Fruitafossor: now a Late Jurassic echidna from Colorado

While reviewing the terrestrial descendants of tree shrews
yesterday, the Late Jurassic Fruitafossor (Figs. 1, 2) stuck out as a chronological misfit as it nested in the otherwise Tertiary edentates (= Xenarthrans).

Here is the problem,
and the solution.

A Jurassic edentate? No.
Fruitafossor windscheffeli (Luo and Wible 2005) used to nest in the LRT with digging edentates, like the armadillo-mimic, Peltephilus (Miocene), and for good reason…

Wikipedia reports,
“The teeth of Fruitafossor bear a striking resemblance to modern armadillos and aardvarks. Its vertebral column is also very similar to armadillos, sloths, and anteaters (order Xenarthra). It had extra points of contact among similar to the xenarthrous process that are only known in these modern forms.”

By contrast, Wikipedia concludes,
“Its shoulder-girdle is similar to a platypus or reptile, but many other features are more similar to most other modern mammals.”

What would Larry Martin say?
Run a complete analysis. Don’t rely on one, two or a dozen traits. And the Late Jurassic is so early in mammal evolution that it becomes important, too. There were fewer mammal clades back then. Edentates had not yet arrived.

Figure 1. Several drawings from Zhou and Wible that one must trust for accuracy. The verification data is too fuzzy to validate.

So is Fruitafossor a Late Jurassic edentate?
Or an edentate-mimic in the Late Jurassic?
With current scoring in the LRT, shifting Fruitafossor from the edentates to the base of the Monotremata adds 23 steps. Shifting to Early Cretaceous Lactodens within the Monotremata adds just 17 steps, the lowest number I could find. Lactodens has typical differentiated teeth and five fingers with small, sharp claws, traits not shared with Fruitafossor + edentates. Lactodens nests with the echidnas, Tachyglossus (extant, Figs. 3–5) and Cifelliodon (Early Cretaceous; Fig. 3). The latter has simple blunt teeth and the former is a known digger.

Figure 2. Fruitafossor in situ from Digimorph.org and used with permission and here colorized to an uncertain extent.

So let’s reexamine scored traits… and solve this conundrum.
Has the LRT met its match? Very few skull traits are known from Fruitafossor. Even so, earlier I overlooked or mis-scored the following that gain importance in hindsight:

Fruitafossor:

1. orbit contacts the maxilla
2. 4 rather than 5 sacrals,
3. coracoid present
4. I could not score hind limb length without a pes and estimates won’t do
5. proximal sesamoid of fibula present
6. fibula diameter greater than half of tibia
7. dorsal osteoderms absent (I misinterpreted scattered elements at Digimorph.org)

Tachyglossus:

1. retroarticular process present as in Fruitafossor
2. metacarpal 1 and 2 are the longest as in Fruitafossor
3. longest manual digit 3 as in Fruitafossor
4. manual digit 4 narrower than 3 as in Fruitafossor

Cifelliodon:

1. three molars, as in Fruitafossor

Figure 3. Early Cretaceous Cifelliodon is ancestral to the living echidna, Tachyglossus according to the LRT. The lack of teeth here led to toothlessness in living echidnas. The skull of Tachyglossus is largely fused together, lacks teeth and retains only a tiny lateral temporal fenestra (because the jaws don’t move much in this anteater. Compared to Cifelliodon the braincase is greatly expanded, the lateral arches are expanded and the two elements fuse, unlike most mammals.

Figure 4. Tachyglossus skeleton, manus and x-rays. Note the perforated pelvis.

Figure 5. The echidna (genus: Tachyglossus) in life. This slow-moving spine-covered anteater has digging claws.

Results (as you might imagine, given these changes):
Fruitafossor is an edentate-mimic nesting basal to Cifellidon and Tachyglossus as a Late Jurassic echidna and monotreme in the LRT. Glad to get rid of that problem!

In their original description of Fruitafossor,
Luo and Wible 2005 nested their discovery between a monotreme clade and a clade with the mammal-mimic, Gobiconodon at its base, then a clade with another egg-laying mammal, Tinodon at its base, then a pangolin ancestor, Zhangheotherium, then a rabbit ancestor Henkelotherium, then two other monotremes, Dryolestes, Amphitherium and the carnivorous marsupial, Vincelestes.  Luo and Wible tested Tachyglossus, but not Cifelliodon, which was published in 2018. Note the simple, blunt teeth in Cifelliodon, nearly matching those in Fruitafossor. Given that the only fossil of Fruitafossor is a bit jumbled, it is possible that it, too, had five fingers in vivo, like other monotremes. With only four fingers (Fig. 1) Fruitafossor had a good excuse for pretending to be an edentate.

