Hollow-cheeked Euchambersia nests alongside puffy-cheeked Charassognathus

Unique among synapsids, Euchambersia
(Broom 1931, Benoit et al. 2017; Fig. 1) had an antorbital fenestra (= maxillary fenestra and fossa, Fig. 1) that may have housed a venom gland posterior to the canine root.

Reported by Brian Switek in Scientific American online,
“Because of the uniqueness of its skull anatomy,” Benoit and coauthors conclude, “Euchambersia mirabilis is and will remain a puzzling species.”

The ability to be unique in a world of gradual accumulations of derived traits 
made this taxon interesting. I wondered, which taxon did Euchambersia nest alongside? And did that taxon have anything like the antorbital fenestra found in Euchambersia?

The two answers are 1) Charassognathus and 2) yes.

Figure 1. Euchambersia skull with colors and shifting bones added.

Turns out Euchambersia was not unique among synapsids
for reasons stated above because its sister in the Therapsid Skull Tree (TST, 75 taxa) Charassognathus (Fig. 2) has a skull bulge posterior to the canine root.

Figure 4. Charassognathus (SAM-PK-K10369) does not share more traits with Abdalodon than other taxa, like Bauria and Promoschorhynchops in the TST. Note the bulge posterior to the canine root.

According to Wikipedia, citing Botha, Abdala and Smith 2007
Charassognathus is a basal cynodont.

By contrast, in the TST, Charassognathus is a cynodont-mimic nesting with therocephalians. Given the state of taphonomy documented in Euchambersia, the possibility that the unique maxillary fenestra was in life covered by a thin bulge of bone, as in Charassognathus, should be considered a possibility.

Wikipedia notes,
“Charassognathus has a snout that makes up slightly less than half of the total length of its skull and a long facial process on its septomaxilla. Other than these two features its skull is that of a typical cynodont. The odd shape of its septomaxilla is more typical of therocephalians than other cynodonts indicating that it may be close to a common ancestor between the two groups.”

The same is true of Euchambersia.

Figure 4. Therapid Skull Tree with the addition of Euchambersia and Charassognathus apart from cynodonts.

Nomenclature tidbit.
According to Wikipedia, “Broom named the genus Euchambersia, which he considered “the most remarkable therocephalian ever discovered”, after the eminent Scottish publisher and evolutionary thinker Robert Chambers, whose Vestiges of the Natural History of Creation was considered by Broom to be “a very remarkable work” though “sneered at by many.”

Chambers was probably happy to get the honor and compliment from Dr. Broom, while others sneered.

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References|
Benoit J, Norton LA, Manger PR and Rubidge BS 2017. Reappraisal of the envenoming capacity of Euchambersia mirabilis (Therapsida, Therocephalia) using μCT-scanning techniques. PLoS ONE 12(2): e0172047. doi:10.1371/journal.pone.0172047
Botha J, Abdala F and Smith R 2007. The oldest cynodont: new clues on the origin and diversification of the Cynodontia. Zoological Journal of the Linnean Society. 149: 477–492.
Broom R 1931. Notices of some new Genera and species of Karroo Fossil Reptiles. Rec Albany Mus. 1931; 41: 161–166.

It’s not often that all the references fall within the range of one letter. The odds against that are approximately one in 26 cubed or 17.576.

https://blogs.scientificamerican.com/laelaps/did-this-protomammal-have-a-venomous-bite/

wiki/Euchambersia
wiki/Charassognathus
wiki/Akidnognathidae

Lobalopex: Finally another therapsid enters the TST!

It’s been awhile
since the last therapsid was traced and scored. Today two taxa enter the TST.

Figure 1. Cladogram of ten taxa employed by Sidor et al. 2004.

Sidor, Hopson and Keyser 2004
introduced a new ‘biarmosuchian’ and ‘burnetiamorph” therapsid from the Permian of South Africa. They named their new find Lobalopex mordax (CGP/1/61, Fig. 2, skull length 15cm). They reported, “a cladistic analysis including ten biarmosuchian taxa indicates that Lobalopex is the sister taxon to Burnetiidae and that Lemurosaurus is the most primitive burnetiamorph.” (Fig. 1).

After testing
in the Therapsid Skull Tree (= TST, 73 taxa, Fig. 3) Lobalopex nested  as a member of the Burnetia clade, matching the nesting of Sidor, Hopsona and Keyser 2004.

