely on vague citations, rather than running tests themselves or citing ongoing and self-repairing studies like the LRT. Publishing a mistake is to be avoided no matter how trivial.

Figure 2. Dorsal, lateral and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Click to enlarge. Note the tiny remnant of the antorbital fenestra. The squamosal has been broken into several parts.

Figure 4. Dorsal, lateral and palatal views of Late Triassic BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Note the tiny remnant of the antorbital fenestra and the long ascending process of the premaxilla.  The squamosal has been broken into several parts. This is a tiny phylogenetically miniaturized sister to the ancestor of Champsosaurus.

Champsosaurus annectens (Cope 1876, Brown 1905) ~1.5 m in length, Late Cretaceous to Eocene. Champsosaurus was derived from a sister to Tchoiria, and was a sister to other choristoderes, such as Cteniogenys and Lazarussuchus. This clade must have originated in the Late Permian or Early Triassic, but fossils are chiefly from late survivors, hence the wide variety in their morphology.


References
Brown B 1905. The osteology of Champsosaurus Cope. Memoirs of the AMNH 9 (1):1-26. http://digitallibrary.amnh.org/dspace/handle/2246/63
Cope ED 1876.
On some extinct reptiles and Batrachia from the Judith River and Fox Hills beds of Montana: Proceedings of the Academy of Natural Sciences, Philadelphia. 28, p. 340-359.
Dudgeon TW, Maddin HC, Evans DC & Mallon JC 2020. 
Computed tomography analysis of the cranium of Champsosaurus lindoei and implications for choristoderan neomorphic ossification. Journal of Anatomy (advance online publication)
doi: https://doi.org/10.1111/joa.13134
https://onlinelibrary.wiley.com/doi/10.1111/joa.13134

http://reptileevolution.com/champsosaurus.htm

Tiny Abdalodon: a basal cynodont, drags in Lycosuchus

Today’s blogpost returns to basal Therapsida,
after several years of ignoring this clade.

Kammerer 2016 reidentifies an old Procynosuchus skull 
as an even more basal cynodont, now named Abdalodon (Fig. 1). The problem is: cynodonts arise from basal theriodonts (Therocephalia) and Abdalodon nests with another flat-head taxon, Lycosuchus (Fig. 1), a traditional therocephalian in every other cladogram, but not the Therapsid Skull Tree (TST, 67 skull-only taxa, Fig. 2), a sister cladogram to the LRT.

So, where is the cynodont dividing line?
(= which tested taxon is the progenitor of all later cynodonts and mammals?)

It would help if we knew the phylogenetic definition
of Cynodontia because we should never go by traits (which may converge), but only by taxon + taxon + their last common ancestor and all descendants to determine monophyletic clades.

From the Kammerer 2016 abstract:
“Phylogenetic analysis recovers Abdalodon as the sister‐taxon of Charassognathus, forming a clade (Charassognathidae fam. nov.) at the base of Cynodontia. These taxa represent a previously unrecognized radiation of small‐bodied Permian cynodonts. Despite their small size, the holotypes of Abdalodon and Charassognathus probably represent adults and indicate that early evolution of cynodonts may have occurred at small body size, explaining the poor Permian fossil record of the group.”

Figure 1. Abdalodon nests with the many times larger therocephalian Lycosuchus in the LRT.

Figure 1. Abdalodon nests with the many times larger therocephalian Lycosuchus in the LRT.

Hopson and Kitching 2001 defined  Cynodontia
(Fig. 2) as the most inclusive group containing Mammalia, but excluding Bauria. In the TT Abdalodon nests with Lycosuchus on the cynodont side of Bauria.

Figure 4. TST revised with new data on Patranomodon and sister taxa.

Figure 4. TST revised with new data on Patranomodon and sister taxa.

So that makes Lycosuchus a cynodont,
by definition.

Figure 2. Procynosuchus, a basal cynodont therapsid synapsid sister to humans in the large reptile tree (prior to the addition of advanced cynodonts including mammals).

Figure 3. Procynosuchus, a basal cynodont therapsid synapsid sister to humans in the large reptile tree (prior to the addition of advanced cynodonts including mammals). This skull has been overinflated dorsoventrally based on the preserved skull, which everyone must have thought was crushed in that dimension.