So, yes, the LRT was up to the challenge.
But it took insight, lacking until now, to provide the correct matrix scoring. I’m happy to announce that the twenty or so corrections made yesterday were added to the 120,000 or so corrections made over the past ten years. With these corrections the LRT gets better and stronger every week. Minimizing taxon exclusion maximizes the opportunity to correctly nest new and enigma taxa with old and established taxa, even if the new and old specimens are incomplete or scattered about.

The earlier August 2017 blogpost for Fruitafossor
was updated yesterday to erase old errors and enter the corrections.

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References
Huttenlocker AD, Grossnickle DM, Kirkland JI, Schultz JA and Luo Z-X 2018. Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature Letters  Link to Nature
Luo Z-X and Wible JR 2005. A late Jurassic digging mammal and early mammal diversification. Science 308:103–107.
Shaw G 1792. Musei Leveriani explicatio, anglica et latina.

wiki/Fruitafossor
digimorph.org/specimens/Fruitafossor_windscheffeli/

 

Asiatherium enters the LRT: mammal nomenclature issues follow

Everyone agrees
that Asiatherium (Figs, 1,2) nests close to Monodelphis, Caluromys and placentals. Trofimov and Szalay 1994 agreed. So did Denyer, Regnault and Hutchinson 2020. So did the large reptile tree (LRT, 1729+ taxa, subset Fig. 3).

Figure 1. Asiatherium in situ from Szalay and Trofimov 1996.

Asiatherium reshetovi (Trofimov and Szalay 1994, Szalay and Trofimov 1996; PIN 3907; Late Cretaceous; 80mya; Figs. 1, 2) is a key Mongolian metathere ancestral to monodelphids and Caluromys, which is ancestral to placentals. It is derived from Triassic sisters to extant late survivors, Didelphis, Gilronia and Marmosops.

Figure 2. Asiatherium skull slightly modified (longer lateral view premaxilla to match dorsal and ventral views) from Szalay and Trofimov 1996. Colors added here.

The problem is,
according to results recovered by the LRT, mammal clade nomenclature needs to go back to basics. Several modern mammalian clade names are found to be junior synonyms of traditional clades in the LRT.

Prototheria (Gill 1872) is a junior synonym
for Monotremata (Bonaparte 1837) in the LRT.

According to Wikipedia, “Prototheria is a paraphyletic subclass to which the orders Monotremata, Morganucodonta, Docodonta, Triconodonta and Multituberculata have been assigned, although the validity of the subclass has been questioned.”

In the LRT Morganucodon is a a marsupial (see below). Docodon is a taxon within Monotremata. Triconodon is a taxon within Monotremata. Multituberculata is a clade within the placental clade Glires (Fig. 4). So, the clade Monotremata is monophyletic and has precedence.

Theria (Parker and Haswell 1897) is a junior synonym
of Marsupialia (Illiger 1811). Metatatheria (Thomas Henry Huxley 1880) is also a junior synonym of Marsupialia.

The late-surviving basalmost marsupial in the LRT (Fig. 4), Ukhaatherium (Fig. 3), has epipubic (marsupial) bones. That long rostrum indicates this taxon is close to monotremes.

Figure 3. Ukhaatherium in situ.

Unlike the monophyletic clade Monotremata,
a series of nested marsupial clades are present. The last of these gives rise to Placentalia, only one of several that lose the pouch (Fig. 4). New names are proposed here where appropriate:

1. Marsupialia = Ukhaatherium and kin + all descendants (including placentals)
2. Paleometatheria = Morganucodon and kin + all descendants.
3. Didelphimetatheria = Eomaia and kin + all descendants
4. Phytometatheria = Marmosops and kin + all descendants
5. Carnimetatheria = Asiatherium and kin + all descendants
6. Transmetatheria = Caluromys and kin + all descendants
7. Placentalia = Vulpavus and kin + all descendants

Figure 4. Subset of the LRT cladogram of basal Mammalia. Note the new names proposed here.

Basal marsupial taxa are omnivores. 
Derived phytometatheres are herbivores. Derived carnimetatheres are carnivores to hyper-carnivores. Transmetatheres (Carluromys) and basal Placentalia remain omnivores.

In the LRT Eutheria (Gill 1872) is a junior synonym
of Placentalia (Owen 1837). Omnivorous civets like Nandinia are basal placentals. Carnivora is a basal placental clade following basal placental civets.

Competing cladograms
Denyer, Regnault and Hutchinson 2020 recently looked at the marsupial patella, or more specifically the widespread absence or reduction of the kneecap. The authors concluded, “metatherians independently ossified their patellae at least three times in their evolution.”