Figure 2. Lobalopex added to previous nested burnetiidae. It has a longer skull than others in this clade. Look to Herpetoskylax to imagine an uncrushed Lobalopex skull.

Sidor et al.
note the dorsal skull is crushed (it looks melted!) dorsoventrally. The rest of the skull is not crushed. Photos of the original material have not been published. Look to Herpetoskylax (Fig. 2) to imagine an uncrushed Lobalopex skull (Fig. 2).

A tiny horn boss
is present on the rostrum of Lobalopex. That horn boss gets bigger in Proburnetia and Burnetia (Fig. 2), but skips Lemurosaurus. The ventral portion of the Lobalopex retroarticular process wa lost during collection. The supratemporals (bright green) are fused to underlying bones. Lobalpex has a longer rostrum, relative to orbit length, than others in its clade, convergent with Eotitanosuchus.

Strangely,
little to no post-cranial material is known from this clade. Was it an herbivore or carnivore?

Figure 3. TST with the addition of Lobalopex nesting in the Burnetia clade.

Lobalopex is derived from Lemurosaurus
in Sidor, Hopson and Keyser (Fig. 1), but the other way around in the TST (Fig. 3). Ictidorhinus (Fig. 2) is the outgroup in both cladograms. Herpetoskylax was not mentioned by Sidor, Hopson and Keyser 2004 because it was published later, not until 2006.

Figure 4. More recent cladograms that include Herpetoskylax and Lobalopex.

More recent cladograms 
that include Herpetoskylax and Lobalopex (Kruger et al. 2015, Kammerer 2016; Fig. 4) presume Biarmosuchus is the outgroup taxon. By contrast in the TST (Fig. 3) Biarmosuchus is the outgroup to more derived therapsid taxa, as is Rubidgina (Fig.5), which also enters the TST (Fig. 3) today, nesting basal to gorgonopsids and therocephalians + cynodonts + mammals.

Figure 5. Rubidgina skull in 4 views. Note the wide cheeks rotating the orbits anteriorly. This taxon is not basal to the Burnetia clade in the TST.

With Rubidgina added to the deep time lineage of humans
it would not be too far off the mark to say, the deep time ancestors of Little Red Riding Hood once looked quite a bit like the big bad wolf. How ironic. ‘Grandma’ really did have such big teeth!

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References
Sidor CA, Hopson JA and Keyser AW 2004. A new burnetiamorph therapsid from the Teekloof Formation, Permian, of South Africa. Journal of Vertebrate Paleontology 24(4):938–950.

wiki/Lobalopex

Albian South Korean tracks do not match Monjurosuchus

Lee et al. 2020 describe
“a new quadrupedal trackway found in the Lower Cretaceous Daegu Formation (Albian) in the vicinity of Ulsan Metropolitan City, South Korea, in 2018. A total of nine manus-pes imprints show a strong heteropodous quadrupedal trackway (length ratio is 1:3.36). Both manus and pes tracks are pentadactyl with claw marks. The manus prints rotate distinctly outward while the pes prints are nearly parallel to the direction of travel. The functional axis in manus and pes imprints suggests that the trackmaker moved along the medial side during the stroke progressions (entaxonic), indicating weight support on the inner side of the limbs. There is an indication of webbing between the pedal digits. These new tracks are assigned to Novapes ulsanensis, n. ichnogen., n. ichnosp., which are well-matched not only with foot skeletons and body size of Monjurosuchus but also the fossil record of choristoderes in East Asia, thereby N. ulsanensis could be made by a monjurosuchid-like choristoderan and represent the first possible choristoderan trackway from Asia.”

Not sure why they say they have a “well-matched”
foot skeleton and body size for Monjurosuchus. That does not appear to be true (Fig. 1). Other coeval mammal-mimic trackmakers, like Repenomamus, appear to match better (Fig. 1).

Figure 1. Novapes tracks from Lee et al. 2020 matched to little Monjurosuchus (lower left) and Repenomamus (upper right) and Repenomamus overall. Croc tracks are similar but the pes lacks digit 5.

Images provided by Lee et al.
indicate digits of nearly equal length on both manus and pes. Unfortunately the choristoderan, Monjurosuchus (Fig. 1) is too small and digit 4 on both manus and pes the longest on a sprawling (not erect) hind limb. Not a good match.

