Phylogeny of the Carnivora – its topsy-turvy!

The large reptile tree
(LRT) presents a novel topology for many clades within the Reptilia. Among them is the Carnivora (Fig.1). The LRT uses fossil taxa and, you’ll note by comparison, is virtually upside-down (topsy-turvy, backwards) when it comes to trees recovered in molecular studies. That major difference MIGHT be traced to the choice of outgroup, as you will see…

Figure 1. Carnivora subset of the LRT with Monodelphis, a basal placental, as the outgroup, not Manis the pangolin.

Figure 1. Carnivora subset of the LRT with Monodelphis, a basal placental, as the outgroup, not Manis the pangolin.

Using molecular phylogenetics
(no fossils) Eizirik et al 2010 recovered a cladogram of the Carnivora that used Manis, the pangolin (Fig. 2), as the outgroup. Does this surprise you? …especially considering the fact that Manis has bounced around various nodes on the mammal family tree for decades. …and since it is toothless! And since it has scales instead of hair! etc. etc.

Figure 2. Manis, the Chinese Tree Pangolin along with other views of other pangolins

Figure 2. Manis, the Chinese Tree Pangolin along with other views of other pangolins

That, in itself, is very strange
to have a highly derived taxon used as a plesiomorphic outgroup. By contrast, in the LRT the outgroup is Monodelphis (Fig. 3), a tiny very plesiomorphic, opossum-like basal placental with origins in the Jurassic. And it has teeth!  And hair!

Figure 4. Entire skeleton of Monodelphis from Digimorph.org and used with permission.

Figure 3. Entire skeleton of Monodelphis from Digimorph.org and used with permission. This little taxon makes a great outgroup for the Carnivora that will flip topologies on their head when employed.

Using a pangolin as the outgroup
the Eizirik team recovered a basal split between feliforms and caniforms.

Feiliforms include Nandinia, then a split between cats and civets + hyenas + mongooses + fossas.

Caniforms include a basal split between wolves and bears + seals + raccoons + minks. Essentially these topologies are quite similar to the LRT, only in the opposite order with cats and dogs nesting in basal nodes, while minks and mongooses nest in derived nodes.

Notice the relatively flipped topologies
Can we blame this on the choice of an outgroup? On the lack of fossil taxa? On the inadequacies of DNA analyses across large clades? Or a little of all three?

Note that
Talpa, the extant Eastern mole, and Mondelphis, the extant gray short-tailed opossum were excluded a priori from the Eizirik study, but revealed by the large gamut analysis of the LRT, which minimizes a priori assumptions such as these.

Also using molecules
Wesley-Hunt and Flynn 2005 found a similar topology to the Eizirik study, turning the order recovered by the LRT on its head, with opossum-like carnivores (civets, minks) in derived nodes. This study used a variety of outgroups (Manis, ElephasLoxodonta, Equus, Bos, Sus, Homo) rather than Monodelphis. Results did not change the topology within the Carnivora.

Now is a good time to ask yourself,
Why did they use such silly, useless and obviously wrong outgroups rather than seek the one true plesiomorphic outgroup?

This is exactly why
this blog and ReptileEvolution.com were created — to throw back the curtain on such odd practices, methods and choices — AND produce viable alternative answers. These are experiments you can repeat yourself, BTW.

Let’s not forget
moles (Fig. 3) are carnivores, too!

Figure 2. Talpa the Eastern mole nests in the LRT with Herpestes the mongoose.

Figure 2. Talpa the Eastern mole nests in the LRT with Herpestes the mongoose.

References
Eizirik E, Murphy WJ, Koepfli KP, Johnson WE, Dragoo JW and O’Brien SJ 2010. Pattern and timing of the diversification of the mammalian order Carnivora inferred from multiple nuclear gene sequences. Molecular Phylogenetics and Evolution 56:49–63.
Wesley-Hunt GD and Flynn JJ 2005. Phylogeny of the Carnivores. Journal of Systematic Palaeontology. 3:1–28.

Mammal evolution analyses using molecules

Now that
the large reptile tree (LRT) has grown to encompass a large gamut of mammals based on shared morphological traits, it’s time to compare it with prior studies based on molecules. Some scientists say that molecular studies that do not include fossil taxa should take precedence over morphological studies that do include fossils. Some studies combine extant DNA and extinct morphological data. In any case, it is important that all pertinent taxa are included and that unrelated taxa are excluded — and that suprageneric taxa are avoided. And finally, stand back and check your work to make sure it makes sense (more on that below).

The base of the Placentalia in the LRT
begins with small, omnivorous. plesiomorphic Monodelphis. This taxon gives rise to a number of small fur balls, all similar in size and shape, but differing subtly and nesting at the bases of more diverse and derived clades. In succession the following clades split off: Carnivora (includes moles), Glires, arboreal mammals, tenrecs/odontocetes, edentates and finally the large herbivores splitting mesonychids, desmostylians and mysticetes from elephants, sirenians and ungulates. This study provides a gradual accumulation of traits from small plesiomorphic generalists to large derived specialists and includes extinct taxa. Importantly, the basalmost taxon is very much like a basal marsupial — as it should be!

By comparison
Meredith 2011
 – begins with Afrotheria (elephants/ sirenians/ elephant shrews/ tenrecs/ golden moles) + edentates, arboreals (sans bats)/ Glires, and finally moles/shrews/hedgehogs + pangolins/carnivores + bats + artiodactyls (including hippos + whales).  This study does not provide a gradual accumulation of traits from small plesiomorphic generalists to large highly derived specialists and does not include extinct taxa. The basalmost taxa are not close to any marsupials in appearance.

Margulies et al. 2007 – essentially repeat this topology. This study has the same problem.

Tree of Life project 1995 – begins with edentates + pangolins, then Glires + arboreals + insectivores + (carnivores + creodonts) + artiodactyls and whales +  aardvarks, + perissodactyls + hyracoids + tethytheres (elephants, embrithopods, desmostylians and sirenians).. This study has the same problem.

Song et al. 2015 – begins with edentates + elephants/ tenrecs, insectivores + bats + ungulates + carnivores + other ungulates + whales, Glires, tree shrews, primates. This study has the same problem.

In a condescending tone
Asher, Bennett and Lehmann 2009 added their research to the topic of mammal phylogeny. Note how often these authors use the word ‘believe’ with regard to the best efforts of prior scientists, none of whom put faith ahead of evidence.