Unfortunately, Denyer et al. tested Caenolestes, the ‘shrew opossum’. Not surprisingly it nested close to placentals in their cladogram. Caenolestes was earlier nested in the LRT within the placental clade, Glires, closer to shrews than to opossums. It has no pouch, but converges with marsupials in several aspects. Inappropriate taxon inclusion, like Caenolestes, occurs due to taxon exclusion. Excluded taxa would have attracted and removed the inappropriate taxon. Taxon exclusion plagues Denyer et al.

Historically, you may remember,
Bi et al. 2018, while presenting Early Cretaceous Ambolestes, suffered from massive taxon exclusion and traditional bias in attempting to produce a cladogram of mammals. Bi et al. recovered Sinodelphys (Early Cretaceous) and Juramaia (Late Jurassic) as ‘eutherians’. In the LRT both are monotremes.

Other basal mammal cladograms
depend too much on tooth traits. Convergence in tooth traits creates problems, as documented earlier. We’ll look at this problem in more detail soon.

The above subset of the LRT appears to be a novel hypothesis
of interrelationships. If not, please provide a citation so I can promote it.

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References
Bi S, Zheng X, Wang X, Cignetti NE, Yang S, Wible JR. 2018. An Early Cretaceous eutherian and the placental marsupial dichotomy. Nature 558(7710):390395 DOI 10.1038/s41586-018-0210-3.
Denyer AL, Regnault S and Hutchinson JR 2020. Evolution of the patella and patelloid in marsupial mammals. PeerJ 8:e9760 http://doi.org/10.7717/peerj.9760
Szalay FS and Trofimov BA 1996. The Mongolian Late Cretaceous Asiatherium, and the early phylogeny and paleogeography of Metatheria. Journal of Vertebrate Paleontology 16(3):474–509.
Trofimov BA and Szalay FS 1994. New Cretaceous marsupial from Mongolia and the early radiation of Metatheria. Proceedings of the National Academy of Sciences 91:12569-12573

Middle Jurassic moonrat: Asfaltomylos patagonicus

Ever since the LRT nested multiberculates within Glires,
we’ve been looking for non-multituberculate members of Glires (rats, rabbits, tree shrews, etc.) from the Jurassic to support that novel hypothesis. Here’s one.

Martin and Rauhut 2005
redescribed the mandible and teeth belonging to Asfaltomylos (Rauhut et al. 2002; Fig. 1) famous for being the first Jurassic mammal from South America and for apparently lacking a canine and incisors.

The question:
Is this an egg-laying monotreme (clade: Prototheria)? That’s what both Rauhut et al. 2002 and Martin and Rauhut 2005 thought based on tooth shape and a post-dentary groove in the medial dentary. They also excluded taxa listed below (and shown in figure 1). Such bias is a too common fault in traditional paleontology, as long time readers are well aware.

Figure 1. Asfaltomylos (MPEF-PV 1671) is a tiny mandible with teeth from in Jurassic strata in South America. Note the shape of the posterior premolar and how it relates to the giant posterior premolar in Carpolestes. The canine is tall. Not sure if Asfaltomylos had large incisors. Either way it does not matter, based on comparisons to Echinosorex and Erinaceus, the living moonrat and hedgehog. Only the posterior molar in Erinaceus looks like the two molars in Asfaltomylos, separated in time by 166 million years.

Based primarily on tooth morphology,
Rauhut et al. 2002 considered Asfaltomylos a member of the Australosphenida, a clade of southern Jurassic mammals that is said to convergently evolve tribosphenic molars with northern mammals and probably gave rise to monotremes. Their taxon restricted cladogram nested Asfaltomylos between Shuotherium (Fig. 2) and several untested taxa leading to several platypus-like  taxa (including genus: Ornithorhynchus; Fig. 3.)

Question for you, dear readers:
Do the mandibles of Asfaltomylos (Fig. 1) and Shuotherium (Fig. 2) resemble one another? They should, given their proximity in the Rauhut et al. and Martin and Rauhut cladograms. If you think they don’t look similar, perhaps we need to expand the taxon list.

Figure 2. Medial view of Shuotherium. The last premolar is similar to the first molar, the coronoid process is tiny and the retroarticular process is absent, all distinct from Asfaltomylos (Fig. 1).

As a test, let’s add all the mammals in the LRT.
When we do, and based on very few mandible characters, Asfaltomylos foregoes the Prototheria and nests with derived members of Glires, derived from moonrats, the only members of Glires that sometimes do not have large gnawing incisors (yet another reversal).

