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.

---

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.

 

A juvenile Anteosaurus? No.

Kruger et al. 2017
reported on a newly discovered ‘juvenile Anteosaurus‘ skull BP/1/7074 (Figs. 1,2). This was also the subject of Kruger’s 2014 Masters thesis.

Unfortunately
in the therapsid skull tree, BP/1/7074 did not nest with Anteosaurus, but with Austraolosyodon (Figs. 1,2). Neither Kruger nor Kruger et al. presented a phylogenetic analysis.

So let’s talk about
this discrepancy and the importance of phylogenetic analysis. We’re long past the age of ‘eyeballing’ taxa.

Figure 1. The purported juvenile Anteosaurus skull, BP/1/7074 compared to he coeval Australosyodon. DGS colors have been applied to the bones of BP/1/7074.

From the 2017 abstract
“A newly discovered skull of Anteosaurus magnificus from the Abrahamskraal Formation is unique among specimens of this taxon in having most of the individual cranial bones disarticulated, permitting accurate delimitation of cranial sutures for the first time. The relatively large orbits and unfused nature of the cranial sutures suggest juvenile status for the specimen. Positive allometry for four of the measurements suggests rapid growth in the temporal region, and a significant difference in the development of the postorbital bar and suborbital bar between juveniles and adults. Pachyostosis was an important process in the cranial ontogeny of Anteosaurus, significantly modifying the skull roof of adults.”

Without a phylogenetic analysis,
it is not wise to assume you have a juvenile of any taxon, especially if you describe it as unlike the adult due to allometry when allometric growth has not been shown in related taxa. All of what Kruger et al. said about pachyostosis may be true, but it awaits a real juvenile Anteosaurus skull to present as evidence. Kruger et al. cited these:

Kammerer et al. 2011 reported that that Stenocybus acidentatus (IGCAGS V 361, Middle Permian, Cheng and Li 1997) is a juvenile Sinophoneus. Phylogenetic analysis nested that smaller skull lower on the therapsid tree.

Liu et al. 2013 thought they had found several short-faced juvenile Sinophoneus skulls. Phylogenetic analysis nested those smaller skulls lower on the the therapsid tree.

Figure 2. Kruger et al. 2017 figure 21. provided “Ontogenetic changes in the skull of Anteosaurus; A. juvenile; B, intermediate sized; C, adult sized, redrawn from Kammerer 2011. Their figure 20 labeled the intermediate sized skull as Titanophoneus. So this is a phylogenetic series, not an ontogenetic one.

 

Misdirection
In Kruger et al. 2017 their figure 21 provided “Ontogenetic changes in the skull of Anteosaurus; A. juvenile; B, intermediate sized; C, adult sized, redrawn from Kammerer 2011” (skulls with colored bones in Fig. 2). However, their figure 20 labeled the intermediate sized skull as Titanophoneus (redrawn from Kammerer 2011), even though it is not a close match to the real Titanophoneus (Fig. 2). So they presented a phylogenetic series, not an ontogenetic one. That intermediate skull is not Anteosaurus and neither is the juvenile.

Given the choice of describing
the first known Anteosaurus juvenile skull or just another Australosyodon skull, Kruger 2014 and Kruger et al. 2017 opted for the former.

Figure 3. From Kruger et al. 2017 the parts of BP/1/7074 colorized to show how the bones were ‘disarticulated.’ This is not disarticulation. This is disassembly of articulated bones.

More misdirection
The abstract describes the bones as ‘unfused’ and therefore juvenile. However the bones did not come out of the ground separate from one another (Fig. 3) and the bones of Syodon are also unfused as an adult. If the bones are indeed juvenile, then they are related to Australosyodon and Syodon, not Anteosaurus.

Statistics, graphs, CT scans and all the high tech data in the world
won’t help you if you don’t have a phylogenetic analysis as your bedrock. You have to know what you have before you can describe it professionally.

From the conclusion
“The ontogenetic series of Anteosaurus magnifies is represented by skull lengths varying from 280 to 805 mm. The most important morphological modifications of the skull are the development of pachyostosis, the positive allometries of the temporal opening, and the postorbital and suborbital bars, which become increasingly robust in adults (Fig. 21). The anterior portion of the snout also grew relatively faster. Adults show proportionally smaller orbits and an increase in the angle between the nasal and the frontal. On the skull roof, the pineal boss increases in height and there is a greater degree of pachyostosis around it. The cranial morphology of juvenile Anteosaurus appears broadly similar to that of the Russian Syodon.”