Earlier we looked at
some Wikipedia writers when they stated, “Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved fossilize poorly…” 

Contra that baseless assertion,
with phylogenetic analysis and clades defined by taxa it is easy to determine which taxa are the last common ancestors, sisters to the progenitors of every derived clade in the TT, LRT or LPT. We can tell exactly which taxon was the first to lay amniotic eggs, without having direct evidence of eggs, simply because all of its ancestors in the LRT laid amniotic eggs. In the same way, we can figure out which taxon, among those tested, is the basalmost cynodont. Adding Bauria to the LRT made that happen today.

Let’s talk about size
The extreme size difference between Abdalodon and Lycosuchus (Fig. 1) brings up the possibility of cynodonts going through a phylogenetic size squeeze… retaining juvenile traits into adulthood… neotony… essentially becoming sexually mature at a tiny size for more rapid reproduction, reduced food needs, ease in finding shelters, etc. We’ve seen that before in several clades here, here and here, to name a few.

Figure 4. Charassognathus does not share more traits with Abdalodon than other taxa, like Bauria and Promoschorhynchops in the TT.

Figure 4. Charassognathus does not share more traits with Abdalodon than other taxa, like Bauria and Promoschorhynchops in the TT.

Kammerer 2016 mentioned another small taxon,
Charassognathus (Fig. 4). In the TST (Fig. 2) Charassognathus nests with Bauria and Promoschorhynchops, within the Therocephalia, distinct from, and not far from Abdalodon and the Cynodontia. So no confirmation here for Kammerer’s proposed clade, ‘Charassognathidae’ (see above).


References
Hopson JA and Kitching JW 2001. A Probainognathian Cynodont from South Africa and the Phylogeny of Nonmammalian Cynodonts” pp 5-35 in: Parish A, et al.  editors, Studies in Organismic and Evolutionary biology in honor of A. W. Crompton. Bullettin of the Museum of Comparative Zoology. Harvard University 156(1).
Kammerer CF 2016. A new taxon of cynodont from the Tropidostoma Assemblage Zone (upper Permian) of South Africa, and the early evolution of Cynodontia. Papers in Palaeontology 2(3): 387–397. https://doi.org/10.1002/spp2.1046

wiki/Bauria
wiki/Abdalodon
wiki/Lycosuchus

A pre-Cosesaurus: the BES SC111 specimen

Earlier
here and here we looked at the pterosaur traits found in the lepidosaur tritosaur fenestrasaur, Cosesauru aviceps (Fig. 1).

Today
let’s look at Cosesaurus traits found in the more primitive BES SC 111 specimen (Fig. 1) traditionally assigned to the larger set of specimens traditionally attributed to Macrocnemus (Fig. 2).

Traditional paleontologists
consider the BES SC111 specimen a juvenile based on its size and short rostrum relative to other Macrocnemus specimens. The large reptile tree (LRT, 1412 taxa) nests the BES SC111 specimen apart from Macrocnemus, basal to Langobardisaurus + Fenestrasauria. Since tritosaurs mature isometrically, juvenile Macrocnemus specimens should have adult proportions, but none are known at present.

Phylogenetic miniaturization
produce smaller tritosaur specimens with a shorter rostrum via neotony. Rather than juvenile traits, late stage embryo (= pre-hatchling) traits are retained into adulthood. Phylogenetic bracketing indicates the BES SC111 specimen was close to adult size.

Figure 1. The BES SC111 specimen attributed to Macrocnemus compared to Cosesaurus, the taxon transitional to pterosaurs. See text for detais.

Figure 1. The BES SC111 specimen attributed to Macrocnemus compared to Cosesaurus, the taxon transitional to pterosaurs. See text for detais.