Only the posterior molar
in the hedgehog, Erinaceus (Fig. 1), looks like the two molars in Asfaltomylos, separated in time by 166 million years. The premolar is nearly identical.

Moonrats
(Fig. 4) have an appropriately primitive appearance, and are different from other members of Glires in being chiefly carnivorous.

Rougier et al. 2007
considered Henosferus another member of the clade ‘Australosphenida’. With its  complete dental formula on a low profile mandible, Henosferous (Fig. x) nests with other basalmost therians, like Morganucodon (Fig. 3) in the LRT, not close to Asfaltomylos. So members of the invalidated clade ‘Australosphenida’ are polyphyletic in the LRT.

Figure x. Henosferus mandible restored by Rougier et al. 2005 from several broken specimens.

Phylogenetic miniaturization and neotony
answer the problems posed by the low number of molars and the retention of the postdentary trough in Asfaltomylos. As you may recall, mammals recapitulate their phylogeny during ontogeny and Asfaltomylos matured at an earlier stage of development due to its small size.

Tooth morphology is something else to be ware of in phylogenetic analyses.
As an example, whale teeth devolved from multi-cusped in a square in their four-limbed terrestrial ancestors, to multi-cusped in a row in archaeocetes with flukes, to simple cones and toothlessness in derived odontocetes.

Figure 3. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus, and Monodelphis, a living tree opossum.

The problem is,
the high coronoid process and retroarticular (angular) process of Asfaltomylos are not found in Ornithorhynchus (Fig. 3) nor in other Prototheres in the large reptile tree (LRT, 1631+ taxa, Fig. 2). Prototheria are notable for their long rostra, lots of teeth and low coronoid process, traits that don’t match the  Asfaltomylos mandible. The medial surface of Asfaltomylos does include a dentary trough in which tiny posterior jaws bones would soon evolve to become ear bones… except that happens by convergence in highly derived arboreal mammals, like multituberculates, that experience that reversal in the auditory region, to the chagrin of Jurassic mammal workers worldwide.

Figure 4. Echinosorex, the extant moonrat, looks like an opossum, but nests with Deinogalerix in the large reptile tree.

In the LRT
Asfaltomylos nests with the moonrat Echinosorex, not far from Carpolestes (Fig. 1), a plesiadapiform in the LRT. 

Here’s a thought:
Take a look at that tall, narrow, posterior premolar in Asfaltomylos. That’s what turns into a similar posterior premolar in moonrats and hedgehogs. That’s what turns into a large cutting premolar in Carpolestes and multituberculates. 

Figure 5. Subset of the LRT focusing on Glires and subclades within. Moonrats and hedgehogs are not too far from Carpolestes and arboreal taxa like aye-aye.

Once again, the LRT shows why it is so important
to test all enigma taxa against a wide gamut of taxa, like the LRT. The LRT minimizes bias in the choice of the inclusion set of taxa. The number of characters for the mandible in the LRT comes down to less than dozen. Tooth cusp characters are largely omitted. So character count is, once again, shown to be not nearly as important, contra the opinions of workers who ask for more characters to no advantage.

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References
Martin T and Rauhut OWM 2005. Mandible and dentition of Asfaltomylos patagonicus (Australosphenida, Mammalia) and the evolution of tribosphenic teeth. Journal of Vertebrate Paleontology 25(2):414–425.
Rauhut OWM, Martin T Ortiz-Jaureguizar E and Puerta P 2002. A Jurassic mammal from South America. Nature 416:165–168.
Rougier, GW, Martinelli AG, Forasiepi AM and Novacek M J 2007. New Jurassic mammals from Patagonia, Argentina : a reappraisal of australosphenidan morphology and interrelationships. American Museum novitates, no. 3566. online here.

wiki/Asfaltomylos

https://pterosaurheresies.wordpress.com-henosferus/

Morganucodon and Kuehneotherium are mammals, not stem-mammals

Newham et al. 2019 report,
“Surprisingly long lifespans and low femoral blood flow suggest reptile-like physiology in key Early Jurassic stem-mammals.“

Abstract:
“There is uncertainty regarding the timing and fossil 5 species in which mammalian endothermy arose, with few studies of stem-mammals on key aspects of endothermy such as basal or maximum metabolic rates, or placing them in the context of living vertebrate metabolic ranges. Synchrotron X-ray imaging of incremental tooth cementum shows two Early Jurassic stem-mammals, Morganucodon and Kuehneotherium, had lifespans (a basal metabolic rate 10 proxy) considerably longer than comparably sized living mammals, but similar to reptiles, and that Morganucodon had femoral blood flow rates (a maximum metabolic rate proxy) intermediate between living mammals and reptiles. This shows maximum metabolic rates increased evolutionarily before basal rates, and that contrary to previous suggestions of a Triassic origin, Early Jurassic stem-mammals lacked the endothermic metabolism of living mammals.”