A better match can be found
in the mammal-mimic Repenomamus. It is the correct size, shape and coeval with the trackmaker of Novapes. Repenomamus is not mentioned by Lee et al. 2020. A Repenomamus relative, Liaoconodon, better preserves the extremities, but the manus and pes are similar in size.

Repenomamus and Liaoconodon are found in
the nearby Yixian Formation, NE China, Albian, late Early Cretaceous, 125 mya. Novapes is also from the Albian, late Early Cretaceous, nearby in South Korea.

Novapes diagnosis from Lee et al. 2020:
Monjurosuchus (M: yes, no); Repenomamus (R: yes, no)

1. Quadrupedal tracks with a pronounced heteropody; (M no; R yes)
2. Pentadactyl manus impression with claw marks and semi-symmetrical outline (M yes; R yes)
3. Manus wider than longer (M no; R yes)
4. Divergence between digit I and V imprints ranges 180° to 210°; (M no; R yes)
5. Digit IV imprint slightly longer than digit II; (M yes; R yes)
6. Entaxonic manus (medial digits more robust than lateral digits); (M no; R no; Novapes no)
7. Pentadactyl pes impression with claw marks and asymmetrical outline (i.e., lateral digits are more developed) (M yes; R yes)
8. Longer than wide; (M yes; R yes)
9. Webbing between the proximal portion of slender digits; (M ?; R?)
10. The subequal digits III and IV imprints longer than others (M 4>3; R 4=3)
11. Digit I imprint only 30% in length of the digit IV imprint); (M yes; R yes)
12. The sole pad impression is elongate with a U-shaped “heel”; (M no; R yes)
13. Entaxonic pes (M no; R no; Novapes no)

Ichnites are sometimes difficult to match to trackmakers, 
but some trackmakers can be eliminated. The possibility of a mammal-mimic trackmaker, like Repenomamus, should not be omitted from consideration.

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References
Lee Y-N, Kong D-Y and Jung SH 2020. The first possible choristoderan trackway from the Lower Cretaceous Daegu Formation of South Korea and its implications on choristoderan locomotion. Nature Scientific Reports 10:14442 https://doi.org/10.1038/s41598-020-71384-1

Asaphestera: the earliest amniote? …No

Summary if you’re in a rush:
Mann et al. 2020 mistakenly reassessed their ‘microsaur’, Asaphestera platyris (Fig. 1), as at ‘the earliest synapsid’. The LRT nests this taxon as a microsaur after demonstrating interpretation and reconstruction errors.

Mann et al. 2020 bring us
their view of ‘microsaurs’ from Joggins, Nova Scotia (Westphalian, Late Carboniferous) with the recognition that “Asaphestera platyris as a synapsid provides the earliest unambiguous evidence of ‘mammal-like reptiles’ in the fossil record.”

Unambiguous?
No. Just because they say so, does not mean it is true.

By contrast (1):
In the large reptile tree (LRT, 1685+ taxa) the earliest amniote (determined by the last common ancestor method) is Silvanerpeton, from the Viséan (Early Carboniferous) at least 15 million years earlier. Gephyrostegus is more primitive in the LRT, but appears as a late survivor in the Westphalian (Late Carboniferous, coeval with Asaphestera platyris) of an earlier radiation. Archaeothyris, another slightly younger Westphalian taxon, was widely considered the earliest known synapsid, and remains so in the LRT. These three taxa are not mentioned in the Mann et al. text.

Figure 1. Asapehestera platyris in situ, traced by Mann et al. 2020, then traced and reconstructed using DGS methods. There is no tall dorsal process to the maxilla, contra Mann et al. The ‘palbebral’ (PB) is below several loose dentary teeth, so it is a palate or mandible element. The ‘dorsal process’ of the maxilla is not represented by bone. The reconstruction nearly matches Kirktonecta (figure 2).

By contrast (2): 
When added to the LRT, ‘Asaphestera platyris’ (RM 2.1192, Steen 1934) nests with and is not much different from the microsaur, Kirktonecta (Fig. 2), far from any amniotes or synapsids. Kirktonecta is mentioned only once in the Mann et al text as part of a list that “do not fit clearly into this [microbrachomorph] framework.”

Figure 2. Kirktonecta is a Viséan taxon nesting with Asaphestera platyris in the LRT.