Traits shared in the BES SC111 specimen and Cosesaurus:

  1. The skulls are virtually identical, including orbit size, antorbital fenestra, tooth size
  2. Torsos quite similar, both with many more gastralia than in ancestors
  3. Tail attenuated
  4. Interclavicle cruciform
  5. Sternum present
  6. Clavicles short, relatively straight and robust
  7. Scapula with longer posterior process (even longer in Cosesaurus)
  8. Metacarpal 4 is the longest, so is manual digit 4
  9. Ilium anterior process present (longer in Cosesaurus)
  10. Prepubis present (larger in Cosesaurus)
  11. Metatarsal 4 is the longest, so is pedal digit 4
  12. Metatarsal 5 is short
  13. Pedal 1.1 is elongate (longer in Cosesaurus)

Derived traits in Cosesaurus relative to BES SC111

  1. Overall smaller in Cosesaurus (neotony)
  2. Epipterygoid absent in Cosesaurus (neotony)
  3. Shorter neck in Cosesaurus (neotony)
  4. 5 sacrals in Cosesaurus (3 in BES SC111)
  5. Sternal complex in Cosesaurus with shifted elements
  6. Coracoid reduced to a curved stem in Cosesaurus (neotony, less ossification)
  7. Hand much larger in Cosesaurus (slightly longer than antebrachium)
  8. Centrale bones migrate to become preaxial carpal and pteroid in Cosesaurus
  9. Thyroid fenestra absent in Cosesaurus
  10. Pedal unguals rounded in BES SC111 

Tanystropheus and kin going back to Huehuecuetzpalli.

Figure 2. Tanystropheus and kin going back to Huehuecuetzpalli. Cosesaurus is not shown here (see figure 1).

Due to convergence,
adding taxa is, perhaps, the only way to split protorosaurs (= prolacertiformes) from tritiosaurs. Make sure you add Huehuecuetzpalli (Fig. 2) to any such analysis.

Figure 3. BES SC111 pectoral region. Colors correspond to figure 1.

Figure 3. BES SC111 pectoral region. Colors correspond to figure 1. The left scapula(?) is incomplete. The interclavicle and sternum are largely hidden beneath the vertebrae. Not sure what that elliptical bone is at upper left and blue. It may be two.

The shifting of pectoral elements
from Huehuecuetzpalli to pterosaurs was detailed earlier here and here.

Figure 4. BES SC111 pelvic region. Colors correspond to those in figure 1. Note the tiny blue prepubes.

Figure 4. BES SC111 pelvic region. Colors correspond to those in figure 1. Note the tiny blue prepubes.

Several indicators of bipedal ability
are present in the BES SC111 specimen, as in the extant Chlamydosaurus kingii.

  1. Elongate ilium anterior process
  2. More than two sacral vertebrae
  3. Prepubes + stiff belly (more gastralia)
  4. Attenuated tail
  5. Elongate cervicals

Figure 6. Green iguana demonstrating the curling of pedal digit 5 in tendril-toed arboreal lepidosaurs, as hypothesized in the BES SC111 specimen and pterosaurs.

Figure 5. Green iguana demonstrating the curling of pedal digit 5 in tendril-toed arboreal lepidosaurs, as hypothesized in the BES SC111 specimen and pterosaurs.

Cosesaurus and Rotodactylus, a perfect match.

Figure 5. Cosesaurus and Rotodactylus, a perfect match. Elevate the proximal phalanges along with the metatarsus, bend back digit 5 and Cosesaurus (left) fits perfectly into Rotodactylus (right).

The curling of pedal digit 5
in the Rotodactylus trackmakers (Fig. 6) is a lepidosaur trait (Fig. 5) carried to extremes in basal pterosaurs, like ‘Sauria aberrante’ and Dimorphodon.


References
Ellenberger P and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier 12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Peabody FE 1948.  Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah.  University of California Publications, Bulletin of the  Department of Geological Sciences 27: 295-468.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Renesto S and Avanzini M 2002. Skin remains in a juvenile Macrocnemus bassanii Nopsca (Reptilia, Prolacertiformes) from the Middle Triassic of Northern Italy. Jahrbuch Geologie und Paläontologie, Abhandlung 224(1):31-48.
Sanz JL and López-Martinez N 1984. The prolacertid lepidosaurian Cosesaurus aviceps Ellenberger & Villalta, a claimed ‘protoavian’ from the Middle Triassic of Spain. Géobios 17: 747-753. 

wiki/Cosesaurus
wiki/Macrocnemus

What is Rhamphocephalus? An earlier bird.

Some confusion in the academic literature today
as a Middle Jurassic fossil known since the 19th century is grossly misidentified.