From the Kruger thesis
“Only two genera of anteosaurs, Australosyodon and Anteosaurus, are recognised from the Karoo rocks of South Africa.” Once again, phylogenetic analysis brings us to a different conclusion. We have to put away our assumptions until the analysis is complete.

We’ve seen before
how the lack of a rigorous large gamut phylogenetic analysis can affect conclusions.

1. Liu et al 2013 and Kammerer2011 (listed above) eyeballed their purported juveniles without a large gamut analysis.
2. Several of Bennett’s papers (listed below) on Pteranodon, Rhamphorhynchus, Pterodactylus and Germanodactylus concluded that specimens were varied due to gender or ontogeny, without testing them phylogenetically.
3. Hone and Benton 2007, 2009 deleted key taxa, introduced typos into the dataset and switched citations to support their contention that pterosaurs were related to erythrosuchid archosauriforms and Cosesaurus was close to Proterosuchus among many other foibles.
4. Ezcurra and Butler 2015 lumped several Proterosuchus/Chasmatosaurus specimens together in an ontogenetic series without testing them phylogenetically.
5. I’m leaving out the many small gamut phylogenetic analyses that suffered from taxon exclusion or inappropriate taxon inclusion that messed up results. Use keyword: ‘taxon exclusion‘ to locate them in this blog.

References
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 1992. Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Bennett SC 1994. Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occassional Papers of the Natural History Museum University of Kansas 169: 1–70.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen limestone of Germany: year classes of a single large species. Journal of Paleontology 69, 569–580.
Bennett  SC (2012) [2013] New information on body size and cranial display structures of Pterodactylus antiquus, with a revision of the genus. Paläontologische Zeitschrift (advance online publication) doi: 10.1007/s12542-012-0159-8
Ezcurra MD and Butler RJ 2015. Post-hatchling cranial ontogeny in the Early Triassic diapsid reptile Proterosuchus fergusi. Journal of Anatomy. Article first published online: 24 APR 2015. DOI: 10.1111/joa.12300
Kammerer CF 2011. Systematics of the Anteosauria (Therapsida: Dinocephalia). Journal of Systematic Palaeontology, 9: 2, 261—304, First published on: 13 December 2010 (iFirst) To link to this Article: DOI: 10.1080/14772019.2010.492645\
Liu J 2013. Osteology, ontogeny, and phylogenetic position of Sinophoneus yumenensis(Therapsida, Dinocephalia) from the Middle Permian Dashankou Fauna of China, Journal of Vertebrate Paleontology, 33:6, 1394-1407, DOI:10.1080/02724634.2013.781505
Kruger A 2014. Ontogeny and cranial morphology of the basal carnivorous dinocephalian, Anteosaurus magnificus from the Tapinocephalus assemblage zone of the South African Karoo. Masters dissertation, University of Wiwatersand, Johannesburg.
Kruger A, Rubidge BS and Abdala F 2017. A juvenile specimen of Anteosaurus magnificus Watson, 1921 (Therapsida: Dinocephalia) from the South African Karoo, and its implications for understanding dinocephalian ontogeny. Journal of Systematic Palaeontology. http://dx.doi.org/10.1080/14772019.2016.1276106
Rubidge BS1994. Australosyodon, the first primitive anteosaurid dinocephalian from the Upper Permian of Gondwana. Palaeontology, 37: 579–594.

Mammal phylogeny employing fossil and extant taxa

Current hypotheses of relationships among mammals
may cause one to develop a large cartoon question mark over one’s cranium. Traditional paleontologists still cannot recover the closest sisters to bats and they continue to suggest that hippos are the closest sisters to known pre-whales with legs.

Unfortunately
very few fossil taxa are employed in these analyses (send them along if you know of any). DNA studies don’t completely match morphological studies and they ignore fossil taxa. Wikipedia sums up the current thinking dividing paleontologists from molecular biologists.