From the abstract:
‘‘Microsaurs’ are traditionally considered to be lepospondyl non-amniotes, but recent analyses have recovered a subset of ‘microsaurs’, the fossorially adapted Recumbirostra, within Amniota.”

The LRT does not support this nesting.

Recumbirostra = pantylids, gymnarthrids, brachystelechids, ostodolepids, and rhynchonkids. In the LRT all these taxa are in the clade Microsauria, a sister clade to the Reptilomorpha. Kirktonecta is basal to the microsaur clade that ultimately produced the extant caecilian, Dermophis.

From the Mann et al. description:
“Most of the right maxilla and portions of both temporal regions are known only from impressions of the bones that have weathered away; nevertheless, valuable information is present in what remains. Parts of the dorsal margins of both temporal fenestrae are preserved on either side of the cranium, but the morphology is more completely represented on the right side.”

A reconstruction (Fig. 1) based on the same specimen does not support this description. There is no tall dorsal process to the maxilla, contra Mann et al. The ‘palbebral’ (PB) is below several loose dentary teeth, so it is a palate or mandible element. The ‘dorsal process’ of the maxilla is not represented by bone. The reconstruction nearly matches Kirktonecta (Fig. 2).

From the text:
“As a result, we tentatively attribute RM 2.1192 (Fig. 1) to the Eothyrididae. If this identification is correct, RM 2.1192 would extend the record of eothyridids substantially.”

Co-author, B Gee,
writing on his blogpost (link below) reported, “Among synapsids, this specimen most closely resembles the eothyridids, although it shares a number of features with acleistorhinid parareptiles, which were often confused for eothyridids in their earlier history of study (perhaps they still are eothyridids?).”

In the LRT, even eothyrids are not synapsids. They are basal caseasauria derived from the lepidosauromorph, Milleretta.

In summary:
Mann et al. 2020 mistakenly reassessed their microsaur, Asaphestera platyris, as a synapsid. The LRT nests it as a microsaur close to Kirktonecta, a taxon essentially overlooked by the authors. Nearly coeval Archaeothyris remains the earliest known synapsid, but several synapsids are more primitive, indicating an earlier radiation. So, they’re out there somewhere! Mann et al. did not find them…yet.

Postscript:
A reader (J) wondered how I was able to reconstruct Kirktonecta if, given the limitations provided by another reader (DM) that only the inside of the skull bones were visible. Here (Fig. 3) I show the method and the data, a crushed skull in which the bones are slightly separated along their sutures and sometimes split during taphonomic crushing. I traced the skull bones of Kirktonecta, then reassembled them using the DGS method (color tracing using Photoshop). The first step was to invert the colors (creating a negative) of the original image, something a paleontologist with firsthand access to the specimen would be unable to do without repeating this method. The original image had higher resolution, reduced here for online publication. Apparently the insides were little different from the outsides given the two-dimensional, plate-like shapes of the skull bones with few-to-no complex curves in the bones of this taxon. I leave it to the reader to decide whether or not the DGS method was successful in this case, whether inside or not.

Figure 3. Kirktonecta in situ and traced using the DGS method.

 

References
Mann A et al. (7 co-authors) 2020. Reassessment of historic ‘microsaurs’ from Joggins, Nova Scotia, reveals hidden diversity in the earliest amniote ecosystem. Papers in Palaeontology 2020:1–17.
Steen MC 1934. The amphibian fauna from the South Joggins. Nova Scotia. Journal of Zoology, 104, 465–504.

wiki/Kirktonecta
wiki/Asaphestera
wiki/Asaphestera2
wiki/Carboniferous

https://bryangee.weebly.com/blog/new-publication-reassessment-of-historic-microsaurs-from-joggins-nova-scotia-reveals-hidden-diversity-in-the-earliest-amniote-ecosystem-mann-et-al-2020-papers-in-palaeontology

Kenomagnathus: what you can do with only 2 bones

Spindler 2020
reports on a new basal pelycosaur, Kenomagnathus scottae (ROM 43608; Upper Pennsylvanian, Late Carboniferous, Garnett, KS, USA; Figs 1-3) known from a single lacrimal and maxilla (with teeth) exposed in medial view (Fig. 1).

Figure 1. Kenomagnathus in situ from Spindler 2020. The halo of organic matter is interesting.