Figure 2. Rhamphocephalus in situ, traced by Seeley, traced by O'Sullivan and Martill and Rhamphorhynchus graphic from Wellnhofer 1975.

Figure 1. Rhamphocephalus in situ, traced by Seeley, traced by O’Sullivan and Martill and, for comparison sake, Rhamphorhynchus graphic from Wellnhofer 1975, all appearing in O’Sullivan and Martill 2018. Rhamphocephalus has been traditionally identified as a pterosaur. That paradigm was challenged by O’Sullivan and Martill 2018, but that challenge is challenged again here.

Today a paper by O’Sullivan and Martill 2018
redescribes several fossils from the Middle Jurassic (165–166 mya) of England, traditionally ascribed to the wastebasket pterosaur taxon, Rhamphocephalus prestwichi (type, Seeley, 1880;  OUM J.28266; Figs. 1–4). Most of the disassociated specimens (individual jaws, limbs) are clearly pterosaurian. One (the goose-sized skull roof) is clearly not pterosaurian.

Figure 2. Rhamphorhynchus compared to a large choristodere, Simoedosaurus, and to a large thalattosuchian, Pelagosaurus. There is absolutely no match here.

Figure 2. O’Sullivan and Martill compared Rhamphocephalus to a large choristodere, Simoedosaurus, and to a large thalattosuchian, Pelagosaurus. There is absolutely no match here, either in size or morphology. Colors and ‘to scale’ Rhamphocephalus images added for clarity.

The holotype of Rhamphocephalus prestwichi,
“an isolated skull table, is found to be a misidentified crocodylomorph skull,” according to O’Sullivan and Martill, who illustrated the 10x smaller specimen alongside a dorsal view of the 3m long thalattosuchian (marine) croc, Pelagosaurus, from the Lower Jurassic of England and, perhaps to cover all their bases, flipped anterior-to-posterior alongside the Paleocene choristodere, Simoedosaurus (Fig. 2). Note: the authors did not illustrate their comparative taxa to scale (as shown above), perhaps because the taxa are 10x larger and are morphologically dissimilar. So why make such comparisons? I don’t understand the logic of these paleontologists making such readily disprovable comparisons.

Figure 1. The skull roof named Rhamphocephalus here with bones and teeth colored.

Figure 3. The in situ specimen of Rhamphocephalus here with bones and teeth colored. At standard monitor 72 dpi resolution, this image is 2x life size. Perhaps this skull can be µCT scanned for buried data. Some palatal elements are peeking out from the antorbital fenesrae and nares. The dentary teeth make a few appearances, too. This is a sharp-tipped taxon.

Traced here
using DGS methods (Fig. 3) and phylogenetically tested in the large reptile tree (LRT, 1321 taxa) goose-sized Rhamphocephalus nests with the hummingbird-sized, Hongshanornis (Fig. 2), an Early Cretaceous toothed bird from China. Hongshanornis is one of the few toothed birds in which the orbits are further forword, creating a longer cranium to match that of Rhamphocephalus. A suite of other skull traits are likewise most closely matched to Hongshanornis. The Rhamphocephlaus specimen appears to be complete without obvious breaks either at the toothy tip of the skull or the occiput. More teeth and bones were identified here.

Figure 2. Rhamphorcephalus in situ compared to Hongshanornis in situ to scale and enlarged to match.

Figure 2. Rhamphorcephalus in situ compared to Hongshanornis in situ to scale and enlarged to match skull length. To scale image (above) is 1.25x actual size, much too small for sea crocs. similar in size to pre-birds. Hongshanornis is a tiny bird, similar in size to a hummingbird.

Ironically
the authors report, “The earliest known record of Bathonian pterosaurs is an account of “fossil bird bones” from the Taynton Limestone Formation of Stonesfield by an anonymous author A.B., appearing in the March edition of the Gentleman’s Magazine of 1757.” For this specimen, and only this specimen, A.B. got it right. The other specimens are clearly pterosaurian.

Historically
the authors report, “This specimen is exposed on a limestone slab in dorsal view and was assigned to Pterosauria based on its perceived thin bone walls. Seeley (1880) noted that the arrangement of bones was more crocodilian than pterosaurian and considered this construction diagnostic of the new taxon. Significantly he (Seeley 1880: 30) stated: “I shall be quite prepared to find that all the ornithosaurians from Stonesfield belong to this or an allied genus which had Rhamphorhynchus for its nearest ally.” In the LRT crocodilians are closer to birds than pterosaurs are.