Molecular Biologists recover this tree (highly simplified):

1. Atlantogenata =
   Xenartha (sloth + anteater + armadillo)
   Afrotheria (golden mole + elephant shrew + tenrec + aardvark + hyrax + elephant + sea cow) Afroinsectiphilia (golden mole + elephant shrew + tenrec),
2. Boreoeutheria (Boreotheria) =
   Euarchontoglires (rodent +  rabbit + tree shrew + flying lemur + primate + Plesiadapis)
   Laurasiatheria (hedge hog + mole + shrew + bat + pangolin + carnivores + odd-toed ungulates + even toed ungulates + whales)

Traditional paleontologists recover this tree (highly simplified):

1. Xenarthra (sloth + anteater + armadillo)
2. Afrotheria (hyrax + elephant + sea cow)
3. Boreoeutheria (Boreotheria) =
   Euarchontoglires (rodent +  rabbit + tree shrew + elephant shrews + flying lemur + primate + Plesiadapis)
   Laurasiatheria (hedge hog + mole + shrew + bat + pangolin + carnivores + odd-toed ungulates & aardvark + even toed ungulates & whales)

So there is broad agreement between the two camps.
The problem comes when one tries to replicate the experiment (Fig. 1). Adding mammal taxa to the large reptile tree delivers a different hypothesis of relationships. It preserves some traditional relationships and recovers some new ones. If you’re not sure why this keeps happening here, it happens on the professional level, too. Some prior workers employ suprageneric taxa. Others do not employ fossil taxa. I also wonder if there is substantial convergence in the dental traits of included taxa. Teeth vary greatly within the Mammalia.

The ReptileEvolution.com tree (built taxon by taxon)
presented here (Fig. 1) seems to make more sense (more sister taxa look similar to one another) with a basal split between large arboreal omnivores (that ultimately produced carnivores) and small arboreal omnivores (that ultimately produced large and small herbivores and some insectivores).

Figure 1. The family tree (cladogram) of mammal interrelationships. Here the basal division is between slightly larger arboreal omnivores and slightly smaller arboreal omnivores. Of course, evolution kicks into high gear once  the dinosaurs are gone.

Similarities with traditional trees:

1. Outgroups include cynodonts, monotremes and marsupials in that order.
2. Eomaia is a basal placental (eutherian) despite retaining prepubic bones.
3. Primates nest together and with a flying lemur
4. Bats nest with carnivores
5. Civets nest together
6. Rodents nest with rabbits and one tree shrew
7. Xenarthans nest together
8. Ungulates nest together with Phenacodus (55 mya) at their base
9. Elephant nests with hyrax

Differences with traditional trees:

1. There are no geography-based divisions here, only morphology
2. The basic division is between arboreal civet-like omnivores and arboreal rodent-like omnivores.
3. Pangolin nests with primates and Plesiadapis does not
4. Tree shrews are not a single clade. Tupaia nests with rodents, rabbits and Plesiadapis. Ptilocercus nests with flying lemur.
5. Bats nest with a specific carnivore: Chriacus.
6. Whales nest with tenrecs and pre-tenrecs, not ungulates
7. Aardvark nests with armadillo derived from Pantolambda, which has a carnivore-like appearance).
8. Ungulates nest with elephant + hyrax all derived from a sister to Phenacodus.

In the large reptile tree, all sister taxa share a long list of traits
and look similar to one another. You can’t say that about competing hypotheses which don’t always include fossil taxa. The mammals subset of the large reptile tree (725 taxa) has grown so far without tree topology changes. Taxa just drop in between existing nodes.

Very few tooth characters are used here.
And none distinguish one post-canine tooth from another other than [absent, blunt, sharp, multi cusp and multi cusp with constricted base] and another trait notes where the posterior maxillary tooth erupts relative to the orbit.

Wikipedia suggests that:
“Paleontologists naturally insist that fossil evidence must take priority over deductions from samples of the DNA of modern animals. More surprisingly, these new family trees have been criticised by other molecular phylogeneticists, sometimes quite harshly.”

We’ve come a long way…
Simpson 1945 was able to list several subclasses and orders, but was unable to show interrelationships. Novacek 1992 discussed several problems right before computer-assisted phylogenetic analysis came along.

Suggestions for pertinent mammal taxa that need to be added to the LRT?
Please, send them along.