That conclusion would be true
if their cladogram was correct. Unfortunatley, it was not.

Figure 1. Subset of the LRT from 2018 focusing on Basal Mammalia including Morganucodon and Kuehneotherium.

According to
the large reptile tree (LRT, 1579 taxa; subset Fig. 1), Kuehneotherium (Fig. 2) is a basal protothere mammal (= monotreme) in the lineage of echidnas and platypuses. Morganucodon is a very basal metathere mammal (= marsupial). The Virginia opossum, Didelphis, is the most closely related extant taxon in the LRT.

Figure 2. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Here’s a data point of interest:
Newham et al. report, “Only the short-beaked echidna Tachyglossus aculeatus, a monotreme with long lifespan and low metabolic rate, exceeds the Kuehneotherium, but not Morganucodon, distance above the mammalian mean.” And THAT is reflected in the LRT. I also note the platypus, Ornithorhynchus, is not mentioned in the text, only in the citations. Same with Didelphis.

So what does that do to the results?
Seems like the Newham et al. study is suffering from taxon exclusion and an invalid traditional understanding of basal mammal interrelations. Unfortunately Professor MJ Benton is a co-author, infamous for taxon exclusion and guiding his students and any protégé to do the same.

Please tell Elis Newham et al.
to add the platypus and opossum to their study and get back to us! Don’t let this work become another waste of time due to taxon exclusion.

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References
Newham E et al. (19 co-authors) 2019. Reptile-like physiology in Early Jurassic stem-mammals. bioRxiv preprint http://dx.doi.org/10.1101/785360

Mesozoic mammals: Two views

Smith 2011 reported,
at the beginning of the Eocene, 55mya, “the diversity of certain mammal groups exploded.” These “modern mammals”, according to Smith, ‘ consist of rodents, lagomorphs, perissodactyls, artiodactyls, cetaceans, primates, carnivorans and bats. Although these eight groups represent 83% of the extant mammal species diversity, their ancestors are still unknown. A short overview of the knowledge and recent progress on this research is here presented on the basis of Belgian studies and expeditions, especially in India and China.’

Contra the claims of Smith 2011
in the large reptile tree (LRT, 1354 taxa, subsets Figs. 2–4) prototherians are known from the late Triassic (Fig. 1). Both metatherians and eutherians are known from the Middle Jurassic. Many non-mammal cynodonts survived throughout the Mesozoic. In addition, the ancestors of every included taxon are known back to Devonian tetrapods.

Noteworthy facts after an LRT review (Fig. 1):

1. All known and tested Mesozoic mammals (Fig. 1) are either small arboreal taxa or small burrowing taxa (out of sight of marauding theropods).
2. All Mesozoic monotremes are more primitive than Ornithorhynchus and Tachyglossus (both extant).
3. All Mesozoic marsupials are more primitive than or include Vintana (Late Cretaceous).
4. All Mesozoic placentals are more primitive than Onychodectes (Paleocene).

Figure 1. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

Given those parameters
we are able to rethink which mammals were coeval with dinosaurs back on phylogenetic bracketing (= if derived taxa are present, primitive taxa must have been present, too).

Smith reports, “The earliest known mammals are about as old as the earliest dinosaurs and appeared in the fossil record during the late Trias around two hundred and twenty million years ago with genera such as Sinoconodon (pre-mammal in the LRT), Morganucodon (basal therian in the LRT) and Hadrocodium (basal therian in the LRT). However, the earliest placental mammals (Eutheria) were not known before the Early Cretaceous. Eomaia scansoria (not eutherian in the LRT) from the Barremian of Liaoning Province, China is the oldest definite placental and is dated from a hundred and thirty million years ago.”

Mesozoic Prototherians

1. All included fossil taxa are Mesozoic. Two others are extant (Fig. 2).

Figure 2. Mesozoic prototherians + Megazostrodon, the last common ancestor of all mammals. Only two taxa (gray) are post-Cretaceous.

Mesozoic Metatherians (Marsupials)

1. Derived Vincelestes is Early Cretaceous, which means Monodelphis and Chironectes were present in the Jurassic.
2. Derived Didelphodon is Late Cretaceous, which means sisters to Thylacinus through Borhyaena were also present in the Mesozoic.
3. Derived Vintana is Late Cretaceous, which means sisters to herbivorous marsupials were also present in the Mesozoic.