Figure 6. Rhamphocephalus chronologically precedes the Solnhofenbirds by several million years making it the oldest known bird.

Figure 6. Rhamphocephalus chronologically precedes the Solnhofenbirds by several million years making it the oldest known euornithine bird.

Is the Middle Jurassic too early for a toothed bird?
Perhaps not. Remembet that all of the Late Jurassic Solnhofen birds, traditionally named as one genus, Archaeopteryx, already represent a diverse radiation of taxa, suggesting an earlier genesis for that radiation. Rhamphocephalus indicates that the original bird radiation had its genesis at least 15 million years earlier. 

It is unfortunate
that O’Sullivan and Martill attempted to force fit the skull specimen into a crocodilian clade when no aspect of the thin-walled, goose-sized skull of Rhamphocephalus is crocodilian (Fig. 2)… or choristoderan (when flipped backwards!!). Adding Rhamphocephalus to the LRT gives it a single most parsimonious sister among all the toothed birds and a special Middle Jurassic place in the origin of birds story. All the details fit.

Working with a high-resolution image
of Rhamphocephalus (Fig. 3) copied from a PDF of the paper by O’Sullivan and Martill made this all possible.

Once again, to determine the affinities of a specimen it is more important to have a wide gamut of taxa to work with than to have firsthand access to the specimen itself. No one likes this method, but it clearly works time after time and to not use it invites discredit.

USE THE LRT. That’s what it is here for.

References
O’Sullivan M and Martill DM 2018. Pterosauria of the Great Oolite Group (Bathonian, Middle Jurassic) of Oxfordshire and Gloucestire. Acta Palaeontologica Polonica 63 (X): xxx–xxx, 2018 https://doi.org/10.4202/app.00490.2018
Seeley HG 1880. On Rhamphocephalus prestwichi Seeley, an Ornithosaurian from the Stonesfield Slate of Kineton. Quart. J. Geol. Soc. 36: 27-30.

wiki/Rhamphocephalus

Dual origin of the mammalian-type jaw joint

Today: we look at a new paper
by Lautenschlager et al. 2018, who tested transitional synapsid jaw joints evolving into mammal ear bones. Before we begin, let’s remember these five pertinent facts:

1- A monophyletic clade consists of two select members,
their last common ancestor and all of its descendants. A clade does not include taxa that share, by convergence, a particular trait, no matter how ‘key’ that trait is.

2- Linnaeus 1758 decided THE key trait in mammals
is the expression of milk for infants from dermal glands. Since milk glands almost never fossilize several skeletal traits are used instead as ‘lactation markers.’

3- These markers include
the single replacement of milk teeth with permanent teeth. This replacement pattern implies toothless hatchlings dependent on their mother’s milk, a trait common to all living mammals and presumably, all extinct ones. Hatchling and neonate basal mammals only develop teeth and the ability to locomote as they mature in their mother’s care. Derived mammals, like cattle and horses, are ready to locomote at birth, as we learned earlier here.  Sinoconodon, a proximal mammal outgroup, lacked permanent teeth.

4- Another traditional ‘key’ trait in mammals
is the mammalian jaw joint (dentary-squamosal) which gradually (both embryologically and phylogenetically) replaces the basal tetrapod jaw joint (articular-quadrate). For several transitional taxa, both jaw joints operate side-by-side. In mammals the former posterior jaw bones eventually become gracile splints, then tiny ear bones.

5- Ear bone location
in egg-laying mammals (Prototheria), these ear bones are below the jaw joint. In Theria these ear bones are posterior to the jaw joint, demonstrating yet another act of convergence from a common ancestor in which the posterior jaw bones were still connected to a trough in the posterior dentary and a tiny, but robust quadrate, as in Megazostrodon.

So that sets the stage
for today’s discussion. It’s time to reexamine what makes a mammal a mammal.