References
Novacek MJ 1992. Mammalian phylogeny: shaking the tree. Nature Review Article. 356:121-125.
Simpson GG 1945. The principles of classification and a classification of mammals. Bulletin of the AMNH Bulletin of the American Museum of Natural History 85:1-350. online.
Songa S, Liub L, Edwards SW and Wub S 2012. Resolving conflict in eutherian mammal phylogeny using phylogenomics and the multi species coalescent model. PNAS 109 (37): 14942-14947.
Spaulding M, O’Leary MA and Gatesy, J 2009. Relationships of Cetacea (Artiodactyla) among mammals: Increased taxon sampling alters interpretations of key fossils and character evolution. PLoS ONE 4:e7062. doi:10.1371/journal.pone.0007062:1-14.

why evolution is true – mammal tree

 

 

The big and small Estemmenosuchus (Dinocephalia, Therapsida, Synapsida, Archosauromorpha)

Figure 1. There are two known Estemmenosuchus species, the smaller famous one, E. mirabilis, and the larger less famous one, E. uralensis, here shown to scale. The ectopterygoid (Tr here) is oddly placed in E. uralensis, posterior to the pterygoid, but of the standard shape, if I’m reading this drawing correctly. the palatine/pterygoid pads are part of the herbivorous chewing apparatus.

Earlier we looked at
the baroque skull of Estemmenosuchus (late Permian, Tchudinov 1960, 1968) here, here and here. Today they are shown together to scale, with one about twice the size of the other. E. mirabilis is the more famous one because it is the more bizarre one, yet it is smaller than E. uralenesis. Both the postfrontals and the jugals expand distally to proceed these skull ‘horns’. A smaller one is produced by the premaxillary ascending process. And other smaller bumps are produced by the postorbital and frontal.

Looking at the palate drawings
we see a row of palatine teeth, a pterygoid with a row of teeth on the transverse process and more on the medial process, and some oddly placed ectopterygoids posterior to the pterygoids.

Figure3. The skull of Estemmenosuchus mirabilis, the hippopotamus of the Late Permian. Note the large procumbent teeth.

The maxilla has
two parallel rows of post canine teeth in E. mirabilis, essentially one row of marginal maxillary teeth in E. uralensis. But the maxillary/palatine row in E. uralensis is a new twist on the same construction. We’ve seen multiple rows of maxillary teeth in the taxa that lead up to rhynchosaur lepidosaurs.

References
Tchudinov PK 1960. Diagnosen der Therapsida des oberen Perm von Ezhovo: Paleontologischeskii Zhural, 1960, n. 4, p. 81-94.
Tchudinov PK 1968. Structure of the integuments of theriomorphs. Doklady Acad. Nauk SSSR. 179:207-210.

Hipposaurus: close to the ancestry of man, but off a wee bit

Figure 1. Therapsida includes the pangolin, Manis, which nests here with Notharctus. one of only a few mammals tested so far.

At the very base of the Therapsida
(Fig. 1) we have a split between the plant-eating Anomodontia (dicynodonts, dromasaurs and kin) and the meat-eating Kynodontia (new name for a new clade that encompasses all other therapsids, including cynodonts and mammals). At the base of the Kynodontia is the rarely discussed, but obviously important taxon, Hipposaurus boonstrai (Fig. 2, Haughton 1929, 21 cm skull. SAM 8950). Biarmosuchus is a sister.

Figure 2. Published material on Hipposaurus permits one to create a reconstruction like this. Not far removed from its ophiacodont / haptodine / pelycosaur precursors, Hipposaurus had longer, more gracile limbs and a distinct sabertooth canine, like Haptodus or Cutleria on steroids!

Long-legged, saber-toothed Hipposaurus
was originally thought to be a gorgonopsian, but in a note from Dr. Jim Hopson  (U Chicago) who xeroxed Boonstra 1965 for me, Hipposaurus (“horse lizard”) has been considered a biarmosuchian within the Ictidorhinidae since the 1980s.

Figure 3. The skull of Hpposaurus was larger than that of its sisters and predecessors among the basal Therapsida, including Stenocybus and Cutleria. The fangs were longer too.

There are some odd details
in the manus and pes of this mid-sized carnivore that indicate this is a derived late survivor of an earlier radiation.

