Laosuchus naga enters the LRT

The question today is:
what are chroniosuchians? Are they reptiles or not? Arbez, Sidor and Steyer 2018 say: ‘not’ (Fig. 1). Here that mistake is due to tradition and taxon exclusion, based on their cherry-picked outgroups. Heretically. chroniosuchians are amphibian-like reptiles.

Figure 1. Cladogram from xx 2018 with Laosuchus nesting with chroniosuchians in the absence of Solenodonsaurus.

Figure 1. Cladogram from Arbez, Sidor and Steyer 2018 with Laosuchus nesting with chroniosuchians in the absence of Solenodonsaurus.

Arbez, Sidor and Steyer report from their abstract:
“Chroniosuchians were a clade of non-amniotic tetrapods known from the Guadalupian (middle Permian) to Late Triassic, mainly from Russia and China.” Asaphestera is the proximal outgroup followed by Limnoscelis, Seymouria, Gephyrostegus and other taxa.

By contrast and using more outgroup taxa
the large reptile tree (LRT 1301 taxa, subset Fig. 2) nests chroniosuchians within the base of the archosauromorph branch of reptiles. When more taxa are included in the LRT, Limnoscelis and Gephyrostegus nest as reptiles (= amniotes) while Asaphestra and Seymouria nest as unrelated traditional microsaur lepospondyls and seymouriamorphs respectively.

Figure 1. Laosuchus nests with untested Solenodonsaurus rather than the extra fenestrated chroniosuchians.

Figure 2. Laosuchus nests with untested Solenodonsaurus rather than the extra fenestrated chroniosuchians. Note that all these reptiles precede the traditional first dichotomy in amniote cladograms: the splitting of synapsida from the other amniotes.

Arbez, Sidor and Steyer 2018 introduce a new taxon,
Laosuchus naga (Fig 3), a long-snouted chroniosuchian without an antorbital fenestra. The authors did not include Solenodonsaurus (Fig. 3), which nests with Laosuchus as basal chroniosuchians in the LRT (Fig. 2). Both lack an antorbital fenestra, distinct from Chrionosaurus and Chroniosuchus.  

Figure 2. Laosuchus compared to Bystrowiella and Solenodonsaurus to scale (presuming that the scale bar for Laosuchus is 10cm.)

Figure 2. Laosuchus compared to Bystrowiella and Solenodonsaurus to scale (presuming that the scale bar for Laosuchus is 10cm.) Distinct from many reptiles, the lacrimal no longer contacts the orbit in this taxon and other chroniosuchians, but continues to contact the naris.

Laosuchus naga traits include:

  1. an extremely reduced pineal foramen
  2. absence of palatal dentition
  3. well-developed transverse flange of the pterygoid that contacts the maxilla
  4. internal crest on and above the dorsal side the palate
  5. otic notch closed by the tabular horn and the posterior part of the squamosal, forming a continuous wall
  6. thin and high ventromedial ridge on parasphenoid.
Figure 4. Solenodonsaurus skull in situ and reconstructed. That brown bone on top of the frontal/parietal suture is a displaced lacrimal that nicely fills the gap in the reconstruction.

Figure 4. Solenodonsaurus skull in situ and reconstructed using DGS. That brown bone on top of the frontal/parietal suture are displaced pieces of the lacrimal that nicely fills the gap in the reconstruction. A few bones are always ignored. I’m just trying to get the basics here.

Something I learned while reexamining Solenodonsaurus
The displaced bone atop the skull is actually part of the broken lacrimal. The quadratojugal is displaced on the posterior mandible. The prefrontal is broken but not very displaced. The posterior jugal is broken into several pieces. Using DGS allows one to cut and paste and fit these puzzle pieces back into the missing parts of the skeleton where they belong. If they don’t fit, they don’t belong, but they never fit perfectly. It’s like putting Humpty Dumpty together again. There are always a few pieces left over.

References
Arbez T, Sidor CA and Steyer J-S 2018. Laosuchus naga gen. et sp. nov., a new chroniosuchian from South-East Asia (Laos) with internal structures revealed by micro-CT scan and discussion of its palaeobiology. Journal of Systematic Palaeontology DOI: 10.1080/14772019.2018.1504827

http://zoobank.org/urn:lsid:zoobank.org:pub:11D07FA3-0F4C-4EF9-A416-E8E6BE76C970

 

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The short-faced bear (Arctodus) is a giant wolverine in the LRT.

Yesterday we looked at three bears, Ursus, Arctodus (Fig. 1) and Ailuropoda (the polar bear, the short-faced bear and the panda bear). They do not form a single bear clade in the large reptile tree (LRT, 1299 taxa), but each is more closely related to small weasels and grew to bear-size by convergence.