In the large reptile tree
(LRT, 1293 taxa; subset Fig. 1) the last common ancestor of all living mammals is Megazostrodon from the Latest Triassic. The first dichotomy splits egg-laying mammals (Prototheria) from live-bearing mammals (Theria). So that happened early,

The smallest mammals were not the first mammals.
In the LRT tiny Early Jurassic Hadrocodium nests at the base of a small clade of basal therians that includes Morganucodon and Volaticotherium. Following in the pattern of basal reptiles, which also had smaller taxa after the genesis of the clade, basal mammals slowly evolved new reproductive structures and made improvements following the first tentative appearances of novel reproductive membranes and structures.

Six traditional mammals,
Gobiconodon (Trofimov 1978), Maotherium (Rougier et al. 2003); Spinolestes (Martin 2015); Yanaconodon (Luo et al. 2007) Liaoconodon (Meng et al. 2011) and Repenomamus (Li et al. 2001; Hu et al. 2005) nest outside the clade of crown (all living) mammals in the LRT, despite the fact that they all had single tooth replacement and a dentary-squamosal jaw joint, as in mammals. Traditionally these traits have caused taxonomic confusion as workers assumed no convergence.

Figure 1. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

Figure 1. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

A new paper by Lautenschlager et al. 2018
discusses “The role of miniaturization in the evolution of the mammalian jaw and middle ear.” Phylogenetic miniaturization prior to the appearance of mammals (Fig. 3) has been widely known for decades and was discussed earlier here. Putting their own twist on this hypothesis, Lautenschlager et al. report, “Here we use digital reconstructions, computational modeling and biomechanics analyses to demonstrate that the miniaturization of the early mammalian jaw was the primary driver for the transformation of the jaw joint. We show that there is no evidence for a concurrent reduction in jaw-joint stress and increase in bite force in key non-mammaliaform taxa in the cynodont–mammaliaform transition, as previously thought.”

Unfortunately,
Lautenschlager et al. begin their paper with a false statement: “The mammalian jaw and jaw joint are unique among vertebrates.” No. The LRT documents that this happened twice in parallel near the genesis of the clade Mammalia (Fig. 1). The authors’ error appears due to taxon exclusion in their phylogenetic analysis, creating a tree topology (Fig. 2) different from the LRT (Fig. 1). A larger taxon list would have rearranged the taxa in the Lautenschlager et al. cladogram as it does as the LRT continues to grow.

Figure 1. Modified from figure 1 in Lautenschlager et al. 2018 with the addition of a cyan and magenta band keyed to pre-mammals and mammals in the LRT. Note the oddly large Repenomamus and Vincelestes in the original work. They don't belong where they are placed.

Figure 2. Modified from figure 1 in Lautenschlager et al. 2018 with the addition of a cyan and magenta band keyed to pre-mammals and mammals in the LRT. Note the oddly large Repenomamus and Vincelestes in the original work. They don’t belong where they are placed here.  Zhangheotherium is a pangolin ancestor. Vincelestes is a top predator marsupial. Rugosodon is a multituberculate rodent. Massive taxon exclusion is the problem here. Worse yet, the red dotted line indicating “Jaw-joint transition” really should have started at the top of the graph, as shown in figure 3.

In the Lautenschlaer et al. 2018 cladogram
(Fig. 2) the last common ancestor of all mammals is tiny Hadrocodium. In their cladogram Megazostrodon, Morganucodon and Brasilitherium are not mammals, but Mammaliaformes (= the most recent common ancestor of Morganucodonta and Prototheria + Theria). The current definition of Mammaliaformes turns out to be a junior synonym for Mammalia because in the LRT Morganucodon and kin are all mammals.

Figure 3. Kynodontia to scale. The miniaturization of the ancestors of mammals had its genesis long before the proximal ancestors of mammals.

Figure 3. Kynodontia to scale. The miniaturization of the ancestors of mammals had its genesis long before the proximal ancestors of mammals, like Therioherpeton.

The LRT
(Fig. 1) documents the final stages of the evolution of the dentary-squamosal joint actually occurred twice: once in the lineage of mammals that led to all extant mammals (Fig. 4) and again in the lineage that led to Repenomamus and kin (Fig. 5).