From the abstract:
“This is the oldest known diastema in synapsid evolution, and the first reported from a faunivorous member that lacks a precanine step, aside from Tetraceratops. This unique precanine morphology occurred independently from similar structures in Sphenacodontoidea.” 

See Spindler’s freehand drawing
of the ‘true diastema’ (Fig. 2). 

Figure 2. Kenomagnathus maxilla and lacrimal with the rest of the skull restored in lateral view. Note the deeper jugal (cyan), though not as deep as in Ophiacodon (Figs. 3, 4). For that reason the mandible of Ophiacodon was used in this restoration. Spindler’s freehand drawing indicates a deeper orbit, shallower jugal and smaller naris along with a larger mandible.

It is worth noting
that maxillary teeth shrink toward the naris in Ophiacodon (Fig. 3). A diastema may be present in Pantelosaurus (formerly Haptodus saxonicus, Fig.3). These pertinent taxa were not illustrated in Spindler 2020.

Figure 3. Pertinent synapsid skulls to scale. The origin of the Pelycosauria + Therapsida is marked by phylogenetic miniaturization, as in so many other clade origins. Note the depth of the jugal in basal taxa here.

Spindler’s freehand restoration
increased the size of the orbit and decreased the depth of the restored jugal. So this is yet another cautionary tale highlighting the danger in using freehand drawings in scientific studies.

The shallow jugal depth in the Spindler freehand restoration
is a key oversight. When repaired (Fig. 2) the semi-deep jugal of Kenomagnathus transitionally links deeper jugal Ophiacodon (Fig. 3) to shallower jugal Pantelosaurus and Haptodus (Fig. 3) at the base of Pelycosauria + Therapsida in the large reptile tree (LRT, 1642+ taxa). While running the risk of ‘Pulling a Larry Martin’, there are so few traits to consider here (Fig. 1) and none contradict the present hypothesis of interrelationships. All that puts Kenomagnathus in the lineage of synapsids leading to therapsids, mammals, primates and humans.

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References
Spindler F 2020. A faunivorous early sphenacodontian synapsid with a diastema. Palaeontologia Electronic 23(1):a01. doi: https://doi.org/10.26879/1023
https://palaeo-electronica.org/content/2020/2905-early-sphenacodontian-diastema

Styracocephalus x2 enters the TST

Fraser-King et al. 2019
bring us new data on Styracocephalus (Fig. 3), a purported dinocephalian therapsid from Late Permian South Africa. Unfortunately the Fraser-King et al. phylogenetic analysis (Fig. 1) excludes relevant taxa (like Phthinosuchus) and includes one unrelated taxon, Tetraceratops. The authority for this criticism is a larger study, the therapsid skull tree (TST, 72 taxa, subset Fig. 2) a side branch of the large reptile tree (LRT, 1579 taxa). It includes the relevant taxa in the Fraser-King et al. study, and many more excluded from Fraser-King et al.

Figure 1. Cladogram from Fraser-King et al. 2019. Compare to figure 2 where many more taxa are included. Tetraceratops is not related to any therapsid, but nests closer to Limnoscelis.

FIgure 2. TST with the addition of two specimens attributed to Styracocephalus and Raranimus. Compare to fewer taxa in figure 1. Here Styracocephalus is not related to tapinocephalids. The TST is fully resolved.

Both the holotype of Styracocephalus
and the new referred specimen nest together in the LRT despite their many morphological differences (Fig. 3). Even so, I think the differences are strong enough to erect a new genus for the new specimen. The two nest with the Phthinosuchus clade in the LRT, a taxon not included in the Fraser-King et al. study.

Figure 3. At left, the holotype of Sclerocephalus SAM PK 8936. At right the distinctly different referred specimen to scale BPI I 7141.

So, in this case,
I’m a splitter, not a lumper. And I wish Fraser-King et al. had included a few more taxa.

PS
Raranimus also entered the TST because Fraser-King included that taxon. Raranimus nested with Ictidorhinus, and ironically, could be congeneric given how little is known of Raranimus.

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References
Fraser-King S, Benoit J, Day MO and Rubidge BS 2019. Cranial morphology and phylogenetic relationship of the enigmatic dinocephalian Styracocephalus platyrhynchus from the Karoo Supergroup, South Africa. Palaeontologia africana 54: 14–29.