For instance,
Arctodus is most closely related to today’s wolverine (Gulo gulo, Figs. 1, 2) among tested taxa, and the similarities are immediately apparent. Have they ever been tested together before? Let me know if this is so.

Figure 1. Arctodus (shor-faced bear) skeleton compared to the smaller Gulo (wolverine) skeleton. Both have similar proportions. Arctodus is larger than 3m, while Gulo is about 1m in length.

Figure 1. Arctodus (shor-faced bear) skeleton compared to the smaller Gulo (wolverine) skeleton. Both have similar proportions. Arctodus is larger than 3m, while Gulo is about 1m in length.

Arctodus simus (Leidy 1854; Cope 1874; up to 3 to 3.7m tall) is the extinct short-faced bear, one of the largest terrestrial mammalian carnivores of all time. Long limbs made it a fast predator. Being related to the wolverine made it short-tempered and dangerous.

Figure 2. Long-legged Gulo, the wolverine, is most similar to Arctodus, the short-faced bear in the LRT.

Figure 2. Long-legged Gulo, the wolverine, is most similar to Arctodus, the short-faced bear in the LRT. That’s a penile bone, not a prepubis.

Gulo gulo (Linneaus 1758; up to 110 cm in length) is the extant wolverine, a ferocious predator resembling a small bear. Note the tail length is midway between the long tail of weasels and the short tail of birds.

Figure 1. Subset of the LRT focusing on the Carnivora with tan tones on the bears newly added.

Figure 3. Subset of the LRT focusing on the Carnivora with tan tones on the bears newly added.

The red panda
(Ailurus) was also added to the LRT (Fig. 3) and, to no one’s surprise, nests with the raccoon, Procyon apart from the giant panda.

Figure 4. Gulo skull in lateral and dorsal views. Compare to Arctodus in figure 5.

Figure 4. Gulo skull in lateral and dorsal views. Compare to Arctodus in figure 5. The male skull has the larger and longer parasagittal crest.

The skulls of Gulo and Arctodus
(Figs. 4, 5) despite their size differences, are quite similar. Both display sexual dimorphism.

Figure 5. Arctodus (short-faced bear) skull in lateral view. Compare to figure 4.

Figure 5. Arctodus (short-faced bear) skull in lateral view. Compare to figure 4.

Taxon inclusion
sheds light on phylogenetic interrelationships.

If you have an interest in wolverine evolution,
I suggest you use the keyword “Gulo” or you’ll end up learning about Marvel’s superhero, also named Wolverine.

References
Cope ED 1879. The cave bear of California. American Naturalist 13:791.
Leidy 1854. Remarks on Sus americanus or Harlanus americanus, and on other extinct mammals. Proceedings of the Academy of Natural Sciences of Philadelphia 7:90.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

wiki/Gulo
wiki/Short-faced_bear

These three ‘bears’ are not monophyletic

More heresy:
When I added three bears (polar, short-face and panda (= Ursus, Arctodus and Ailuropoda) to the large reptile tree (LRT, 1298 taxa) they nested apart from dogs and bear-dogs AND they did not nest together. Rather all three arose from separate types of weasels (= Mustela, Puijila, and Gulo).

Figure 1. Subset of the LRT focusing on the Carnivora with tan tones on the bears newly added.

Figure 1. Subset of the LRT focusing on the Carnivora with tan tones on the bears newly added.

Ailuropoda melanoleuca (David 1869) is the extant giant panda. Here it nests between the weasel, Mustela and the rest of the bear/seal clade. This odd carnivore eats bamboo. The centrale extends beyond the wrist forming a new medial digit, the panda’s ‘thumb’.

Ursus maritimus (Phipps 1774; up to 3m in length) is the extant polar bear, a hypercarnivore restricted to cold climates. Bears split from other Carnivora, specifically the giant weasel, Puijila, some 38mya.

Arctodus simus (Leidy 1854; Cope 1874; up to 3 to 3.7m tall) is the extinct short-faced bear, one of the largest terrestrial mammalian carnivores of all time. Here it nests as a giant wolverine (genus: Gulo, also newly added). Long limbs made it a fast predator.

More details on these taxa coming soon.

