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).

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.

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.

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

Rhyncholestes: it’s supposed to be a marsupial…

…but Rhyncholestes raphanurus (Osgood, 1924; long-nosed shrew-opossum, Chilean shrew opossum, extant; snout-vent length 20cm), nests in the large reptile tree (LRT, 1259 taxa) between the squirrel-like tree shrew, Apatemys, and a large living shrew, Scutisorex, all within the placental clade, Glires. Wikipedia and other sources consider this shrew-like South American mammal a marsupial, but Wiki also notes that Rhyncholestes lacks a marsupium (pouch). Females have seven nipples.

Figure 1. Skull of Rhyncholestes along with in vivo photo.

Figure 1. Skull of Rhyncholestes along with in vivo photo.

Rhyncholestes appears to be terrestrial,
nocturnal and an omnivore with a very restricted range (central Chile). Unlike the common shrew, Scutisorex, Rhyncholestes has a complete zygomatic arch and 3 large molars + a fourth vestige molar. That may be why it was considered a marsupial… but that would be pulling a Larry Martin in the LRT, where you need hundreds of traits to determine where a taxon nests.

 

Figure 1. The shrew Scutisorex compared to the apatemyid, Labidolemur from the early Eocene. Despite the difference in time, the teeth are still quite comparable.

Figure 2. The shrew Scutisorex compared to the apatemyid, Labidolemur from the early Eocene. Despite the difference in time, the teeth are still quite comparable. More to the point of today’s blogpost, fewer teeth on a shorter rostrum here than on Rhyncholestes, but otherwise, scores about the same in the LRT.

References
Osgood WH 1924. Review of living caenolestids with description of a new genus from Chile. Field Museum of Natural History Zoological Series 14, 165–173.

wiki/Long-nosed_caenolestid
https://sib.gob.ar/ficha/ANIMALIA*rhyncholestes*raphanurus

What is Periptychus carinidens?

Figure 1. Subset of the LRT focusing on the nesting of Periptychus.

Figure 1. Subset of the LRT focusing on the nesting of Periptychus.

Short answer:
In the large reptile tree (LRT, 1253 taxa, subset Fig. 1) Periptychus (Figs, 2–4) nests between basal phenacodonts like Phenacodus, Thomashuxleya and Pleuraspidotherium, and derived phenacodonts, like Gobiatherium + Arsinoitherium and Coryphodon + Uintatherium. These are all extinct herbivores from a clade that was recovered here first. Some derived taxa had ornate skull bumps/horns.

Previously known
as a condylarth from less complete materials (Cope 1881), the latest academic paper on Periptychus (Shelley, Williamson and Brusatte 2018) was still unable to determine closest relatives based on new data. No cladogram was presented. Sisters listed above were not listed in the text. Rather, the authors called it, “A robust, ungulate-like placental mammal.” 

Figure 2. Periptychus skull in 3 views.

Figure 2. Periptychus skull in 3 views ftom Shelley, Williamson and Brusatte 2018, colors added.

Think of Periptychus as a placental herbivore with very primitive feet…

Figure 3. Periptychus skeleton restored.

Figure 3. Periptychus skeleton restored from Shelley, Williamson and Brusatte 2018.

… and hands (no reduced digits). This mammal is remarkable for its long list of unremarkable traits.

Of course,
this was only the ‘warm-up act’ for the big, bizarre uintatheres to follow.

Figure 4. Manus and pes of Periptychus with some bones restored.

Figure 4. Manus and pes of Periptychus with some bones restored.