“In the not so distant past, there was a lot of uncertainty regarding how clades of living mammals were interrelated. Many mammalian systematists believed that sengis (Macroscelididae or ‘elephant shrews’) were closely related to rabbits and rodents, that pangolins (Pholidota) were ‘edentates’ along with anteaters, or that tenrecs (Tenrecidae) and golden moles (Chrysochloridae) were ‘insectivorans’ along with shrews and hedgehogs. Some believed that hyraxes (Procaviidae) were part of the Perissodactyla, and others thought that bats were so close to primates that the non-echolocating ones actually were primates, or at least close enough to make Chiroptera paraphyletic. In contrast, the consensus today on each of these issues is not only quite different, but also resolved with a substantial level of confidence. Questions regarding character evolution among living mammals now have the decisive advantage of a relatively well-resolved tree.”

Asher, Bennett and Lehmann 2009 – begin with a basal split between Atlantogenata (edentates + elephants + elephant shrews) and Boreoeutheria (primates/ rodents + insectivores + carnivores + bats + ungulates (including whales). This study has the same problem(s). And I, for one, have no ‘substantial level of confidence’ in its results. A ‘relatively well-resolved tree’ that does not provide a series of taxa with gradually accumulating derived traits is no match for a completely resolved tree topology that does provide that gradual accumulation. Let’s keep our thinking caps on. 

Does anyone else see
that in each of these studies, bats and ungulates nest as closely related? That the highly specialized edentates and elephants nest basal to the little furry opossum-like omnivores? The LRT does not have these problems. And yes, I’m picking the low-hanging fruit, but those kinds of problems are your clue that it is best to ditch DNA for major clade interrelationships (but keep DNA for congeneric and criminal studies) and stick to morphology when you create your own tree topology). That way you can visually check your results! Stand back from your cladogram before you publish it and see if all nodes and branches form a continuous and logical sequence with only gradual changes apparent between sister taxa. And that basal taxa look like outgroup taxa. That’s why I show my work.

When it comes to whales
Geisler et al. 2011 – nested fossil and extant odontocetes and mysticetes arising from Zygorhiza. and Georgiacetus, two archaeocetes. The toothed taxa, Janjucetus, Mammalodon and Aetiocetus were nested as basal mysticetes. Sus (pig), Bos (cattle) and hippopotamidae (hippos) were outgroup taxa. This study appears to be accurate when it comes to extant whales. But this team assumed whales were monophyletic and thus haven them a common ancestor with fins and flukes. By contrast the LRT found toothed whales arising from toothed tenrecs and baleen whales arising from desmostylians, all of which have a long diastema (toothless region of the jawline) and dorsal nares.

References
Asher RJ, Bennett N and Lehmann T 2009. The new framework for understanding placental mammal evolution. BioEssays 31:853–864.
Geisler JH, McGowen MR, Yang G and Gatesy J 2011. A supermatrix analysis of genomic, morphological, and paleontological data from crown Cetacea. BMC Evolutionary Biology 11:112.
Margulies EH et al. 2007. Analyses of deep mammalian sequence alignments and constraint predictions for 1% of the human genome.
Meredith RW et al. 2011. Impacts of the Cretaceous terrestrial revolution and KPg Extinction on Mammal Diversification. Scence  334(6055):521-524.
Song S, Liu L, Edwards SV and Wu S 2015. Resolving conflict in eutherian mammal phylogeny using phylogenomics and multi species coalescent model. PNAS 109(37)14942-14947.

Docodon and molar count

The genus Docodon (Marsh 1881; Late Jurassic, 170 mya; Fig. 1) is represented by a jaw with several more molars than typical (Fig. 1). Hard to tell the premolars from the molars in lateral view. See the dorsal view for the distinct difference. Even so, the count may be off, because molars are not molars based on shape, but on the fact that they appear once and are not replaced during growth. I cannot tell, note have I found references that say where the division is in Docodon.

Figure 1. The holotype of Docodon has 4 incisors, 4 premolars and a whopping 7 molars.

Figure 1. The holotype of Docodon has 4 incisors, 4 premolars and a whopping 7 molars. diplocynodon has 8 according to Osborn, who confirms the 7 in Docodon.

More than four molars in a mammal jaw is relatively rare.
But not rare in ancestral monotremes. Among tested taxa  Amphitherium, Kuehneotherium and Akidolestes have 6.

Figure 1. The addition of teeth in Kuehneosaurus and Akidolestes led to the loss of teeth in Ornithorhynchus.

Figure 1. The addition of teeth in Kuehneosaurus and Akidolestes led to the loss of teeth in Ornithorhynchus.

There is also “a rule” that says
only one canine appears, but the other teeth can vary greatly in number. I’m wondering if that is true. Sometimes there will be a small, simple tooth arising between the canine and the double-rooted premolars. Is that a tiny canine? or a tiny premolar? Maybe someone out there has not only the answer, but the reason why.

And yes,
I’m aware of the convention that numbers premolars 1-4. But the anterior one, is almost always the smallest, as if it just arrived.

References
Marsh OC 1881. Notice of new Jurassic mammals. American Journal of Science. (3) xxi: 511-513.

wiki/Docodon

 

The Eutriconodonta is also paraphyletic in the LRT

Triconodon, Docodon, and Kuehneotherium are known form dentary bones with most of their teeth in place. Generally I avoid adding such partial specimens to the large reptile tree (LRT-updated at 848 taxa) because so few scores are generated for them with the current character list that they lead to loss of resolution at their nodes…

But curiosity won out
when wondering about members of the putative clade Eutriconodonta (Kermack et al. 1973), a clade that ostensibly replaces the paraphyletic Triconodonta. Some of these mandible-only taxa I added to the tree only to delete them later, just to see where they nested (often at the base of the Monotremata, as one would guess given their Mid-to Late Jurassic ages).

According to Wikipedia
“The Eutriconodonta  is a [presumeably monophyletic] order of mammals” broadly, though not exclusively characterized by molar teeth with three main cusps on a crown that were arranged in a row.  “Eutriconodonts retained classical mammalian synapomorphies like epipubic bonesvenomous spurs and sprawling limbs. Eutriconodonts had a modern ear anatomy, the main difference from therians being that the ear ossicles were still somewhat connected to the jaw via the Meckel’s cartilage.