References
Cope ED 1879. The cave bear of California. American Naturalist 13:791.
Kellogg AR 1931. Pelagic mammals of the Temblor Formation of the Kern River region, California. Proceedings of the California Academy of Science 19(12):217-397.
Kohno N, Barnes LG and Hirota K 1994. Miocene fossil pinnipeds of the genera Prototaria and Neotherium (Carnivora; Otariidae; Imagotariinae) in the North Pacific Ocean: Evolution, relationships and distribution. The Island Arc. 3(4): 285–308. doi:10.1111/j.1440-1738.1994.tb00117.x
Kumar V et al. 2017. The evolutionary history of bears is characterized by gene flow across species. Nature.com/scientificreports. 7:46487
Leidy 1854. Remarks on Sus americanus or Harlanus americanus, and on other extinct mammals. Proceedings of the Academy of Natural Sciences of Philadelphia 7:90.
Linneaus C von 1761. Fauna Suecica sistens Animalia Sueciae Regni: Mammalia, Aves, Amphibia, Pisces, Insecta, Vermes. Distributa per Classes, Ordines, Genera, Species, cum Differentiis Specierum, Synonymis Auctorum, Nominibus Incolarum, Locis Natalium, Descriptionibus insectorum. Editio altera, auctior. Stockholmiae: L. Salvii, 48 + 578 pp.,
Phipps CJ 1774. A voyage towards the North Pole: undertaken by His Majesty’s command. J. Nourse, London.

wiki/Ursus
wiki/Puijila
wiki/Neotherium
wiki/Mustela
wiki/Giant_panda
wiki/Short-faced_bear

 

Jinguofortis perplexus: not so perplexing after all

Figure 1. Jinguofortis perplexus in situ with interpretive drawing from Wang, Stidham and Zhou 2018.

Figure 1. Jinguofortis perplexus in situ with interpretive drawing from Wang, Stidham and Zhou 2018. Red lines are added over the fibula and digit 5.

Jinguofortis perplexes (Wang, Stidham and Zhou 2018; Early Cretaceous, 127 mya; IVPP V24194; Figs. 1-3) was described as “an unusual mosaic of bird and dinosaur features”. In the large reptile tree (LRT, 1297 taxa) it nests within the toothed bird clade (Odontornithes) of Neognathae alongside Longicrusavis. Jinguofortis has a fused scapulocoraocid, distinct from sister taxa. Although it has a pygostyle, no fan of tail feathers was preserved, even though wing feathers were preserved.

Figure 2. Jinguofortis perplexus reconstruction based on drawings in Wang, Stidham and Zhou 2018.

Figure 2. Jinguofortis perplexus reconstruction based on drawings in Wang, Stidham and Zhou 2018.

Phylogenetically, the authors report:
Jinguofortis is resolved as the sister to Chongmingia (another bird known from fewer bones but also having a fused scapulocoracoid), and they form the out group to Sapeornis and Ornithothoraces.” 

The authors erected a new clade,
Jinguofortisidae within the clade Pygostylia. Several theropods and bird clades developed pygostyles by convergence, but this was not known to the authors due to taxon exclusion (see below). The authors reported, “the earliest evidence of reduction in manual digits among birds.” Note that all the phalanges in manual digit 3 are missing except the ungual.

Figure 3. Jinguofortis skull in situ and reconstructed using DGS

Figure 3. Jinguofortis skull in situ and reconstructed using DGS. Many of the skull bones are mere strips, making identification difficult, except, perhaps by comparison to sisters, like Longicrusavis (Fig. 4).

The LRT does not confirm this nesting.
What they identified as the quadrate, is here identified as the quadratojugal. What they identified as a splenial is here identified as a hyoid. The authors employed only one Solnhofen bird (= Archaeopteryx) in their phylogenetic analysis. They should have used more as we talked about earlier here. When that happens enanthiornithine, confusiornithine, sapeornithine birds all have Late Jurassic origins and that changes the tree topology they presented in their SuppData.

Figure 2. It's always valuable to see what the taxon looks like with scale bars. This is a tiny specimen, but rather completely known.

Figure 4. This is Longicrusavis, a coeval sister in the LRT to the newly described Jinguofortis.

Employing more Solnhofen birds in phylogenetic analysis
is getting to be the key concept in repairing traditional bird tree topologies. If I can do it (Fig. 5), anyone can. It may surprise them to find the Odontornithes nesting with the Neognathae.

Figure 1. More taxa, updated tree, new clade names.

Figure 5. Bird origins from several months ago. Although many birds have been added since then, the tree topology shown here has not changed. Updated LRT is posted.

References
Wang M, Stidham TA and Zhou Z 2018. A new clade of basal Early Cretaceous pygostylian birds and developmental plasticity of the avian shoulder girdle. PNAS https://doi.org/10.1073/pnas.1812176115

NatGeo Jinguofortis

Rapetosaurus: my what a big pubis you have!!

Rapetosaurus krausei
(Curry, Rogers & Forster, 2001) is a Late Cretaceous titanosaur sauropod that is known from several bits and pieces from 3 adults, plus the majority of a juvenile specimen (Fig. 1). Adult lengths are estimated up to 15 m.

Figure 1. Rapetosaurus in traditional quadrupedal and imagined bipedal poses. Here that giant pubis is carrying a big gut.

Figure 1. Rapetosaurus in traditional quadrupedal and imagined bipedal poses. Here that giant pubis is carrying a big gut.