 

References
Cope ED 1881. The Condylarthra (Continued). American Naturalist 84;18: 892–906.
Shelley SL, Williamson TE and Brusatte SL 2018. The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America. PLoS ONE 13(7): e0200132.

https://doi.org/10.1371/journal.pone.0200132

Marmosa, Caluromys and Chironectes: the living, breathing origin of the Eutheria

These are the mouse, wooly and water opossums:
Marmosa (Fig. 1), Caluromys (Fig. 4) and Chironectes (Figs. 8, 9). As traditional didelphids, they’ve received too little attention. In a world in love with DNA phylogenetic analysis, they’ve received too little attention. In the large reptile tree (LRT, 1252 taxa, subset Fig. 3) these are the much sought after transitional taxa between Metatheria (marsupials) and Eutheria (placentals).

As simple and logical as this sounds
the present hypothesis of interrelationships (Fig. 3) is heretical. From Novacek 1989, 1992 to Tarver et al. 2016 other workers have placed armadillos, pangolins and elephants at the base of the Eutheria using gene analyses. As mentioned earlier, it is discouraging to see serious paleontologists (references below, including a certain science blogger) among the ‘believers’ as they embrace and put their faith in a method (gene analysis) that fails to deliver a gradual accumulation of derived traits at every node in large phylogenetic analyses, hoping for eventual redemption. They just accept the results without questioning. And that is surprising, because as a professor, you can’t really explain to students how these results gradually evolve. Rather these studies mix up and confuse the placental clades as others have mixed up the bird clades using DNA. We’ll take a look at these influential placental DNA papers and list their problems in detail in a few days. It’ll be horrible, untenable and illogical, so prepare yourself.

FIgure 1. Marmosa murina in vivo.

FIgure 1. Marmosa murina in vivo. Yes, this pouch less marsupial is carrying babies in front of its thigh. This is what basal placentals, like bats and flying lemurs do.

 

Marmosa murina
(Gray 1821, Voss and Jansa 2009) 
is one of 19 species of ‘mouse opossums’ native from Mexico to Argentina. In the large reptile tree (LRT, 1252 taxa, Fig. 3) Marmosa nests at the base of the last metatherian clade prior to the origin of eutherians (placentals), the clade that includes Monodelphis, and Chironectes (a swimmer). Like other mouse opossums (Fig. 5) Marmosa lacks a marsupium (= pouch) like its sisters (Fig. 5).

Marmosa waterhousi (Gray 1821) skull is shown below (Fig. 2).

Figure. 2. Marmosa waterhousi skull.

Figure. 2. Marmosa waterhousi skull.

 

Caluromys derbianus
(Allen 1904; Fonseca and Astúa 2018; Fig. 4) is the living ‘wooly opossum’, native to Central America. Sometimes it feeds inverted as seen in bats and hypothesized for pre-bats. It is an omnivore, like related placental carnivore, Nandinia.

Caluromys nests just inside of the first placental clade, Carnivora, alongside Vulpavus (Fig. 6), a taxon omitted from all prior papers on didelphids. Basal Carnivora are larger than other basal shrew- and mouse-sized placentals. In like fashion, Caluromys is the largest of these opossums, similar in size and shape to Vulpavus.

Figure 4. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

Figure 3. Subset of the LRT focusing on the Metatheria (=Marsupials). Here the diprotodont dentition evolved twice.

As we discussed earlier
here regarding Mondelphis (a genus including 22 species of short-tailed opossum) and the origin of bats and dermopterans, the transition from metatherians to eutherians was a gradual one that took place at this phylogenetic transition. So there is no great revelation here, just more evidence piling on.

Figure 1. Caluromys skull and mandible (sized to fit).

Figure 4 Caluromys skull and mandible (sized to fit).



Voss and Jansa found a ‘pouch’ in Caluromys,
but no pouch in the slightly more primitive and perhaps more plesiomorphic, Marmosa and Monodelphis. They report, “The marsupium of Caluromys philander uniquely consists of deep lateral skin folds that enclose the nursing young and open in the midline.” 

But wait!
In this regard the marsupium of Caluromys more closely resembles that of placental dermopterans and bats, taxa that expand these deep lateral skin folds to create newborn nurseries and ultimately, gliding membranes. Voss and Jansa do not mention the term ‘Eutheria’ and do not mention placentals as descendants of mouse opossums in their paper. This was an opportunity missed, but resolved here.