 

Pseudotherium enters the LRT

From the Wallace et al. 2019 abstract:
“We describe a new probainognathian cynodont, Pseudotherium argentinus, from the early Late Triassic Ischigualasto Formation of Argentina.”

Figure 1. Pseudotherium from Wallace, Martinez and Rowe 2019, eyeball, premaxilla and mandible from sister Haldanodon added here. Isn’t nice to see all the bones colored? Isn’t that helpful! Note the gave the squamosal and jugal the same color. Best not to do that. 

In the large reptile tree (LRT, 1559 taxa) Pseudotherium (Fig. 1) nests between Haldanodon and Pachygenelus, pretty close to where Wallace et al. nest their new taxon (Fig. 2). Haldanodon is not mentioned in their text. All are descendants of probainognathian cynodonts, so they nailed it! As noted by the authors, the 3D skull of Pseudotherium adds greatly to the dataset of a typically crushed bunch of sister taxa.

Figure 2. Cladogram from Wallasc, Martinez and Rowe 2019. Pink arrow added where LRT nests Pseudotherium. Try to make sure that all basal taxa are to the left (above here). Adding taxa puts Brasilitherium to the Mammalia and Pseudotherium closer to the missing Haldanodon.

Wallace et al. report,
“Our analysis found weak support for Pseudotherium as the sister taxon of Tritylodontidae.”

Not true according to the LRT. More basal in the LRT.

Wallace et al. also report,
“Thus, Pseudotherium may lie just inside or just outside of Mammaliamorpha, and there is also weak character support for its sister taxon relationship with Brasilitherium.”

Not true according to the LRT where Brasilitherium lies in the ancestry of the platypus, Ornithorhynchus, within the Prototheria, within the Mammalia. Pseudotherium nests basal to Pachygenelus in the LRT.

FIgure x. From the Don Prothero lab. They don’t use the LRT which works and we know why: taxon inclusion. 

This was an excellent paper, with lots of details.
Adding a few more taxa would have helped, but just a little.

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References
Wallace RVS, Martinez R and Rowe T 2019. First record of a basal mammailiamorph form the early Late Triassic ischigualasto Formation of Argentina. PLoS ONE 14(8): e0218791. https://doi.org/10.1371/journal.pone.0218791

Microdocodon: If those are hyoids, then where are the fingers?

A new mammaliaform, Microdocodon,
(Zhou et al. 2019; Figs. 1–4; Middle Jurassic, 165 mya) is exceptionally well preserved and complete, down to the smallest details. According to the authors, those details include “complex and saddle-shaped hyoid bones (Fig. 1), like those seen in modern mammals.”

Figure 1. From Zhou et al. 2019, colors added. Microdocodon is in yellow. The two taxa in gray are derived members of Glires and do not nest in the LRT where shown here. It is obvious from looking at this evolutionary progression that the two highly derived gnawing taxa do not document a gradual accumulation of derived traits, like the remaining plesiomorphic taxa do.

Timing?
Microdocodon was found in strata 40 million years into the Jurassic, some 40 million years after the appearance of the first mammal, Megazostrodon in the large reptile tree (LRT, 1545 taxa). Pre-mammal cynodonts lived alongside mammals throughout the Mesozoic.

H-shaped, articulated hyoids were unexpected in such a primitive cynodont
and a dozen news organizations picked up on the unexpectedness of this story. If valid this would suggest that a muscularized throat was present phylogenetically before the genesis of the milk-suckling clade, Mammalia.

Figure 2. Microdocodon throat region. Are those bones hyoids or fingers? If hyoids, then where are the fingers? Note the displaced radius (olive green)  reaching toward the throat. Only impressions of once present (or still buried) fingers are present on the right limb.

Unfortunately,
there may be reason to doubt the identity of these bones. Are they hyoids? Or fingers? If the mystery bones are indeed hyoids, then the fingers are missing. If fingers, then the hyoids are missing, which takes all the surprise and wonder out of the Zhou et al. paper.

FIgure 3. Microdocodon in situ. Plate and counter plate plus colors added. Manus, pelvis and pes reconstructed. The recombining of plate and counter plate is something that does not work as well in print.