“Phylogenetic studies conducted by Zheng et al. (2013), Zhou et al. (2013) and Yuan et al. (2013) recovered monophyletic Eutriconodonta containing triconodontids, gobiconodontids, Amphilestes, Jeholodens and Yanoconodon. The exact phylogenetic placement of eutriconodonts within Mammaliaformes is uncertain.”

“Traditionally seen as the classical Mesozoic small mammalian insectivores, discoveries over the years have ironically shown them to be among the best examples of the diversity of mammals in this time period, including a vast variety of bauplans, ecological niches and locomotion methods.”

Traditional Eutriconodont taxa (see Martin et al. 2015) presently included in the LRT nest in a variety of clades:

  1. Gobiconodon – pre-mammal tritylodontid
  2. Repenomamus – pre-mammal tritylodontid
  3. Spinolestespre-mammal tritylodontid
  4. Jeholodens – pre-mammal tritylodontid
  5. Volaticotherium basal placental 
  6. Triconodonbasal monotreme
  7. Trioracodon – basal monotreme
  8. Yanoconodonpre-mammal (Fig. 1).

We’ve seen this sort of splitting
of traditionally established clades based chiefly on tooth traits before with the Docodonta (Fig. 1). As in molecule trees, tooth trees are not replicated in the LRT, which recovers a distinctly new tree topology without the odd logic jumps that traditional clades, like Afrotheria, produce.

Figure1. Repeat of an early subset of the LRT, this time highlighting putative eutriconodonts and where they nest. No wonder they are described as a diverse clade!

Figure1. Repeat of an early subset of the LRT, this time highlighting putative eutriconodonts and where they nest. No wonder they are described as a diverse clade!

For the most part
eutriconodonts nest more or less together very close to the base of the Mammalia, whether in or out. Those slender posterior jaw bones are not typically preserved, but the long groove in which they are attached is typically preserved. Caution must be exercised, as fully mammalian taxa like Monodelphis, can also preserve a remnant of this groove despite the complete evolution of the post-dentary bones into tiny ear bones.

Figure 4. Mondelphis domestics with its posteromedial jaw groove highlighted in red. The ear bones are tiny and enclosed within the auditory bulla beneath the cranium.

Figure 2. Mondelphis domestics with its posteromedial jaw groove highlighted in red. The ear bones are tiny and enclosed within the auditory bulla beneath the cranium.

So.. about those venomous ankle spurs…
Ornithorhynchus, the platypus, has them and likely so do its sisters. The authors of the Volaticotherium paper make no mention of either venom nor spur and score it as a “?”. The Yanoconodon tarsi are a challenge to reconstruct based on their preservation. The authors and yours truly note no spur-like bones present.

A new evolution website has launched
Check out www.TimeTree.org for a tremendous amount of phylogenetic information. For instance, one can input two well-known taxa, like Gallus and Homo, and the tree will determine the estimated date of their last common ancestor, in this case Vaughnictis (which is a taxon not in their current database).

References
Editors: Carrano MT et al. 2006. Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds and Reptiles. University of Chicago Press.  online here.
Kermack KA, Mussett F, Rigney HW 1973. The lower jaw of Morganucodon. Zoological Journal of the Linnean Society.53 (2): 87–175.
Martin T et al. 2015. A Cretaceous eutriconodont and integument evolution of early mammals. Nature 526:380-384. online.

wiki/Eutriconodonta

Imagining the unknown: the skulls of Amphitherium and Docodon

Often enough
tiiny Jurassic synapsids, like Amphitherium prevosti  (von Meyer 1832; Middle Jurassic, 170 mya) and Docodon victor (Marsh 1881; Late Jurassic, 2 cm skull length), are known only from mandibles with teeth (Fig. 1).

We can guess what the skull looks like
because the molars occlude and the rest of the teeth interlock, slide past one another or meet at or near their tips. Plus we have clues from sister taxa that set parameters for possibilities in a method known as phylogenetic bracketing. In such cases some scores are less risky to guess, like the number of molars. Others are more risky, like the presence of caniniform canines.

Figure 1. Amphitherium and Docodon with skulls imagined.

Figure 1. Amphitherium and Docodon with skulls imagined. The large number of molars nests both these taxa with Monotremata.

References|
Marsh OC 1881. Notice of new Jurassic mammals: American Journal of Science, ser. 3, 21: p. 511-513.
Meyer H von 1832. Palaeologica, zur Geschichte der Erde und ihrer Geschöpfe. Schmerber, Frankfurt a/M, xi, 560 pp.

Mammalian nomenclature problems

Several putative stem mammal clades
have not been recovered in the LRT, like the ‘Notoungulata’ and the ‘Allotheria.‘ Similarly several putative reptile clades were also not recovered.

Now
the base and stem of the mammal clade are showing some nomenclature problems relative to traditional results.

First, I added a few mammals
(Mus, Taeiniolabis, Paulchaffatia) just be sure I was comparing listed taxa (see below) with listed taxa. If you know of any pertinent taxa that will change the current tree topology back to traditional topologies, please let me know. So far, I’m coming up short. These are the changes recovered so far:

Carrano et al. (editors) 2006
reports the following pertinent definitions. Comments follow (not in boldface).

Mammalia Linneaus 1758
The least inclusive clade containing Ornithorhynchus and Mus. In the LRT, Sinoconodon is the last common ancestor and Pachygenelus nests at the base of the outgroup clade, the Trithelodontidae (including the Tritylodontidae). So, no problems with this definition.

Trithelodontidae Broom 1912
The most inclusive clade containing Pachygenelus, but not Tritylodon and Mus. In the LRT Pachygenelus is basal to both Tritylodon and Mus, so the most inclusive clade containing Pachygenelus includes Mammalia and Tritylodontidae, contra prior studies.

Tritylodontidae Kühne 1956|
The most inclusive clade containing Tritylodon, but not Pachygenelus or Mus. In the LRT, this clade is monophyletic, and now includes Repenomamus.

Mammaliamorpha Rowe 1988
The least inclusive clade containing Tritylodon, Pachygenelus and Mus. In the LRT this clade is a junior synonym of the Trithelodontidae (see above).

Mammaliformes Rowe 1988
The most inclusive clade containing Mus, but not Tritylodon or Pachygenelus. In the LRT, this clade is a junior synonym for the clade Mammalia because Pachygenelus is the proximal outgroup taxon to Mammalia.