Didelphids
Take a look at the nesting of Didelphis in the LRT (subset in Fig. 3) and you’ll see that this is the primitive clade from which all other metatherians evolved. Most large carnivores and herbivores split off on a separate clade, leaving the mouse-sized didelphids (the Proeutherians) a more direct route to the Eutherian grade. This hypothesis of interrelationships has not been noticed or published before.

Pouch-less marsupials?
Why not just call them what they are? Transitional taxa. This is exactly how the Eutheria evolved from the Metatheria. Is this a heretical hypothesis? Or is it just another overlooked hypothesis that should have been proposed a century ago.

Figure 6. Mammary glands in pouchless marsupials. These taxa have not been tested in the LRT.

Figure 5. Mammary glands in pouch-less marsupials (mouse opossums). Pouch-less marsupials? Why not just call them what they are? Transitional taxa.

Other hypotheses
In the pre-cladistic era, Lillegraven et al. 1987 described the origin of Eutherian mammals “with high intensity food habits, small body masses and adaptations to very cold climates.” The authors focused on soft tissue traits the involve reproduction and metabolismn and put forth a hypotheses as to how nonspecific eutherians could have arisen from nonspecific metatherians… when they could have just studied mouse, wooly and water opossums and removed the guesswork. As mentioned above, modern authors delved far astray in their search for taxa at this transition.

In a very real sense
when you look at these images of mouse, wooly and water opossums you’re looking at an excellent example of the last common ancestors of all placental mammals, probably originating in the Early Jurassic (based on the first appearance of placental multituberculate Megaconus in the Middle Jurassic, in the LRT). These small didelphids are not terminal taxa. They are living breathing late-surviving representatives of an Early Jurassic split between pouch-less metatherians and pouch-less eutherians.

Figure 8. Caluromys, the largest of the mouse opossums, to scale with its LRT sister, Vulpavus, a basal member of Carnivora.

Figure 6. Caluromys, the wooly opossum, to scale with its LRT sister, Vulpavus, a basal member of Carnivora.

Here’s an unexpected finding:
Caluromys, the woolly opossum, nests as the basalmost member of the Carnivora (Fig. 3), but it retains a pouch. Time in the pouch is not particularly short. Size at birth is not particularly large. Sister taxa, including Vulpavus and Deltatherium, are both extinct, so we don’t know whether they had a pouch, but we know that on the main branch of carnivores, starting with Nandinia, the pouch was gone, convergent with mouse opossums (Fig. 5). Caluromys also has more molars than other carnivores and a longer nasal bone.

But remember,
in phylogeny it’s not the particular cherry-picked traits that determine what clade a taxon is a member of, its the nesting within a clade based on a suite of traits that is paramount.

So, similar to mammal-like reptiles,
amphibian-like reptiles, walking whales and dinosaur-like birds, Caluromys was a very basal metathere-like carnivore. And that’s how evolution really works in trait analysis.

Figure 8. Chironectes minimus skull.

Figure 8. Chironectes minimus, the water opossum, skull.

We didn’t spend much time with the water opossum, Chironectes.
It’s important to note that it, too, has a pouch. This sole aquatic marsupial has a water-proof pouch with a unique sphincter for access. And it nests in the LRT as the proximal outgroup taxa to the Eutheria, although the aquatic niche and webbed feet are autapomorphies not retained in descendant taxa among the placental mammals. These traits have had the entire Cretaceous and Cenozoic to develop after that phylogenetic split.

When you’re looking for transitional taxa,
keep looking for the little, plain, brown taxa and you will often find them.

Figure 9. Chironectes minimus, the water opossum, in vivo.

Figure 9. Chironectes minimus, the water opossum, in vivo. This sole aquatic marsupial has a water-proof pouch with a unique sphincter for access.