Figure 3. Mesozoic metatherians (in black), later taxa in gray. Whenever derived taxa are present in the Mesozoic (up to the Late Cretaceous) then ancestral taxa, or their sisters, were also present in the Mesozoic. Didelphis is extant, but probably unchanged since the Late Jurassic/Early Cretaceous.

Mesozoic Eutherians (= Placentals)

1. Rarely are placental mammals identified from the Mesozoic, because many are not considered placentals.
2. Placentals (in the LRT) are remarkably rare in the Mesozoic, but sprinkled throughout the cladogram, such that all taxa more primitive than the most derived Mesozoic taxon (Anagale and derived members of the clade Glires, Fig. 4, at present a number of multituberculates) must have had Mesozoic sisters (Carnivora, Volitantia, basal Glires). 

Figure 4. Mesozoic euthrerians (placentals, in black). Later taxa in light gray. All taxa more primitive than Mesozoic taxa were likely also present in the Jurassic and Cretaceous. None appear after Onychodectes. Madagascar separated from Africa 165-135 mya, deep into the Cretaceous with a population of tenrecs attached. No rafting was necessary.

The above represents what a robust cladogram is capable of,
helping workers determine the likelihood of certain clades appearing in certain strata, before their discovery therein, based on their genesis, not their widest radiation or eventual reduction and extinction. In other words, we might expect sisters to basal primates, like adapids and lemurs, to be present in the Mesozoic, but not sisters to apes and hominids. We should expect sisters to all tree shrews and rodents to be recovered in Mesozoic strata. We should expect to see sisters to Caluromys, Vulpavus and other small arboreal therians/carnivorans in Mesozoic strata, but not cat, dog and bear sisters.

References
Smith T 2011. Contribution of Asia to the evolution and paleobiogeography of the earliest modern mammals. Bulletin des séances- Académie royale des sciences d’outre-mer. Meded. Zitt. K. Acad. Overzeese Wet.57: 293-305

Mammal taxa: origin times

A few days ago, we looked at a revised and expanded cladogram of the Mammalia based on skeletal traits (distinct from and contra to a cladogram based on DNA). Today we add chronology to the cladogram to indicate the first appearance of various mammals and estimate the origin of the various clades (Fig. 1).

Note that derived taxa
that chronologically precede more primitive taxa indicate that primitive taxa had their genesis and radiation earlier than the first appearance of fossil specimens, which always represent rare findings usually during wide radiations that increase the chance the specimen will fossilize in the past and be found in the present day.

Looking at time of mammal taxa origin categories:

Figure 1. Cladogram with time notes for the Mammalia (subset of the LRT).

Some notes:

1. Both prototheres and basal therians were present (and probably widespread) in the Late Triassic.
2. Derived prototheres appear in the Late Triassic, suggesting an earlier (Middle Triassic?) origin for Mammalia and an earlier (Middle Triassic?) split between Prototheria and Theria.
3. Both fossorial metatherians and basal arboreal eutherians were present (and probably widespread) in the Late Jurassic. These were small taxa, out of the gaze of ruling dinosaurs.
4. Large derived eutherians eolved immediately following the K-T boundary in the Paleocene and radiated throughout the Tertiary.
5. A large fraction of prototherians, metatherians and eutherians are known only from extant taxa, some of which are rare and restricted, not widespread.
6. Multituberculates and kin are derived placentals close to rodents by homology, not convergence.

 

Cladogram of the Mammalia (subset of the LRT)

A summary today…
featuring a long cladogram (Fig. 1), a subset from the large reptile tree (LRT, 1259 taxa) focusing on the Mammalia. This is how this LRT subset stands at present. Not much has changed other than the few node changes from the past week.

The transition from Prototheria to Theria (Metatheria)
includes long-snouted taxa, like Ukhaatherium. Nearly all Prototheria are also long-snouted (Cifelliodon is the current sole exception).

The transition from Metatheria to Eutheria (simplified)
includes small omnivorous didelphids arising from the carnivorous/herbivorous split among larger metatherians. Basal Carnivora, the most basal eutherian clade, are also omnivores. Caluromys, the extant wooly opossum, has a pouch, but nests at the base of all placental taxa (the LRT tests only skeletal traits), so it represents the size and shape of the earliest placentals (contra O’Leary et al. 2013)… basically didelphids without pouches, and fewer teeth, generally (but not always).

Basal members of most placental clades
are all Caluromys-like taxa, with a rapid radiation in the Late Triassic/Early Jurassic generating most of the major placental clades in the LRT (Fig. 1). Larger members of each of these placental clades appeared in the fossil record only after the K-T extinction event. So hardy where these basal taxa, that many still live to this day.

As shown earlier, higher eutheria are born able to able to walk or swim. They are no longer helpless with arboreal parents (tree-climbing goats the exception). Basal eutherians reproduce more like their metatherian ancestors, with helpless infants.