Theria  Parker and Howell 1897
The least inclusive clade containing Mus and Didelphis. In the LRT this clade is monophyletic and unchanged.

Theriimorpha Rowe 1988
The most inclusive clade containing Mus but not Ornithorhynchus. In the LRT this clade is a junior synonym for Theria.

Metatheria Huxley 1880
The most inclusive clade containing Didelphis, but not Mus. This definition was meant to include all marsupials, but in the LRT the clade that includes most marsupials does not include Didelphiswhich nests basal to and outside both monophyletic Marsupialia and Placentalia. So, strictly speaking, Metatheria in the LRT currently includes only Didelphis and perhaps its sister, Ukhaatherium.

Allotheria Marsh 1880
The most inclusive clade containing Taeniolabis, but not Mus or Ornithorhynchus. This was meant to indicate that Taeniolabis nested outside the Mammalia, but in the LRT Taeniolabis nests with Plesiadapis and Carpolestes and this clade is a sister to the clade containing Mus and the Multituberculata — within the Glires and Placentalia.

Multituberculata Cope 1884
The least inclusive clade containing Taeniolabis and Paulchofattia. This was meant to  include all the multituberculates and have them nest outside of the Mammalia, but in the LRT Taeniolabis nests with Plesiadapis and Paulchofattia nests with Carpolestes. So that is a clade of four taxa at present and it does not include Ptilodus and other multituberculates, the clade with a large and grooved lower last premolar. These traditional multis now need a new clade name. They are derived from a sister to the rodent clade in the LRT and they leave no descendants. Carpolestes is a sister to the ancestor of rodents and multis and Carpolestes (Fig. 1) has a large and barely-grooved lower last premolar, a precursor to that identifying trait in that second clade of multis.

Figure 1. Carpolestes simpsoni skull shows that large lower precursor premolar.

Figure 1. Carpolestes simpsoni skull shows that large lower premolar with just a few grooves. Here in the LRT Carpolestes nests close to the base of the traditional multituberculates that emphasize this trait. But see text for strict definitions of this clade.

References
Editors: Carrano MT et al. 2006. Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds and Reptiles. University of Chicago Press.  online here.
Kermack KA, Mussett F, Rigney HW 1973. The lower jaw of Morganucodon. Zoological Journal of the Linnean Society.53 (2): 87–175.
Martin T et al. 2015. A Cretaceous eutriconodont and integument evolution of early mammals. Nature 526:380-384. online.

The Stem-Mammals–a Brief Primer (with remarks)

Preface added the day after posting: M. Mortimer gratefully informed me that some authors consider all taxa closer to mammals than to other living taxa as ‘stem’ mammals. Perhaps that is how ‘stem’ taxa are defined. That came as news to me because I understood the term ‘stem’ to refer to immediate outgroups only based on the terms usage in other works. So you learn as you go. The broad definition quickly loses relevance and adds to confusion. In M. Mortimer’s example, Diplodocus is a ‘stem’ bird. Please read the following with these caveats in mind. 

Preface added 10/26/2016: Just found out there is are two definitions for ‘stem’ taxa, one in the wider sense and one in the narrower sense, the one is was familiar with. Learn more at Wikipedia here

Usually I cover published academic papers
here at PterosaurHeresies.WordPress.com. Today we’ll cover a Tetrapod Zoology blog post published online by Dr. Darren Naish a month ago. Unfortunately the post was sprinkled with traditional misconceptions.

Below
the Naish text is copied in italic and his captions are copied in their original ALL CAPS. Remarks are in red. You can see the original blogpost here. This is how all good referees mark up submitted manuscripts, with precise comments intended to help the writer improve the next draft. To that end, Naish notes he is currently writing a book that includes this subject.

The Stem-Mammals–a Brief Primer
Mammals are but the only surviving members of a far grander, older lineage
By Darren Naish on September 20, 2016

Figure 1. Strangely Naish labeled this illustration "Non-synapsid-mammal-montage" Most of these taxa, caseids and Tetraceratops exempted, are indeed synapsids. The problem is, all of the red taxa are not stem mammals, nor are they in the mammal lineage at any node. Rather they represent extinct offshoots.

PROVISIONAL AND IN-PREP MONTAGE (FOR MY TEXTBOOK ON VERTEBRATE HISTORY) DEPICTING A SELECTION OF STEM-MAMMALS. I’VE DRAWN FAR MORE THAN THE SELECTION SHOWN HERE. CREDIT: DARREN NAISH Strangely Naish labeled this illustration “Non-synapsid-mammal-montage” Most of these taxa, caseids and Tetraceratops exempted, are indeed synapsids. The problem is, all of the red taxa are not stem mammals, nor are they in the mammal lineage at any node. Rather they represent extinct and distant offshoots. Virtually all science journalists accept what they read in publication without criticizing it. But Naish is also a PhD, so it is his duty to keep a laser focus on his headline topic, not to stray off subject, and most importantly, to clarify for his readers the inconsistencies present. Otherwise, as above, there is confusion and lack of clarity for the reader.

“For some considerable time now I’ve been promising that one day — one day — I’ll devote time and energy to coverage of that enormous, diverse, long-lived tetrapod group that we variously term the non-mammalian synapsids or stem-mammals. The most traditional term for them is ‘mammal-like reptiles’: while still in use, this term should be avoided given that the animals concerned are simply not part of the reptile lineage. Not true. According to the large reptile tree (LRT) all descendants of the first reptile/amniote, Gephyrostegus, are also reptiles, and that includes mammals and their long list of descendants. Unfortunately Naish is repeating an old and invalid tradition. The vernacular terms protomammal and paramammal have both been used for the group as well, though both have problems. Stem-mammals will be used here. If so, it is important that Naish restrict his discussion to just the immediate precursors of mammals, not the long list going back to basal synapsids, but that is not what he does.