References
Allen JA 1904. Mammals from southern Mexico and Central and South America. Bulletin American Museum of Natural History 20(4): 29-80.
Burnett GT 1830. Illustrations of the Quadrupeda, or Quadrupeds, being the arrangement of the true four-footed Beasts indicated in outline. Quarterly Journal of Science, Literature and Art, July to December, 1829, 336–353.
Cifelli RL 1993. Theria of metatherian-eutherian grade and the origin of marsupials. In FS Szalay, MJ Novacek, and MC McKenna (editors), Mammal phylogeny: Mesozoic differentiation, multituberculates, monotremes, early therians, and marsupials, 205–215. New York: Springer.
Gray JE 1821. On the natural arrangement of vertebrose animals. London Medical Repository 15(1):296–310.
Hallstrom BM, Kullberg M, Nilsson MA and Janke A 2007. Phylogenomic data analyses provide evidence that Xenarthra and Afrotheria are sister groups. Molecular Biology and Evolution 24, 2059–2068.
Lillegraven JA, Thompson SD, McNab BK and Patton JL 1987. The origin of eutherian mammals. Biological Journal of the Linnean Society 32:281–336.
Murphy WJ, et al. 2001. Molecular phylogenetics and the origins of placental mammals. Nature 409, 614-618.
Naish D 2015. The Refined, Fine-Tuned Placental Mammal Family Tree. scientificamerican.com/tetrapod-zoology/
Novacek MJ 1989. Higher mammal phylogeny: the morphological-molecular synthesis. In Fernholm, B., Bremer. K. & Jornvall, H. (eds) The Hierarchy of Life. Elsevier, Amsterdam, pp. 421-435.
Novacek MJ 1992a. Fossils, topologies, missing data, and the higher level phylogeny of eutherian mammals. Systematic Biology 41, 58-73.
Novacek MJ 1992b. Mammalian phylogeny: shaking the tree. Nature 356, 121-125.
Pine RH, Flores DA and Bauer K 2013. The second known specimen of Monodelphs unistriata (Wagner) (Mammalia: Didelphimorphia), with redescription of the species and phylogenetic analysis. Zootaxa3640 (3):425-441.
Tarver JE et al. 2016. The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference. Genome Biol. Evol. 8(2):330–344. doi:10.1093/gbe/evv261
Voss RS and Jansa SA 2009. Phylogenetic relationships and classification of didelphid marsupials, an extant radiation of New World metatherian mammals. Bulletin of the American Museum of Natural History, no. 322. PDF
Wildman et al. 2007. Genomics, biogeography, and the diversification of placental mammals. Proceedings of the National Academy of Sciences of the United States of America 104, 14395-14400 PDF.

wiki/Monodelphis
wiki/Marmosa
wiki/Caluromys derbianus
https://animaldiversity.org/accounts/Chironectes_minimus/
wiki/Water_opossum

Eutherian phylogeny and niches

Over the past two weeks
I’ve been attracted to poor Bootstrap scores in the large reptile tree (LRT, 1151 taxa, subset Fig. 1) reexamining data and re-scoring where necessary. The result is a tree with improved Bootstrap scores. Herewith, the eutherian (placental) mammal subset of the LRT.

Figure 1. Subset of the LRT focusing on eutherian mammals. Colors refer to niches.

Figure 1. Subset of the LRT focusing on eutherian mammals. Colors refer to niches.

Sharp-eyed readers
will find the one node that is not resolved in this tree. Hint: the specimens lacking resolution are known from damaged skulls and a few post-cranial bones, so they can be scored for a relatively few character traits.

Curious readers
seeking more information for any genera listed above need only use it for a keyword in the search feature of this blog post (above).

Even though
the present tree has been improved, there is still room for improvement, probably around the weaker Bootstrap scores.