Figure 1. Subset of the LRT focusing on mammals. Extant taxa are colored. Thylacinus is recently extinct.

The latest competing study
(O’Leary et al. 2013, Fig. 2) recovers the highly specialized edentates, aardvarks, elephants and elephant shrews as the most primitive placentals. Carnivora + bats are quite derived in the O’Leary team cladogram, somehow giving rise to ungulates and whales. This is an untenable hypothesis. It doesn’t make sense. Evidently the O’Leary team had faith that smaller didelphid-like ancestors would fill in the enormous phylogenetic gaps in their cladogram. By contrast the LRT has all the operational taxonomic units (OTUs) it needs to produce a series of gradually accumulating derived traits between every taxon in its chart (Fig. 1). The LRT makes sense.

Figure 5. Simplified version of the O’Leary et al 2013 cladogram showing placental relations exploded after the K-T boundary.

References
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of  placentals. Science 339:662-667. abstract
Wible JR, Rougier GW, Novacek MJ, Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary Nature 447: 1003-1006

https://pterosaurheresies.wordpress.com/2016/08/31/another-look-at-the-oleary-et-al-hypothetical-ancestor-of-placentals/

https://pterosaurheresies.wordpress.com/2013/02/15/post-k-t-explosion-of-placentals-oleary-et-al-2013/

ArchibaldEtAl.pdf
protungulatum-donnae website

A new place to nest Volaticotherium: with Morganucodon

Volaticotherium is the epitome
of a roadkill fossil (Fig. 1). It needs to be put together (Fig. 2) like a puzzle.

Figure 1. Volaticotherium in situ, in X-ray, as originally traced (line drawing) and DGS traced (colors). Patagium is the large tan ovoid.

Earlier I made mistakes
in a reconstruction (somewhat repaired here, Fig. 2) and the large reptile tree (LRT, 1158 taxa, subset Fig. 3)) was missing taxa that were close to Volaticotherium: Morganucodon and Ukhaatherium.

Figure 2. A new reconstruction of the skull of Volaticotherium along with the skull of Morganucodon to scale. See text for details.

Ukhaatherium
(Late Cretaceous late survivor of a Triassic radiation) is basal to both Morganucodon and Volaticotherium in the LRT.

Figure 3. Subset of the LRT focusing on basal mammals, including Volaticotherium.

Note that, like Volaticotherium,
Morganucodon also has only two large upper molars and a similar nasal shape. Also note that simpler, linear shape of the Volaticotherium cusps is derived relative to the ancestral Ukhaatherium.

Ukhaatherium nessovi (Novacek et al. 1997; Campanian, Late Cretaceous, 80 mya) is known from eight partial skeletons. Wikipedia reports, 4 premolars and 3 molars, but the break in dental shape happens between premolar 3, typically the largest and last one, and the next tooth, molar 1 of 4. The lumbar region is curved. Traditionally considered an Asioryctidae, close to placentals, but with epipubic bones, here it nests at the base of the Metatheria (see below), between Juramaia+ kin and Eomaia + kin. In size and shape the skull resembles that of Morganucodon.

Morganucodon watsoni (Kühne 1949; Late Triassic, 200–164mya; 2–3cm skull length) is known from fissue fills (abundant, 3D and completely disarticulated) fossils from Wales and China. This insectivore was like a shrew. The anterior teeth had a single replacement. Traditionally considered a mammaliaform, here Morganucodon nests with Ukhaatherium at the base of the Theria, close to the base of the Mammalia (= Prototheria). The jaw retains some tiny rear jaw elements, so the hearing was not perfected yet.

Volaticotherium antiquus (Meng et al. 2006; Middle to Late Jurassic, 164 mya; 5 cm skull length; IVPP V14739) was described a few years back as a gliding mammal of uncertain affiintiy. It is based on a preserved patagium, or gliding membrane, complete with short hair and skin. Here, derived from a sister to Ukhaatherium and Morganucodon, this is the last of its kind, at present. The molars resemble rotary saw blades, the external naris is divided by an ascending process of the premaxilla (rare among higher cynodonts and mammals), proximally the femur has no ‘neck’ and not much of a ‘head’, and the tail is extraordinarily long. We also see a deeper mandible medial to sabertooth fangs in Thylacosmilus, and this may be the reason for the oddly deeper chin here.

References
Kuehne WG 1949. On a triconodont tooth of a new pattern from a fissure-filling in South Glamorgan. Proceedings of the Zoological Society of London 119:345-350
Meng J, Hu YM, Wang YQ, Wang XL and Li CK 2006. A Mesozoic gliding mammal from northeastern China. Nature 444:889-893.
Novacek MJ, Rogier GW, Wible JR, McKenna MC, Dashzev g D and Horovitz I 1997. Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia Nature 389: 483-486.