Anyway, we’re talking about that group of tetrapods that are not mammals but are ancestral to them, and which occupy all those points on the mammal lineage outside of Mammalia. The presence of a laterotemporal fenestra (a single skull opening behind the eye socket) is a key feature distinguishing them from other amniotes. Not true. Several clades by convergence developed such a skull opening including 1. the millerettid clade and their descendants from Oedaleps to Australothyris, including the caseids. Emeroleter and Lanthanosuchus had that fenestra. So did bolosaurids. And then there are the prodiapsids from Heleosaurus to Archaeovenator and the last common ancestor of synapsids and diapsids, VaughnictisThe early members of this segment of the mammal lineage have often been called pelycosaurs while the members of the more mammal-like segment of the lineage are termed therapsids. Actually finback pelycosaurs are an offshoot clade, not in the lineage of mammals, which proceeds from a sister to Ophiacodon to Cutleria without including finbacks. The importance of these animals concerns the fact that their comparatively excellent fossil record charts transition from an ancestral ‘reptile-like’ form to mammals via a near-perfect series of intermediates. Alas, their relative obscurity and the lack of good popular syntheses means that they are not the poster-children of evolution that they really should be… at least, not outside the palaeontological community.  Those animals were featured on both versions of Cosmos.

TETRACERATOPS FROM THE EARLY PERMIAN OF THE USA, AN EARLY SYNAPSID SOMETIMES IDENTIFIED AS ONE OF THE OLDEST THERAPSIDS BUT LATER RE-INTERPRETED AS OCCUPYING A MORE ROOT-WARD POSITION IN THE TREE. CREDIT: DMITRI BOGDANOV WIKIPEDIA CC BY 3.0. The LRT nests Tetraceratops with Tsejaia and Limnoscelis, whether it had a lateral temporal fenestra or not. Massive crushing adds doubt to that. It doesn't look like any other synapsid and it nests better with other reptiles, so why include it?

TETRACERATOPS FROM THE EARLY PERMIAN OF THE USA, AN EARLY SYNAPSID SOMETIMES IDENTIFIED AS ONE OF THE OLDEST THERAPSIDS BUT LATER RE-INTERPRETED AS OCCUPYING A MORE ROOT-WARD POSITION IN THE TREE. CREDIT: DMITRI BOGDANOV WIKIPEDIA CC BY 3.0. The LRT nests Tetraceratops with Tsejaia and Limnoscelis, whether it had a lateral temporal fenestra or not, far from the synapsids. Massive crushing adds doubt to that. It doesn’t look like any other synapsid and it nests with other reptiles, so why include it?

This article is not the time and place to start a group-by-group review of the many lineages concerned… I know from experience how those projects quickly expand into gargantuan multi-part monsters that can never be finished. Rather, this is just a brief primer, a placeholder. If you want to see the lineage of mammals going back to stem tetrapods, click here then peruse at your leisure the taxa that interest you.

COVER OF KEPT (1982). THE BEST BOOK ON THE GROUP OF ANIMALS SO FAR. NOW OUT OF PRINT (BUT AVAILABLE AT REASONABLE PRICES ONLINE. CREDIT: ACADEMIC PRESS LONDON. This is indeed the go-to book for synapsid data.

COVER OF KEPT (1982). THE BEST BOOK ON THE GROUP OF ANIMALS SO FAR. NOW OUT OF PRINT (BUT AVAILABLE AT REASONABLE PRICES ONLINE. CREDIT: ACADEMIC PRESS LONDON. This is indeed the go-to book for synapsid data and has been for more than 30 years. See ReptileEvolution.com for updates since then. 

Before anyone asks, the one crippling, punishing problem with these animals is that – even today – there is no single, good, up-to-date, go-to volume on their diversity, history, evolution and biology. But you can go online here for the latest data. Yes, there are books on these animals, but they’re technical and mostly out of print. The best is Tom Kemp’s Mammal-Like Reptiles and the Origin of Mammals (Kemp 1982). There’s also Nick Hotton et al.’s The Ecology and Biology of Mammal-like Reptiles (Hotton et al. 1986) (a collection of papers by different authors). I have a substantial, well illustrated chapter on these animals in my giant textbook (on which go here, if you wish), but a good, dedicated, modern volume just does not exist. There are several decent review articles on the group as a whole, among the most recent being Angielczyk (2009).

MUCH-SIMPLIFIED CARTOON CLADOGRAM OF STEM-MAMMALS BASED ON TOPOLOGIES RECOVERED IN SEVERAL RECENT STUDIES. EXPANDED VERSIONS BEING PREPARED FOR MY IN-PREP TEXTBOOK (MORE HERE). CREDIT: DARREN NAISH As above, caseids are not related. Pelycosaurs are offshoots. The basal dichotomy of therapsids separated the Anomodonts from the Kynodonts.

MUCH-SIMPLIFIED CARTOON CLADOGRAM OF STEM-MAMMALS BASED ON TOPOLOGIES RECOVERED IN SEVERAL RECENT STUDIES. EXPANDED VERSIONS BEING PREPARED FOR MY IN-PREP TEXTBOOK (MORE HERE). CREDIT: DARREN NAISH As above, caseids are not related. Pelycosaurs are offshoots. The basal dichotomy of therapsids separated the Anomodonts from the Kynodonts.

The oldest stem-mammals date to the Moscovian part of the Carboniferous (here again, an inappropriate use of the term ‘stem’) and have conventionally been depicted as very reptilian in appearance. That’s because they are or were reptiles, as recovered by the LRT.  This is probably true in broad terms but is open to some question, there being indications that their integument and so on was not ‘reptilian’ as we conventionally imagine it. Likely without scales, based on the scant evidence at hand, but living dinosaurs are also without scales, except for those transformed from feathers. These early forms belong to those lineages conventionally lumped together as ‘pelycosaurs’ – a term that clearly refers to a paraphyletic assemblage given that therapsids evolved from somewhere among them. Not true. The LRT recovers a clade of pelycosaurs, a resurrected clade Pelycosauria. 

SOMEWHAT DATED SCHEMATIC REPRESENTATION OF SYNAPSID EVOLUTION WHICH I INCLUDE BECAUSE IT DOES A NICE JOB OF ILLUSTRATING BOTH CRANIAL VARIATION WITHIN THE GROUP, AND SOME OF THE MAIN DIFFERENCES OBVIOUS BETWEEN 'PELYCOSAURS', THEROCEPHALIAN-GRADE ANIMALS, AND CYNODONTS. CREDIT: PALAEOS, ORIGINALLY BY THOMAS KEMP. If Naish is trying to show us what we used to think, he's doing a good job, but wasting time when his whole point was to update his readers on the latest, which can be found at ReptileEvolution.com

SOMEWHAT DATED SCHEMATIC REPRESENTATION OF SYNAPSID EVOLUTION WHICH I INCLUDE BECAUSE IT DOES A NICE JOB OF ILLUSTRATING BOTH CRANIAL VARIATION WITHIN THE GROUP, AND SOME OF THE MAIN DIFFERENCES OBVIOUS BETWEEN ‘PELYCOSAURS’, THEROCEPHALIAN-GRADE ANIMALS, AND CYNODONTS. CREDIT: PALAEOS, ORIGINALLY BY THOMAS KEMP. If Naish is trying to show us what we used to think, he’s doing a good job, but wasting time when his whole point was to update his readers on the latest, which can be found at ReptileEvolution.com.