 

Heuristic testing
of just the basal tetrapods and lepidosauromprhs (370 taxa, 1 tree) took less than 51 seconds for a completely resolved subset of the LRT. Testing of just the archosauromorphs (781 taxa, 2 trees) took 8:45 minutes of computing time. So, 410 more taxa and one more tree take more time.

Taking it to the final step: Testing of the entire LRT (1151 taxa, 14 trees) took 1 hour 50 minutes. You can see computing time rises exponentially with increasing taxa, even with the next best thing to complete resolution.

So where did those 12 extra trees come from?
Should be from no more than 3 unresolved nodes. Here’s where PAUP fails (or becomes exhausted) with high taxon numbers:

Basalmost Synapsida  (Ellioitsmithia, Apsissaurus, Aerosaurus, etc.), Lepidosauria/Sphenodotia/Marine Younginiformes/Diadectomorpha + Pareiasauria/ Caseasauria/Basal Lepidosauromorpha/ Basal Archosauromorpha/ Basal Diapsida/ 13 more little clades/15 single taxa and…Hypuronector/Vallesaurus/Megalancosaurus

So with all those problems
(way more than expected) I ran PAUP again, sans mammals and terrestrial younginiforms (including protorosaurs and archosauriforms): so…. basically all the primitive taxa were included. Result: 565 taxa, 2 trees) took 5:54 minutes with loss of resolution between (Megazostrodon + Hadrocodium) and (Brasilitherium + Kuehneotherium), three of which are skull-only taxa just outside of the deleted mammals. No other tree topology changes are recovered.

Just so you know…
it seems that PAUP does exhaust itself in large cladograms, even in a simple Heuristic search.

 

Speaking of taeniodont origins, there’s another candidate: Cimolestes

Yesterday we looked at the origin of taeniodonts, like Stylinodon. and found it nested with Mustela the mink and Phoca the seal. Other workers (Lillegraven 1969, Rook and Hunter 2013) indicated that Cimolestes (Fig.1, Late Cretaceous) was a suitable ancestor to the taeniodonts. So, let’s look at Cimolestes and compare it to related taxa.

Figure 1. Cimolestes mandible from Lillegraven 1969 compared to a phylogenetically basal eutherian the marsupial without a pouch, Monodelphis, the basal tenrec, Maelestes and Cimolestes. All have a slender mandible.

Figure 1. Cimolestes mandible from Lillegraven 1969 compared to a phylogenetically basal eutherian the marsupial without a pouch, Monodelphis, the basal tenrec, Maelestes and Cimolestes. All have a slender mandible without the anterior depth found in Stylinodon, Mustela and Martes in figure 2.

In comparison
Cimolestes is more like the basal marsupials Monodelphis and Maeilestes (Fig. 1) in having a rather slender mandible with incisors anterior to the canines. By contrast, the carnivores Martes, the martin, and Mustela (Fig. 2), and the taeniodonts, Wortmania (Fig. 3) and Stylinodon have a robust mandible, deep anteriorly with canines to the anterior and incisors between them.

Figure 2. Martes, the extant martin, and Mustela, the extant mink or polecat mandibles. Both are deeper in front, more like the taeniodont, Stylinodon.

Figure 2. Martes, the extant martin, and Mustela, the extant mink or polecat mandibles. Both are deeper in front, more like the taeniodont, Stylinodon. Note the number of teeth varies among these closely related taxa.

Lillegraven 1969 wrote: “A smaller carnivorous species described as new of Cimolestes probably represents a primitive stage in the development of miacids, and subsequently fissiped and pinniped carnivores.” Well, we’re all in the same ballpark and thinking along similar lines. Not sure where Cimolestes nests in the LRT yet. Not much is known of it, other than jaw fragments.

Figure 6. Wortmania as drawn freehand by Schoch compared to bones Photoshopped together.

Figure 6. Wortmania as drawn freehand by Schoch compared to bones Photoshopped together.