1. Hipposaurus has a large pisiform (post axial carpal, Fig. 1)
2. The first centrale is quadrant shaped
3. The second centrale is shaped like a squat chevron
4. The radiale is twice as long as wide
5. The fourth and fifth carpals are fused
6. A small circular sternum present (none in sister taxa)
7. The posterior calcaneum has a hook like tuber
8. Two wedge-shaped centralia extend the width of the tarsus
9. The first distal tarsal is the size of a metatarsal and shifts the proximal metatarsal distally, almost to the mid length of metatarsal 2.
10. Two mid phalanges are fused on pedal digit 4

Speaking of oddities at clade bases…
as we’ve seen before, clade bases are, by definition, when novelties arise. In the case of Hipposaurus, these novel carpal and tarsal oddities went nowhere. A sister taxon without such novelties, Biarmosuchus, produced all the descendants we all know and love. Hipposaurus became a mere footnote and a short Wikipedia page.

References
Boonstra LD 1952. Die Gorgonospier-geslag Hipposaurus en die familie Ictidorhinidae: Tydskr. Wet. Kuns., v. 12, p. 142-149.
Boonstra LD 1965. The girdles and limbs of the Gorgonopsia of the Taphinocephalus Zone. Annals of the South African Museum 48:237-249.
Haughton SH 1929. On some new therapsid genera: Annals of the South African Museum, v. 28, n. 1, p. 55-78.

What it takes to be a mammal, according to the LRT

Earlier we nested
the monotremes, Akidolestes and Ornithorhynchus,  as basalmost mammals (contra traditional nestings) in the large reptile tree (LRT). It is notable that both of these taxa display atavistic (reversed) traits, including splayed hind limbs and the loss of teeth. Akidolestes also appears to converge with therians in molar occlusion patterns, while Ornithorhynchus converges with expansion of the braincase and fusion of the cranial elements.

It is possible
that known monotremes developed advanced molar occlusal patterns by convergence with derived mammals. This may be one reason for the difference in the nesting of monotremes in other studies and the LRT.

Basic and traditional traits
associated with being a mammal include:

1. Mammary glands on the female
2. Single tooth replacement
3. Dentary/squamosal jaw joint

Rowe (1988) defined Mammalia phylogenetically 
as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therians and all descendants of that ancestor. According to Wikipedia, that excludes all pre-Middle Jurassic forms.

Kemp (2005) defined mammals by their key traits:
Synapsids that possess a dentary–squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement. This becomes the last common ancestor of Sinoconodon and living mammals. The earliest known synapsid satisfying Kemp’s definitions is Tikitherium, (Late Triassic, Datta 2005).

Here
in the large reptile tree, not all known taxa are listed. Here monotremes, like Ornithorhynchus and Akidolestes, appear after Chiniquodon. Sinocodon nests within the Mammalia. The following are traits separate monotremes from Chiniquodon in the large reptile tree, which employs traits not specific to synapsids and mammals. Other mammals may not have these traits. Traits specific only to monotremes are not listed.

1. Parietals fused, frontals fused
2. Quadrate becomes an ear ossicle
3. Supraoccipital fused to tabulars and opithotics (data missing for Chiniquodon)
4. Premaxillary teeth, more than four (lost in Ornithorhynchus, of course).
5. Last maxillary tooth below posterior orbit
6. Sacral vertebrae reduced to two (not Akidolestes)
7. Interclavicle evolves from long ‘I’ shape to short ‘T’ shape
8. Sternum present
9. Ulna > 3x radius + ulna width
10. Longest metacarpals: 3 and 4 (except Ornithorhynchus where 2-4 are subequal)
11. Longest manual digit is not 4
12. Ilium posterior process is not longer
13. Pubis orientation is not strictly medial
14. Prepubis bone present
15. Tarsus < 0.6x pedal digit 4 length
16. Longest pedal digits 2-4 (rather than 4, not known from Chiniquodon)
17. Pedal 4.1 proportions: length/width not < 3:1

Most of these, 
it goes without saying, are provisional subject to additional taxa and more precise data.