Animals from this ‘pelycosaur’ part of the tree include the long-snouted, mostly predatory varanopids and ophiacodontids, the omnivorous and herbivorous caseasaurs, and the edaphosaurids and sphenacodontids, the latter including the famous Dimetrodon. Why waste time on these non stem-mammals? While many of these animals (especially the early members of these groups) were small (less than 50 cm long), large size (3 m or more) evolved several times independently. There are lots of other significant events here as well, including the evolution of high-fibre herbivory and the independent evolution of dorsal sails.  Why waste time on these non stem-mammals? Even in these animals there are indications of social behaviour and parental care (Botha-Brink & Modesto 2007, 2009).

RECONSTRUCTION OF AN ASSEMBLAGE (A FAMILY GROUP?) OF THE VARANOPID HELEOSAURUS, PICTURED IN THE POSE IN WHICH THEIR SKELETONS WERE DISCOVERED. CREDIT: BOTHA-BRINK & MODESTO (2009). This is Heleosaurus, which is a pro-diapsid, an outgroup to the Synapsida, but the concept is probably true of young ones nesting with an adult.

RECONSTRUCTION OF AN ASSEMBLAGE (A FAMILY GROUP?) OF THE VARANOPID HELEOSAURUS, PICTURED IN THE POSE IN WHICH THEIR SKELETONS WERE DISCOVERED. CREDIT: BOTHA-BRINK & MODESTO (2009). This is Heleosaurus, which is a pro-diapsid, an outgroup to the Synapsida, but the concept is probably true of young ones nesting with an adult.

Dimetrodon – one of the most familiar and famous of all stem-mammals (Not true, merely an offshoot)– is a fascinating creature that has recently undergone something of an image change: ideas regarding the evolution, function and anatomy of its sail have all been challenged, its ecology and lifestyle have been the source of some debate, and its life appearance and gait have undergone revision in recent years. I plan to devote an article to these issues.

YOU MIGHT HAVE SEEN THIS ANIMAL BEFORE. IT'S DIMETRODON. CREDIT: D'ARCY NORMAN WIKIMEDIA CC BY 2.0 Not sure why Naish is bothering with these popular but irrelevant taxa.

YOU MIGHT HAVE SEEN THIS ANIMAL BEFORE. IT’S DIMETRODON. CREDIT: D’ARCY NORMAN WIKIMEDIA CC BY 2.0 Not sure why Naish is bothering with these popular but irrelevant taxa when so many taxa much closer to mammals, the REAL stem mammals also make for good stories. Seems like he doesn’t know or doesn’t care. 

Animals close to sphenacodontids gave rise to therapsids. A more erect gait and faster metabolism occurred at the time of this transition, numerous additional changes associated with dentition, palatal structure, limb posture and so on occurring as well. It’s within this vast group (Therapsida) that we find the often herbivorous, beak-jawed dicynodonts and kin, the often predatory biarmosuchians, gorgonopsians and therocephalians, and the often striking, often large dinocephalians. That last group includes both predators and herbivores, hippo-sized animals, and species with thickened skull roofs probably used in head-butting. They dominated many continental animal communities in the Permian, being best known from the fossil records of South Africa and Russia. Still not talking about stem mammals here. When are we going to get to them? The text does not follow the headline. 

TAPINOCEPHALID DINOCEPHALIANS - LIKE TAPINOCEPHALUS DEPICTED HERE - HAD THICKENED SKULL ROOFS THAT LIKELY HAD A DISPLAY OR COMBAT FUNCTION. THE BIGGEST OF THESE ANIMALS WERE OVER 3 M LONG. CREDIT: DIBDG WIKIMEDIA CC BY SA 3.0 While fascinating, this is not a stem-mammal.

TAPINOCEPHALID DINOCEPHALIANS – LIKE TAPINOCEPHALUS DEPICTED HERE – HAD THICKENED SKULL ROOFS THAT LIKELY HAD A DISPLAY OR COMBAT FUNCTION. THE BIGGEST OF THESE ANIMALS WERE OVER 3 M LONG. CREDIT: DIBDG WIKIMEDIA CC BY SA 3.0 While fascinating, this is not a stem-mammal, but another offshoot.

Gorgonopsians and therocephalians are exciting groups that include various macropredatory, often ‘sabre-toothed’ species; both have been the subject of various recent revisions. Species within these groups have been likened to weasels, wolves and bears in approximate body form, though any resemblance would have been highly superficial. Sometime during the Late Permian, cynodonts arose from an ancestor closely related to therocephalians (both groups form the therapsid clade Eutheriodontia): Cynodontia is the group that includes mammals as well as a number of additional lineages that have their own histories and evolved their own specializations. Now we’re getting closer to the stem-mammals, members of the clade Tritylodontia within the Cynodontia!

And because this was meant to be a very, very brief primer, that is all I’ll say for now. There is so much more to do… WAIT! Naish never once wrote about or illustrated a stem-mammal here! I read this whole blog post without learning anything new about the stem-mammals, the Tritylodontidae and their immediate predecessors.. As we’ve seen before, Naish sometimes cruises on the invalid past rather than exploring today’s cutting edge data and latest discoveries. Pity, all that talent going for the low-hanging fruit. Darren, as you write your book on synapsid relationships, feel free to reference ReptileEvolution.com and the large reptile tree. It will help you understand the issues and enigmas generated in Kemp’s 1982 book.

Stem-mammals have been covered on scant occasions at Tet Zoo. But see…

Sometimes Dr. Naish referees manuscripts offered for academic publication. With his stuck-in-the-past bias, good luck if he referees your submission. I would not want wish that on my worst enemy, especially if you’re promoting new hypotheses.

Many scientists like to play it safe, resisting and waiting for the tide to shift on advancing new hypotheses before jumping on the bandwagon. Don’t be like that. Follow the data. Test as much as you can yourself. Be a skeptical Scientist, not a nodding Journalist. 