References
Lillegraven JA 1969. Latest Cretaceous mammals of upper part of Edmonton formation of Alberta, Canada, and review of marsupial-placental dichotomy in mammalian evolution. Article 50 (Vertebrata 12) The U. of Kansas Paleontological Contributions. 122pp.
Rook DL and Hunter JP 2013. rooting around the eutherian family tree: the origin and relations of the Taeniodonta. Journal of Mammal Evolution. DOI 10.1007/s10914-013-9230-9

wiki/Cimolestes

Nesting twin-horned Arsinoitherium within the Condylarthra

Figure 1. Famous and enigmatic, Arsinoitherium has been known for over a century, and traditional paleontologists still do not know what it is.

Figure 1. Famous and enigmatic, Arsinoitherium has been known for over a century, and traditional paleontologists still do not know what it is.

FIgure 1. Subset of the large reptile tree, the Condylarthra, featuring Astrapotherium. Note the phylogenetic proximity of Astrapotherium and Tapirus.

FIgure 1. Subset of the large reptile tree, the Condylarthra, featuring Astrapotherium. Note the phylogenetic proximity of Astrapotherium and Tapirus.

Traditionally Arsinoitherium zitteli has been hard to classify.
Wikipedia reports, “Arsinoitherium (Beadnell 1902; Eocene-Oligocene, 36-30mya; 3 m in length; Fig. 1) is related to elephants, sirenians, hyraxes and the extinct desmostylians.” That’s a pretty broad gamut of taxa.

And they’re all wrong according to the large reptile tree (now 812 taxa, subset Fig. 2).

And this came as a surprise to me, too
among 811 other taxa, Arsinoitherium nests with Gobiatherium mirificum (Fig. 3; Osborn and Granger 1932; Middle Eocene), which Wikipedia considers, “one of the last uintatheres” of which Uintatherium is the titular and most famous member. Wikipedia goes on to report, “Gobiatherium lacked knob-like horns, or even fang-like tusks. Instead, it had enlarged cheekbones and an almost spherical snout. Because of the noticeable lack of many diagnostic uintathere features (the horns and tusks), the genus is placed within its own subfamily.” Here’s where tradition and the LRT agree… but let’s push this a little further to see where it takes us within the friendly confines of the current LRT taxon list.

Figure 3. Gobiatherium skull in three views. Though not immediately apparent, Gobiatherium is closest to Arsinoitherium in the LRT.

Figure 3. Gobiatherium skull (A. M. 26624) in three views. Though not immediately apparent, Gobiatherium is closest to Arsinoitherium in the LRT. Image from Osborn and Granger 1932.

Among all tested placental taxa, and despite distinct overall appearances
only Arsinoitherium and Gobiatherium:

  1. redevelop the ascending process of the premaxilla, completely enclosing the naris;
  2. produce a wide, elevated set of nasals, further expanding into horns in Arsinoitherium;
  3. only two molars, rare among placentals;
  4. and no other condylarths have a wide flat cranium, usually a crest or a convex cranium is present.

That premaxillary ascending process
looks so normal. But among marsupial and placental mammals it is very rare indeed! Of course, the LRT does not depend on one or several traits, several dozen nest Arsinoitherium with Gobiatherium and their sisters.

Even without Gobiatherium
Arsinoitherium nests with Uintatherium. Coryphodon nests closer to Uintatherium. All descend from a sister to Thomashuxleya (Fig. 4), which we’ll look at soon in greater detail.

Figure 4. Thomashuxleya is basal to uintatheries and arsionoitheres. It is not a notoungulate, an invalid taxon.

Figure 4. Thomashuxleya is basal to uintatheries and arsionoitheres. It is not a notoungulate, an invalid taxon.

We hold as an ideal
a gradual accumulation of derived traits in derived taxa, like Gobiatherium and Asinoitherium. In this clade, unfortunately we don’t have enough taxa to make that gradual accumulation of traits any more gradual than it currently is. This is the best we can do, at present, with available data and the present taxon list.