 

Dryolestes, a tiny mammal mandible with many molars

Figure 1. Having 7 or 8 molars is very unusual for mammals. So finding a close match for Dryolestes is easy, once you have Docodon and Amphitherium. Late Jurassic Dryolestes is the most derived of these three based on the anterior lean of the coronoid process and the shamrock-shaped molars, reduced from their Middle and Late Jurassic relatives. 

With only the mandible + teeth to work with
(Fig. 1) the data on Dryolestes is too sparse to enter into the LRT, but having 8 molars in the Late Jurassic is atypical enough to make finding a close relative relatively easy—if you have Docodon and Amphitherium (Fig. 1) in your list. The best place to look for tiny Jurassic mammals with so many molars is within the Prototheria (Monotremata), which now includes extant toothless taxa.

Wikipedia offers
a cladogram that nests Dryolestes close to Amphitherium, but the next taxon at the next node is Vincelestes, which has very few molars and large saber teeth and therefore is in no way related. Docodon does not appear on that cladogram. The plasticity of the tooth shapes is quite apparent here. Dryolestes looks like to was more interesting in sieving than in cracking beetle shells. But who knows?

All these taxa have been known for over 150 years. Wikipedia reports, “It has been suggested that this group [Dryolestoidea] is closely related to modern therian mammals.  Dryolestid dentition is thought to resemble the primitive mammalian dentition before the marsupial-eutherian differentiation and dryolestids are candidates to be the last common ancestor of the two mammalian subclasses.”

The burrowing placental, Necrolestes, is listed as a dryolestoid in Wikipedia.

References

wiki/Dryolestes
wiki/Dryolestidae
wiki/Dryolestoidea

Sinodelphys: not a marsupial in the LRT

2003 was just too early for this taxon to be properly nested.
Sinodelphys (Luo et al., 2003) was considered the oldest known metathere (= marsupial) and was compared with Didelphis, the extant Virginia opossum. Here in the large reptile tree (LRT, 1250 taxa, subset Fig. 1) Sinodelphys nests between Chaoyangodens and Brasilitherium + Kuehneotherium among the prototheres, basal egg-laying mammals. Sinodelphys may have been mistakenly nested because Chaoyangodens and Brasilitherium are newer taxa. Several of the other taxa are also more recently published.

Figure 2. Mesozoic prototherians + Megazostrodon, the last common ancestor of all mammals. Only two taxa (gray) are post-Cretaceous.

The Luo et al. study nests Sinodelphys
just inside the Metatheria, very close to the Eutheria/Metatheria split. Among taxa both analyses have in common, very few have matching sister taxa. Many are not even in the same large clade (Eutheria/Metatheria/Prototheria). This may be due to an over reliance on dental traits in the Luo et all. study and an under reliance of dental traits in the LRT, which employs a wider gamut of taxa (vs. taxon exclusion in the Luo et al. study).

Figure 2. Sinodelphys skeleton in situ with select bones colored using DGS.

Clearly
Sinodelphys has a dorsal naris with short ascending processes on the premaxilla, not a terminal naris opening anteriorly. This trait alone nests Sinodelphys with the egg-laying mammals. Even so, a long list of traits support that nesting. Perhaps if Sinodelphys were described today, after so many other prototheres have been reported, it would have been identified as one of them.

Figure 3. Skull and forelimbs of Sinodelphys in situ. Arrow shows the displacement of the entire hand that otherwise appears to be lost beyond the matrix. How fortuitous!

With an inch-long skull
this is a tiny Early Cretaceous egg-layer, ancestral to today’s platypus and echidna.

Figure 4. Reconstruced skull of Sinodelphys based on DGS methods. This is very close to Brasilitherium, but with a larger set of canines. Like other prototheres, the nares are dorsal, not terminal.

The fingers on both hands are jumbled up (Fig. 3).
If Luo et al. are correct in their manus reconstruction, the only change I would make is to flip it left to right. Note their digit 5 is missing the proximal phalanx (Fig. 5). That is more likely the thumb because then digits 3 and 4 are the longest, as in sister taxa in the LRT.

Figure 5. Manus of Sinodelphys as originally reconstructed. Flipping the hand, as in the revised image, more closely matches sister taxa with digits 3 and 4 the longest.

References
Luo Z-X, Ji Q, Wible JR and Yuan C-X 2003. An Early Cretaceous tribosphenic mammal and metatherian evolution. Science 302:1934–1939.