What do I expect from these remarks?
Based on his vocal antipathy toward the results recovered by the LRT, Dr. Naish will probably cling to his invalid traditions. After all, based on his writings, he has ‘painted himself into a corner’ from which he cannot escape without an about face apology and acknowledgment. That’s something primates, like us, do very very rarely. PhDs are not wired for it. But if Naish did run the tests he would find what I found. If not, I’d like to hear why not.

Refs – –
Angielczyk, K. D. 2009. Dimetrodon is not a dinosaur: using tree thinking to understand the ancient relatives of mammals and their evolution. Evolution: Education and Outreach 2, 257-271.
Botha-Brink, J. & Modesto, S. 2007. A mixed-age classed ‘pelycosaur’ aggregation from South Africa: earliest evidence of parental care in amniotes? Proceedings of the Royal Society B 274, 2829-2834.
Botha-Brink, J. & Modesto, S. 2009. Anatomy and relationships of the Middle Permian varanopid Heleosaurus scholtzi based on a social aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 29, 389-400.
Hotton, N., MacLean, P. D., Roth, J. J. & Roth, E. C. 1986. The Ecology and Biology of Mammal-like Reptiles. Smithsonian Institution Press, Washington and London.
Kemp, T. S. 1982. Mammal-Like Reptiles and the Origin of Mammals. Academic Press, London.

Close cousins at the base of the Placentalia – all went their separate ways

Yes, they all look alike (Fig. 1)
AND, the very subtle differences between them (aka microevolution) took the first steps that led to the major differences (aka macroevolution) seen in later more diverse clade members.

Figure 1. We are very fortunate to have several of these basal placental taxa still living with us, as chronologically long-lived taxa. Starting with the extant Didelphis at the base of the Theria, phylogenetic miniaturization gave us the smaller Monodelphis domestics and the even smaller M. sores and M. kunsi, which gave rise to the larger Nandinia at the base of the Carnivora, Tupaia, at the base of the expanded Glires, Ptilocercus at the base of the expanded Archonta, and Maelestes at the base of the tenrecs + whales and the Condylarthra, aka the rest of the mammals.

Figure 1. We are very fortunate to have several of these basal placental taxa still living with us, as chronologically long-lived taxa. Starting with the extant Didelphis at the base of the Theria, phylogenetic miniaturization gave us the smaller Monodelphis domestics and the even smaller M. sores and M. kunsi, which gave rise to the larger Nandinia at the base of the Carnivora, Tupaia, at the base of the expanded Glires, Ptilocercus at the base of the expanded Archonta, and Maelestes at the base of marsupials and the Condylarthra, aka the rest of the mammals.

AND size does matter. 
Phylogenetic miniaturization was once again at work here. Starting with the extant Didelphis at the base of the Theria, phylogenetic miniaturization gave us the smaller Monodelphis domesticus and the even smaller M. sorex and M. kunsi.

These in turn gave rise to
the larger Nandinia close to the base of the Carnivora, Tupaia, close to the base of the expanded Glires, Ptilocercus close to the base of the Ptilocercia, and Maelestes at the base of the Marsupialia.

This heretical origin hypothesis account runs counter
to all prior cladistic analyses, but I hope you’ll see a certain logic and order here that appears to echo Mesozoic evolutionary events. That order will be reflected in the LRT shortly.

Wonder why I added two more species from the genus Monodelphis?
Actually I was looking for skulls with larger premaxillary teeth, fewer molars, smaller canines, nares that open only anteriorly and other traits found in basal members of the various clades pictured above. I only found fewer molars among the smaller species show above.

But note:
M. sorex has the relatively shorter, taller skull, like that of the Carnivoran, Nandinia.  M. kunis has the smaller naris and angled rostrum seen in Maelestes and Asioryctes. So, the larger trends are just barely present here, not enough to nest the smaller Monodelphis species apart from the the larger M. domesticus. 

These largely overlooked taxa
must be among the most successful and well-adapted placental mammals of all time. After all, when you think about it, they have lasted the longest without having to evolve another morphology or die out. Putting them in the limelight will hopefully lead to future studies.

wiki/Placentalia

Another look at the O’Leary et al. hypothetical ancestor of placentals

This post takes another look at
O’Leary et al. (2013) who created a hypothetical ancestral placental. Now that several dozen mammals have been added to the LRT (including the extant basal placental, Monodelphis), hopefully now we can look at the situation with more insight. Let’s see how close to Monodelphis O’Leary’s team got in creating their own hypothetical ancestor. Comparisons can be made both in the bones and soft tissues between the LRT ancestor, Monodelphis, and the imagined ancestor of the O’Leary team (see below).

BTW, the O’Leary team nested Didelphis and Monodelphis as sisters without descendants using nucleotide data. The LRT found that both were good plesiomorphic models for as yet undiscovered ancestors to the Metatheria and the Eutheria (=Placentalia) respectively.

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

Figure 1. The purported marsupial without a pouch, Monodelphis domestica, nests as the most primitive tested placental in the LRT.

O’Leary et al. (2013) sought to bring new insight into the earliest radiation of placental mammals by creating a hypothetical ancestor to all placentals. They reported this happened in a great radiation of clades right after the K-T extinction event. This counters earlier claims that undiscovered placentals may have been present during the Cretaceous based on data mentioned below. Monotremes and metatherians, the ancestors of today’s egg-laying and marsupial mammals, were present during the Cretaceous. Other lineages of mammals, like Morganucodon, were present as far back as the Triassic. So the O’Leary et al. hypothesis requires relative stasis throughout much of the Mesozoic for mammals, followed by an explosive radiation in the first third of the Paleocene following the K-T extinction event.

Figure 2. Hypothetical ancestor to placental mammals as imagined by O'Leary et al. 2013.

Figure 2. Hypothetical ancestor to placental mammals as imagined by O’Leary et al. 2013.

The O’Leary et al. abstract reports: To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species. Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest members of Afrotheria.”

Figure 2. Monodelphis skull in three views. Note the supra occipital is narrower than the exoccipitals, like other mammals, not like the data from the figure previously used.

Figure 3. Monodelphis skull in three views. Note the supra occipital is narrower than the exoccipitals, like other mammals, not like the data from the figure previously used. From Digimorph.org and used with permission.