But it’s a good start!
And closer than anyone figured out before.

References
Beadnell HGC 1902. A preliminary note on Arsinoitherium zitteli, Beadnell, from the Upper Eocene strata of Egypt. Public Works Ministry, National Printing Department. Cairo: 1–4.
Lucas SG 2001. Gobiatherium (Mammalia: Dinocerata) from the Middle Eocene of Asia: Taxonomy and biochronological Significance. Paläontologische Zeitschrift 74 (4): 591–600.
Osborn HF and Granger W 1932. Coryphodonts and uintatheres from the Mongolian expedition of 1930. American Museum Novitates 552:1-16.

wiki/Arsinoitherium
wiki/Gobiatherium

The differences between Eutheria and Metatheria are a little fuzzy

Updated September 22, 2018
with a note that 1293 taxa now are listed in the LRT. In some of these taxa more rules are broken.

Eutheria are placental mammals. Metatheria are marsupial mammals. Traditionally the latter have a pouch and epipubic bones. Placentals do not. According to Wikipedia eutherians have:

  1. Enlarged malleolus process on the distal tibia.
  2. First metatarsal offset medially compared to second metatarsal
  3. Various features of the jaws and teeth.

Not really much to go on, is it? But all is not lost:
According to Wikipedia, metatherians have:

  1. “Four pairs of molar teeth in each jaw, whereas eutherian mammals (including true placentals) never have more than three pairs.”

UnfortunateIy
I found several eutherians with four pairs of molar teeth, including Asioryctes (Fig. 1), Leptictis and Maiacetus, the stem whale with legs.

Figure 1. Skulls of mammals at or near the Metatheria/Eutheria split. Here Asioryctes has four pairs of molars, like Didelphis and Monodelphis. Leptictis also has four pairs of molars.

Figure 1. Skulls of mammals at or near the Metatheria/Eutheria split. Here Asioryctes has four pairs of molars, like Didelphis and Monodelphis. Leptictis also has four pairs of molars. Note the similarities and differences here as each of these taxa are basal to a long list of other mammals. They should look alike because phylogenetically they are not that far from each other.

On the same subject, Carroll 1988 reports on marsupials:

  1. Tooth replacement is limited to the 3rd premolar, not the molars.
  2. Most have 3 premolars and 4 molars, while most placentals have 4-5 premolars and 3 molars.
  3. The jugal extends to the jaw joint
  4. The reflected angular process is present in marsupials… and some early eutherians.
  5. Tooth details

I noted earlier
that the purported marsupial Monodelphis (Fig. 1) does not have a pouch, but does have epipubic bones. Also note that in Asioryctes the jugal also extends to the jaw joint.

All is not lost.
As we learned earlier with basal reptiles (=amniotes) traditional traits should not be used to identify basal members of any clade. Those traits might not get it right. Rather provisional basal taxa are identified during phylogenetic analysis as the last common ancestors of clade members. >After< nesting their traits can be listed.

In the large reptile tree (776 taxa) the basal eutherians Eomaia and Monodelphis are distinguished from tested outgroup metatherians by the following traits (subject to change/evolve in derived eutherians, and I’m including pre- mid- and post-node changes for these two taxa, since the line of division has not been officially set, or is due to be upset after this):

  1. Post-orbital cranial length greater than pre-orbital rostrum length
  2. Naris opening is anterior, with the premaxilla filling in any anterolateral exposure
  3. The frontal nasal angle is anteriorly oriented, not zig-zag
  4. The squamosal descends only to mid skull, not the tooth row.
  5. An ectotympanic develops to ventrally cover and isolate the tiny ear bones
  6. The opisthotic no longer descends as a separate bone, but becomes fused to surrounding occipital bones or extends laterally.
  7. The second sacral rib (transverse proceess) is fused to the first.
  8. Prepubis (epipubis) bone is absent (but I expect this to change with future discoveries as some derived taxa either redevelop or retain it)
  9. Longest metatarsal(s): 3 and 4