From the abstract
“We report a new Jurassic docodontan mammaliaform found in China that is preserved with the hyoid bones. Its basihyal, ceratohyal, epihyal, and thyrohyal bones have mobile joints and are arranged in a saddle-shaped configuration, as in the mobile linkage of the hyoid apparatus of extant mammals. These are fundamentally different from the simple hyoid rods of nonmammaliaform cynodonts, which were likely associated with a wide, nonmuscularized throat, as seen in extant reptiles. The hyoid apparatus provides a framework for the larynx and for the constricted, muscularized esophagus, crucial for transport and powered swallowing of the masticated food and liquid in extant mammals. These derived structural components of hyoids evolved among early diverging mammaliaforms, before the disconnection of the middle ear from the mandible in crown mammals.”

The big question is:
If those are indeed hyoids, then where are the fingers? EVERYTHING else is present and visible on this perfectly preserved fossil, except, apparently, the fingers of both hands.

Further complication:
I looked closely at the purported hyoids and found they

1. included unguals
2. began at the wrist
3. were articulated like fingers
4. had all the proportions and correct number expected in a typical manus from that node on the LRT (Fig. 5).

Often enough,
when bones you expect are missing AND similar bones you don’t expect are present, you should suspect that a misidentification is taking place.

Figure 4. Microdocodon skull, plate and counter plate, colors added.

After phylogenetic analysis
Microdocodon nests at the base of the Tritylodontidae (Oligokyphus and kin) + (Riograndia + Chaliminia) clade. These are therapsids retaining a primitive quadrate/articular jaw joint, not like a mammal with a squamosal/dentary jaw joint.

At this point it is probably good to remember
that the most primitive mammals do not suckle. Prototherians, like echidnas and platypuses lick their mothers milk from sweat puddles on her belly. Only metatherians and eutherians have infants that suckle on their mothers’ teats, which is several nodes up the ladder from Microdocodon.

A docodont?
The authors considered Microdocodon a small member of the Docodonta, a clade traditionally defined by dental and mandible traits. Unfortunately, Microdocodon does not nest in the LRT with other clade members listed on the Wikipedia page. As we’ve seen many times, dental traits can converge.

The phylogenetic analysis of Zhou et al. employs “tritylodontids” as a suprageneric taxon nesting outside of Pachygenelus, (the opposite of the LRT) derived from Thrinaxodon and Massetognathus. To their peril, Zhou et al. include a long list of multituberculates, but no carpolestid and plesiadapid sister taxa recovered by the LRT. So taxon exclusion is a problem as highly derived multituberculates arise in Zhou et al. prior to primitive prototherians (Fig. 1). Also mis-nested in the Zhou et al analysis, the early and basal metatherian, Eomaia and the basal prototherian, Juramaia, nest as derived eutherians. These are all red flags, probably arrived at by an over-reliance on dental traits and the most typical problem in vertebrate paleontology: taxon exclusion. The LRT minimizes taxon exclusion because it tests such a wide gamut of taxa.

Figure 5. Microdocodon pectoral and forelimb reconstruction from DGS traced elements. Those fingers were originally considered hyoid elements. Yes, those are elongate coracoids, typically found in members of the Tritylodontidae.

But wait! All is not lost.
Microdocodon fills an important gap leading to the Tritylodontidae in the LRT. So it can still be exciting and newsworthy for this overlooked reason.

The pre-mammal/pre-tritylodontid split occurred
by the Middle Triassic, which gives Middle Jurassic Microdocodon plenty of time to evolve distinct traits. And it did. The snout is longer than typical. The medial metatarsals were atypically longer than the others. Tiny phalanges 3.2, 4.2, 4.3 and 5.2 reappear after disappearing several nodes earlier. That bit of atavism is interesting. The limbs are long and gracile with reduced interoseal space between the crural and ante brachial elements, mimicking/converging on more derived mammals.

Figure 6. Subset of the LRT focusing on basal Therapsida and Microdocodon’s nesting in it.

The authors report,
“Phylogenetically, Microdocodon and [coeval] Vilevolodon are the earliest-known mammaliaform fossils with mammal-like hyoids.” Vilevolodon is a highly derived, squirrel-like member of the clade Multituberculata within the rodent/rabbit clade of Glires within the Eutheria in the LRT.

Articulated hyoids
are exceptionally rare in the early fossil record of mammals. So are basal mammals.