References
Datta PM 2005. Earliest mammal with transversely expanded upper molar from the Late Triassic (Carnian) Tiki Formation, South Rewa Gondwana Basin, India. Journal of Vertebrate Paleontology 25 (1): 200–207. doi:10.1671/0272-4634(2005)025[0200:EMWTEU]2.0.CO;2.
Kemp TS 2005. The Origin and Evolution of Mammals. United Kingdom: Oxford University Press. p. 3. ISBN 0-19-850760-7. OCLC 232311794.
Rowe T 1988. “Definition, diagnosis, and origin of Mammalia”(PDF). Journal of Vertebrate Paleontology 8 (3): 241–264. doi:10.1080/02724634.1988.10011708.

How did Moschops take a drink of water?

The dinocephalian synapsid,
Moschops (Fig. 1), looks like it could not bend its neck down to take a drink of water from the shoreline, giraffe-style.

Figure 1. Stiff-necked Moschops did not need to lean down, giraffe-style, to drink water. It could just wade into chin deep water.

But Moschops could wade
into chin-deep water, maybe where it’s plant diet sprouted. Okay, no big deal, but I’ve wondered this and the simplest answer did not come to me for some time, probably because I was giraffe- and wildebeest-biased. Watching my dog walk knee deep into a pond cleared the air for me.

Maybe Anomocephalus had canine fangs, too!

Two dicynodont-mimics,
Tiarajudens (UFRGS PV393P, Cisneros et al. 2011) and Anomocephalus (Modesto et al. 1999) were discovered in the last few two decades. Tiarajudens had sharp teeth and a fang/canine/tusk. Anomocephalus had flat teeth and apparently no tusk (Fig. 1).

Working from the published tracing
I put the scattered teeth of Anomocephalus back into the jaws and discovered that maybe there is a tusk/fang in there, too (Fig. 1). If valid, the fang was broken in half during typhonomy, so it became the same length as the other teeth, all of which had narrow roots, unlike the fang.

Figure 1. Anomocephalus in situ and reconstructed. In situ image from Modesto et al. 199. Apparently a fang/canine/tusk was hiding among the broken teeth.

Tiarajudens and Anomocephalus
are considered middle Permian primitive herbivorous anomodonts by the author(s) of Wikipedia, who also suggest they were ancestral to dicynodonts. By contrast, the large reptile tree (Fig. 2)  nests Tiarajudens and Anomocephalus in a clade close to, but separate from dicynodonts (Fig. 2).

Figure 3. Basal therapsid tree. Note the nesting of the Anomodontia and the dicynodonts here, both derived from smaller dromasaurs.

According to the LRT, the ancestors of dicynodont mimics were 
Venjukovia and Otsheria. The ancestors of dicynodonts include Suminia, a late-survivor of an early radiation. Both were derived from smaller dromasaurs (Fig. 3).

Figure 3. Venjukoviamorphs include the dicynodont mimics, Tiarajudens and Anomcephalus, the latter now with mid-length canines. The Anomocephalus drawing is modified from Modesto et al. 1999 and appears to have certain problems.

References
Cisneros, JC, Abdala F, Rubidge BS, Dentzien-Dias D and Bueno AO 2011. Dental Occlusion in a 260-Million-Year-Old Therapsid with Saber Canines from the Permian of Brazi”. Science 331: 1603–1605.
Modesto S, Rubidge B and Welman J 1999. The most basal anomodont therapsid and the primacy of Gondwana in the evolution of the anomodonts. Proceedings of the Royal Society of London B 266: 331–337. PMC 1689688.

Estemmenosuchus skull sutures

The data I have
for Estemmenosuchus comes from a well-lit by low resolution image complete with halftone dots and moire patterns (Fig. 1). Nevertheless, I think I can find sutures on the image.

Figure1. The skull sutures on Esttemmenosuchus mirabilis (PIN 1758/6). When you score a skull for analysis, you have to have this data. I think that’s the right cheekbone peeking through the naris. See how colors make this interpretation clear, whether accurate or not? The ascending process of the premaxilla look like it spreads laterally. Think of that as an option. That’s the squamosal forming the tip of the cheek horn. Yes, you can still see the moire pattern formed by the halftone dots.

Estemmenosuchus uralensis (Middle Permian, Tchudinov 1960, 1968; Holotype PIN 1758/4 Skull length: 60 cm;  E. mirabilis PIN 1758/6) could be an omnivore. Considering its bulk and short thick legs, it is more likely an herbivore, like a hippo.