In the end
the clade Mammalia is identified (after phylogenetic analysis) by the presence of mammary glands, implying toothless hatchlings/new borns and single tooth replacement, along with a firm dentary squamosal joint and the detachment of the posterior mandible elements that continue to shrink to become middle ear bones). The clade Eutheria is a little more difficult to describe, but I hope the above list leads to further refinement of this issue and the removal of false paradigms that might confuse students reading texts based on Carroll 1988 or other widely read academic works.

References
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.

 

 

Meet the outgroup sister to all eutherian mammals: Monodelphis, and its ALIVE!

Okay, I know this comes as no surprise…
a mouse-like marsupial at the origin of placental (eutherian) mammals. That’s old news.

Figure 1. The marsupial, Monodelphis domestica, nests as a sister to Eomaia, the oldest known placental.

Figure 1. The marsupial, Monodelphis domestica, nests as a sister to Eomaia, the oldest known placental.

But this EXTANT mouse-like marsupial
has traits found in basal eutherian mammals like Eomaia… not found in other tested marsupials… which is exactly the way it should be. We’re always looking for a gradual accumulation of traits in the large reptile tree (LRT, 764 taxa, subset Fig. 2).

So far among marsupials
Didelphis, the Virginia opossum nests basal to all other tested marsupials. Three marsupials nest together in a clade, Thylacinus, Dromiciops and Macropus. One marsupial, Monodelphis, nests between these taxa and the basal eutherian, Eomaia. So marsupials (Metatheria) are forming a grade, not a clade.

Monodelphis is approximately 50 times smaller in body size than Didelphis, and lacks a pouch which is found in the latter.

And
phylogenetic miniaturization strikes again, with a mini-opossum at the base of the Eutheria (if we delete the concept of time, of course).

Figure 2. Mammals and their ancestors as a subset of the large reptile tree placed against their time periods. Note that several primitive taxa are still alive today. It appears that the early radiation of the mammals occurred in the Jurassic, with a second radiation following the Cretaceous.

Figure 2. Mammals and their ancestors as a subset of the large reptile tree placed against their time periods. Note that several primitive taxa are still alive today. It appears that the early radiation of the mammals occurred in the Jurassic, with a second radiation following the Cretaceous.

And
Palaesinopa nests basal to the seal, Phoca. That’s the other new taxon added here.

Time
By placing the appearance of each taxon on their time stratum, we can see that some basal mammals, like Ornithorhynchus, appear only in the present. The first radiation of basal carnivorous and insectivorous mammals was in the Jurassic. A second radiation of derived mammals occurred after the Cretaceous. There was no radiation of rodent-like pre-eutherian multituberculates in the Jurassic. Rather mutituberculates are rodent sisters, implying that rodents were around in the Jurassic, too. We just have not found them.

Future finds
may modify the details, but the tree topology appears to be strong. This is a simplified topology without several pre-eutherian clades. It does not represent traditional hypotheses of interrelationships, but introduces a heretical new hypothesis that appears to indicate that many, but not all, mammal clades had deeper roots than previously thought.

References
Burnett GT 1830. Illustrations of the Quadrupeda, or Quadrupeds, being the arrangement of the true four-footed Beasts indicated in outline. Quarterly Journal of Science, Literature and Art, July to December, 1829, 336–353.
Macrini TE 2004. Monodelphis domestica. Mammalian Species 760:1-8.
Pine RH, Flores DA and Bauer K 2013. The second known specimen of Monodelphs unistriata (Wagner) (Mammalia: Didelphimorphia), with redescription of the species and phylogenetic analysis. Zootaxa3640 (3):425-441.

wiki/Monodelphis
http://digimorph.org/specimens/Monodelphis_domestica/adult/