Everyone is looking for a headline with every new fossil specimen.
Unfortunately, as we’ve seen time and again, you can’t believe everything you read, even after PhD peer review and publication in Nature and Science. Make sure you test all novel hypotheses with careful observation and a wide gamut phylogenetic analysis.

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References
Zhou C-F, Bullar B-A S, Neander AI, Martin T and Luo Z-X 2019. New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones. Science 365(6450):276–279.

https://www.sciencenews.org/article/flexible-bone-helps-mammals-chew-dates-back-jurassic-period

https://www.sciencedaily.com/releases/2019/07/190718140440.htm

For a dozen more popular articles: Google keyword: Microdocon.

 

Therocephalians evolved to smaller size? Large Carnivora did not?

Brocklehurst 2019 reports,
“If these results are reliable, they support the traditional paradigm that therocephalians originated as large predators, and only later evolved small body sizes. The patterns observed in mammals do not appear to apply to therocephalians. Mammalian carnivores, once they have reached large size and a specialized bauplan, are apparently unable to leave this adaptive peak. Therocephalians, on the other hand, retreated from the hypercarnivore niche and evolved small sizes later in the Permian.”

Figure 1. Cladogram from Brocklehurt 2019, colors added. Lycosuchus, listed as a basal therocephalian by Brocklehurst, also nests close to cynodonts in the TST. No gorgonopsids are shown here. Biarmosuchus is the outgroup taxon here, a more distant outgroup taxon in the TST.

Brocklehurst’s cladogram
posits that Therocephalia and Cynodontia arose as sisters from a last common ancestor: Biarmosuchus. In the therapsid skull tree (TST, 67 taxa, Fig. 4), Therocephalia (including Cynodontia) arises from Gorgonopsia (Fig. 2).

Figure 2. Gorgonopsids, therocephalians and cynodonts to scale.

The question arises,
what is a ‘large size’ member of the Carnivora? Certainly big cats and walruses (Fig. 3) fall into this definition and do not give rise to smaller ancestors, as Brocklehurst notes. However, if the basalmost member of the Carnivora, Vulpavus, is considered ‘large’ then it breaks the ‘rule’ because it has smaller descendants in the LRT: Mustela and Procyon (Fig. 3). Talpa, the mole, is the smallest member of the Carnivora in the LRT. Talpa has been traditionally omitted from Carnivora studies while being wrongly lumped with the unrelated shrew, Scutisorex, instead.

Figure 3. Carnivora to scale. Note: one branch does increase in size over time (ignoring toy poodles for the moment), while another branch, the one leading to Talpa the mole, shrinks in size. Brocklehurst is correct: once carnivores achieved large size, few to no examples of phylogenetic miniaturization appear in the fossil record.

I wish Brocklehurst 2019 had added
a few sample reconstructions to scale to help readers visualize the size ranges that he found in his cladogram. After all, the subject was ‘size’. I was unfamiliar with the vast majority of therocephalian taxa in his cladogram (Fig. 1).

Figure 4. TST revised with new data on Patranomodon and sister taxa. Here the therocephalian, Bauria, nests closer to cynodonts than in Brocklehurst 2019 (Fig. 1).

Brocklehurst is correct:
once carnivores achieved large size (Fig. 3), no examples of phylogenetic miniaturization subsequently appear. Brocklehurst contrasted this with therocephalians, presuming that Lycosuchus (Fig. 2) was a basal therocephalian, rather than a basal cynodont by definition.

Remember:
Hopson and Kitching 2001 defined  Cynodontia as the most inclusive group containing Mammalia, but excluding Bauria. In the TST (Fig. 4) Abdalodon and Lycosuchus nest on the cynodont side of Bauria.

In the TST
(Fig. 4), cynodonts show no strong size trends until mammals, like Megazostrodon (Fig. 2), evolved tiny sizes. Therocephalians likewise show no strong size trends either (but then, I have not measured every taxon in the Brocklehurt cladogram, Fig. 1). Those that also appear in the TST are in white boxes, and they appear in several clades within Therocephalia.

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References
Brocklehurst N 2019. Morphological evolution in therocephalians breaks the hyper carnivore ratchet. Proceedings of the Royal Society B 286: 20190590. http://dx.doi.org/10.1098/rspb.2019.0590

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.

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.

So that makes Lycosuchus a cynodont,
by definition.

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.

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).

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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