Take away thought:
One cannot determine what a taxon is by identifying a key trait. That would be ‘pulling a Larry Martin.’ ‘Turtles’, ‘cetaceans’ and ‘pinnipeds’ all have a dual origins, as we learned earlier here, here and here. Only after a wide gamut phylogenetic analysis that tests all possibilities and opportunities can one determine the last common ancestor of a clade. That’s how we identify and guard against the specter and real possibility of convergence.

Figure 5. Basal mammals and their proximal ancestors. Here taxa below Megazostrodon are mammals. Those above are not. Hadrocodium is uniquely reduced, but this occurs within the Mammalia.  The dual jaw joint was tentatively present in Pachygenelus.

Figure 4. Basal mammals and their proximal ancestors. Here taxa below Megazostrodon are mammals. Those above are not. Hadrocodium is uniquely reduced, but this occurs within the Mammalia.  The dual jaw joint was tentatively present in Pachygenelus.

Lautenschläger et al. acknowledge convergence when they report: “New fossil information has suggested that a definitive mammalian middle ear (DMME) evolved independently in at least three mammalian lineages by detachment from the mandible, but the emergence of a secondary jaw joint is a key innovation that unites all mammaliaforms. However, a central question exists as to how, during this transformation, the jaw hinge remained robust enough to bear strong mastication forces while the same bones were becoming delicate enough to be biomechanically viable for hearing.”

That’s a good question,
and the authors did a good job of showing how they tested specimens.

Figure 5. Theriodont pre-mammals to scale. Note the dentary-squamosal jaw joint developed by convergence in this clade.

Figure 5. Theriodont pre-mammals to scale. Note the dentary-squamosal jaw joint developed by convergence in this clade.

Lautenschlager et al continue: “Here we integrate a suite of digital reconstruction, visualization and quantitative biomechanical modelling techniques to test the hypothesis that reorganization of the adductor musculature and reduced stress susceptibility in the ancestral jaw joint facilitated the emergence of the mammalian temporomandibular jaw joint. Applying finite element analysis, we calculated bone stress, strain and deformation to determine the biomechanical behaviour of the mandibles of six key taxa across the cynodont–mammaliaform transition.” (See Fig. 2, but also see Fig. 1)

Lautenschlager et al conclude:
“In our analyses, reduction in mandibular size—rather than alterations of the osteology and the muscular arrangement—produced the most notable effects on minimizing absolute jaw-joint stress. Our results demonstrate that changes to joint morphology and muscle (re)organization have little effect on joint loading.

Key to understanding the situation
and perhaps somewhat overlooked by the authors, is the fact that most of the changes to the posterior jaw bones were already in place in the last common ancestor of Repenomamus and Megazostrodon. a taxon close the Therioherpeton and Pachygenelus (Fig. 4). After these taxa, there was just a little bit left to do. Certainly size reduction had a great impact on all the changes that split mammals and their kin apart from their ancestors. Even so, a correct phylogenetic framework is necessary to build a valid case and not mix up mammals with non-mammals as Lautenschlager et al. did. They did not allow for the possibility of convergence which the inclusion of more taxa uncovered.

The multituberculate issue
Multituberculates, like Kryptobaatar, also have a low, robust jaw joint, just like Repenomamus and kin. So are they related? Not yet. In the LRT multituberculates are still more attracted to rodents and their kin than to pre-mammals.

Side note: While reexamining the data in the LRT, Liaoconodon shifted in the LRT to nest with Gobiconodon and Repenomamus, adding to the long list of corrections I’ve made here over the last seven years. As I’ve said many times before, I’m learning as I go. Sometimes that learning happens a little too long after a taxon’s insertion.

One final question: 
Did Repenomamus and Gobiconodon have tiny toothless neonates? Were the neonates helpless? Did their mothers provide milk to them? Which means, ultimately, did they represent an extinct clade of primitive mammals? Present data indicates the answer to all the above is ‘no’, despite the presence of two ‘key’ mammalian traits: permanent teeth and a dentary-squamosal jaw joint.

This is heretical,
but once discovered needs to be reported and later confirmed and/or refuted.

References
Hu Y, Meng J, Wang Y-Q and Li C-K 2005. Large Mesozoic mammals fed on young dinosaurs. Nature 433:149-152.
Lautenschläger S et al. (4 co-authors) 2018. The role of miniaturization in the evolution of the mammalian jaw and middle ear. Nature.com
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