Estemmenosuchus skin
According to Wikipedia, “The fossil material includes an exceptionally well preserved skin impression. The skin appears to be smooth and undifferentiated with no signs of either hairs or scales but with evidence of being well supplied with glands.”

Figure 3. Estemmenosuchus cover

So I made a big mistake
back in 1991 when I illustrated Estemmenosuchus with scales on the cover of this book. The longer you linger the more you learn.

References
Tchudinov PK 1960. Diagnosen der Therapsida des oberen Perm von Ezhovo: Paleontologischeskii Zhural, 1960, n. 4, p. 81-94.
Tchudinov PK 1968. Structure of the integuments of theriomorphs. Doklady Acad. Nauk SSSR. 179:207-210.

Burnetia sutures revealed with DGS (Digital Graphic Segregation)

Sorry to be away for awhile.
I was updating the basal synapsid portion of the large reptile tree at ReptileEvolution.com. Still working on the website as of this writing, but the tree is more robust with a few added taxa. Notably Nikkasaurus and Niaftasuchus have been removed from the Synapsida. The former is now a basal prodiapsid nesting with Mycterosaurus. The latter now nests as another prodiapsid with Mesenosaurus.

Now that all the hard work is done,
let’s take a fresh look at the basal therapsid, Burnetia (Fig. 1), the most derived member of the Burnetiidae. Sutures delineate bones and in order to correctly score the bones you have to see the sutures. And they have to closely resemble those of clade members (Fig. 2). See what you think of these. And note that those who had the fossil in their hands and presumably under the microscope were not able to provide the sutures shown here, gleaned from published photographs.

A Burnetiidae therapsid, Burnetia skull in four views. 1. the original published drawing (Broom 1923); 2. an updated published drawing (Rubidge and Sidor 2002); neither of which are able to indicate sutures; and 3) a DGS tracing with sutures indicated. Finally I add the mandible of Proburnetia as a stand-in for the missing mandible. Only a few paleontologists colorize bones. It’s the best way to show where the sutures are.

Burnetia mirabilia
(Broom 1923, Rubidge and Sidor 2002; BMNH R5397; Late Permian) had a flat, wide skull with exceptional skull ornamentation. The squamosal cheeks flared widely. The teeth are very small. Derived from a sister to Proburnetia.

Derived from a sister to Hipposaurus,
the Burnetidae were basal therapsids from the Middle to Late Permian that evolved bizarre skull ornamentation. Rubidge and Sidor (2002) report, “The systematic position of the Burnetiidae has been unsure largely because of a poor understanding of the cranial morphology of these two enigmatic skulls. In the past they have been considered gorgonopsians (Boonstra, 1934; Haughton and Brink, 1955; Sigogneau, 1970), dinocephalians (von Huene, 1956), and more recently, biarmosuchians (Hopson and Barghusen, 1986; Sigogneau-Russell, 1989). Like Gorgonopsids, this clade has anterior facing nares and a proparietal by convergence.”

We’ll take a look at the other members of this clade later.
But for now here’s the data for the taxa (Fig. 2). Lemurosaurus and Proburnetia appear to have antorbital fenestrae/foraminae and the lacrimal overlaps the jugal. Note the gradual reduction of the teeth in this clade and the gradual widening of the back of the skull. The supratemporals are supposed to be missing from al therapsids, but I found they are missing from all therapsids more derived than this clade.

The clade Burnetiidae/Ictidorhinidae to scale includes Ictidorhinus, Herpetoskylax, Lemurosaurus, Proburnetia and Burnetia, and a few others not shown. The bones were colorized using Photoshop in a method known as DGS or digital graphic segregation. Note the lacrimal overlapping the jugal. The pre parietal (anterior to the parietal foramen) once nested these taxa with gorgonopsids. Some antorbital fenestrae/foramina are present.

References
Broom 1923. On the structure of the skull in the carnivorous dinocephalian reptiles. Proceedings of the Zoological Society of London 2:661–684.
Rubidge BS and Sidor CA 2002. On the crnial morphology of the basal therapsids Burnetia and Proburnetia (Therapsida: Burnetiidae). Journal of Vertebrate Paleontology 22(2):257–267.