In the large reptile tree (LRT, 1293 taxa) Rapetosaurus nests with the much taller and longer Diplodocus. Rapetosaurus has a much larger pubis for no better reason than to help support its guts when bipedal.

Figure 2. Rapetosaurus skull compared to other sauropods.

Figure 2. Rapetosaurus skull compared to other sauropods. That long antorbital fenestra on Rapetosaurus appears to be a combination of the maxillary fenestra seen in Tapuiasaurus. Note: every facial bone has less bone in Rapetosaurus.

The down-turned snouts here
reflect their angle relative to the occiput and probably the semi-circular canals.

References
Curry Rogers K and Forster CA 2001. The last of the dinosaur titans: a new sauropod from Madagascar. Nature. 412: 530–534. doi:10.1038/35087566

https://en.wikipedia.org/wiki/Rapetosaurus

The most basal mammal in the LRT: Megazostrodon

I thought for many years
that Megazostrodon was known from only a fragment of skull, lacking both the anterior and posterior parts.

Then somehow this paper popped up on the Internet
Gow 1986 illustrated the skull of Megazostrodon (Fig. 1; BPI/1/4983; Crompton & Jenkins, 1968; Latest Triassic; 200 mya). Even without this skull data the large reptile tree (LRT, 1293 taxa) nested Megazostrodon at the base of the Mammalia. There is little  argument among paleontologists that this taxon is a close sister to the last common ancestor of all living mammals.

Often wrongly associated
with Morganucodon, the two are phylogenetically separated from one another by tiny Hadrocodium in the LRT. In Megazostrodon the zygomatic arch is straight (without the ascending arch). The skull lacks a sagittal crest.  As in modern marsupials, carnivores, primates and tree shrews the teeth have a standard incisor, canine, premolar and molar appearance. The permanent molars occlude precisely. Uniquely (as far as I know), the dentary has a coronoid boss and a coronoid process.

Figure 1. Megazostrodon skull in several views. Drawings from Gow 1986. Colors applied here.

Figure 1. Megazostrodon skull in several views. Drawings from Gow 1986. Colors applied here. The upper molars are worn down.

The large reptile tree
(Fig. 2) presents a simple, validated topology of mammals and their ancestors based on hundreds of traits, very few of them dental. It differs in nearly every regard from the Close et al. 2015 study, which employs many dental taxa.

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 3. 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.

The first time I reconstructed Megazostrodon
(Fig. 4) the skull looked legit, and was approved by cynodont expert Jim Hopson, but it had some problems. I’m glad to finally get better data on this, that resolves scoring problems around this node.

Figure 1. Megazostrodon, an early mammal, along with Hadrocodium, a Jurassic tiny mammal.

Figure 4. Megazostrodon, an a Jurassic mammal, along with Hadrocodium, a Jurassic tiny mammal. The Megazostrodon skull shown here is not correct.

On a side note:
Wikipedia reports,Tinodon (Marsh 1887; YMP11843) is an extinct genus of Late Jurassic mammal from the Morrison Formation. It is of uncertain affinities, being most recently recovered as closer to therians than eutriconodonts but less so than allotherians.” 

Figure 1. Tinodon is best represented by an incomplete mandible with affinities to basal mammals.

Figure 5. Tinodon is best represented by an incomplete mandible with affinities to basal mammals and basal metatherians. Image from Morphobank.

Too few characters are present here
to add it to the large reptile tree, but if I have restored the missing parts correctly, then it is close to the base of the Mammalia and Theria near Megazostrodon.

References
Close RA, Friedman M. Lloyd GT and Benson RBJ 2015. Evidence for a mid-Jurassic adaptive radiation in mammals. Current Biology. 25 (16): 2137–2142. doi:10.1016/j.cub.2015.06.047PMID 26190074.
Crompton AW and Jenkins FA Jr 1968. Molar occlusion in late Triassic mammals, Biological Review, 43 1968:427-458.
Gow CE 1986. A new skull of Megazostrodon ( Mammalia, Triconodonta) from the Elliot Formation (Lower Jurassic) of Southern Africa. Palaeontologia Africana 26(2):13–22.
Marsh OC 1887. American Jurassic mammals. The American Journal of Science, series 3 33(196):327-348

wiki/Megazostrodon

 

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. Fruitafossor is an edentate. 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
Li J-L, Wang Y, Wang Y-Q and Li C-K 2001. A new family of primitive mammal from the Mesozoic of western Liaoning, China. Chinese Science Bulletin 46(9):782-785.
Meng J, Wang Y-Q and Li C-K 2011. Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. Nature 472 (7342): 181–185.
Trofimov BA 1978. The first triconodonts (Mammalia, Triconodonta) from Mongolia. Doklady Akademii Nauk SSSR. 243 (1): 213–216.

wiki/Repenomamus
wiki/Gobiconodon