What the O’Leary team imagined vs what the LRT recovered:

  1. Molecular clock analyses indicate crown Placentalia were present by the late Early Cretaceous, 100 mya. The LRT confirms that.
  2. Fossil evidence does not corroborate the presence of early primates and early rodents in the Late Cretaceous. The LRT recovered an Early Cretaceous carnivore, Vinceletes and a Late Jurassic rodent precursor, Shenshou, a sister to the extant Solenodon clade that includes moles and shrews, and a Late Triassic sister to multituberculates, Haramiyavia, which nest as derived rodents in the LRT. 
  3. Phenomic phylogenies incorporating fossils have placed ordinal and intraordinal speciation of Placentalia after the K-Pg (K-T) extinction event. The LRT found two raditions: small arboreal basal mammals in the Triassic and Jurassic, larger Tenreccetaceans and Condylarthra (ungulates and kin) in the Cenozoic. 
  4. Ukhaatherium, Mailestes and Zalambdalestes, all from the Late Cretaceous, nest outside the Placentalia. In the LRT the first two nest with Leptictis a basal Tenreccetacean. The last one indeed does nest outside with wombats in the Marsupialia. So, one out of three. 
  5. The first members of modern placental orders began appearing 2 to 3 million years (My) later during the Paleocene.All recent clock-based estimates for the ages of key clades, with few exceptions, are substantially older than indicated by the fossil record.  The first placentals in the LRT appear in the Late Triassic, which agrees with the clocks, not O’Leary et al.
  6. We recognize Protungulatum donnae as the oldest undisputed species within crown Placentalia (Fig. 1), and this species dates to the earliest Paleocene. Haramiyavia (Late Triassic) was tested in the LRT, but later deleted due to its low number of traits.
  7. We find with 100% jackknife support that Eomaia falls outside of Eutheria as a stem taxon to Theria. So does the LRT.
Figure 4. Entire skeleton of Monodelphis from Digimorph.org and used with permission.

Figure 4. Entire skeleton of Monodelphis from Digimorph.org and used with permission.

O’Leary et al. reconstructed the placental ancestor.
Let’s see how well it conforms to the LRT placental ancestor, Mondodelphis (M).

 

  1. It weighed between 6 and 245 g – M: 58 (f) to 95(m) grams
  2. insectivororous – M: omnivorous – gives rise to carnivores and insectivores, notably it holds prey with its hands, which is the fast track toward the primaries
  3. scansorial – M: scansorial
  4. single young born hairless, eyes closed – M: 6-11 young, hairless, eyes closed
  5. uterus with two horns – M:?
  6. placenta with trophoblast – M:?
  7. sperm with flat head – M:?
  8. abdominal testes just caudal to kidneys M: scrotum after 24 days; several placental clades do not produce a scrotum (see below)
  9. brain has corpus callosum, encephalizatin quotient >0.25 – M:?
  10. facial nerve fibers passed ventral to the trigeminal sensory column – M:?
  11. gyrencephalic cerebral cortex – M:?
  12. separate olfactory bulbs – M: single bulb
  13. hemichorial placenta – M: a rudimentary placenta develops then disappears
  14. separate openings for anus and urogenitalia – M: single cloaca
  15. triangular perforated stapes – M: perforation triangular, externally rectangular
  16. lacked epipubic bones – M: epipubes present, as in other basal placentals
  17. internal carotid artery present, but did not leave its mark on bones – M:?
  18. seven post canine teeth, four premolars and three molars – M: four molars, also in Asrioryctes, Leptictis and other Tenreccetaceans.
  19. premolar 3 is lost in Theria, so:p1,2,4,5, m1,2,3 – M: 3 premolars, 4 molars; in many other placentals: 3 premolars, 3 molars (2-4). Pachygenelus has 6 post canine teeth. Morganucodon has 4 premolars, 4 molars. Juramaia has 5 premolars, 3 molars. Amphitherium has 5 premolars and 6 molars. So did certain mammals add teeth? Or did most lose teeth? Depends on where you start counting…

Look again at #4 above. 
Consider the survival advantage that Monodelphis presents: Six to eleven young born at a time feeding on up to 13 retractable nipples without a pouch, versus one joey in a pouch, the marsupial constraint. Female opossums, including Didelphis, often give birth to very large numbers of young, most of which fail to attach to a teat.

Paleogeography
O’Leary et al. state: “Our relatively younger age estimate for Placentalia means that there is no basis for linking placental interordinal diversification to the Mesozoic fragmentation of Gondwana [thus] Afrotheria did not originate in Africa.” The LRT provides a chronology by which placentals could spread worldwide before the breakup of Pangaea and Gondwana.

The O’Leary et al. cladogram
(Fig. 5) does not resemble the LRT. Gradual accumulations of derived traits are hard to find in the O’Leary et al. cladogram. They found the anteaters and sloths were the most primitive placentals and nested whales with hooved mammals, among other odd pairings.

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.

The LRT provides
a gradual accumulation of derived traits unmatched by the O’Leary et al topology. This is the best test of tree topology validity as it echoes actual evolutionary events, micro step by micro step.

O’Leary et al. taxa not validated by the LRT

  1. Epitheria – all placentals sans Xenarthra.
  2. Sundatheria – (= Scandentia + Dermoptera)
  3. Afrotheria 
  4. Paenungulata
  5. Tethytheria
  6. Boreoeutheria
  7. Laurasiatheria
  8. Euarchontoglires
  9. Euarchonta

A postscript on testicles
According to slate.com, “Platypus testicles, and almost certainly those of all early mammals, sit right where they start life, safely tucked by the kidneys. Nearly all marsupials today have scrotums, Marsupials’ testicles hang in front of their penises.” In the opossum they are about where the ovaries are in females. In some placentals like “elephants, mammoths, aardvarks, manatees, and groups of African shrew- and mole-like creatures…retain their gonads close to their kidneys. Scrotums bounce along between the hind limbs (behind the penis) of primates, cats, dogs, horses, bears, camels, sheep, and pigs. Hedgehogs, moles, rhinos and tapirs, hippopotamuses, dolphins and whales, some [wait a minute, only some?] seals and walruses, and scaly anteaters have gonads inside and away from the kidneys.” You can read the various hypotheses of why testicles descend or not the slate website.

References
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
O’Leary, MA et al. 2013. The placental mammal ancestor and the post-K-Pg radiation of  placentals. Science 339:662-667. abstract

ArchibaldEtAl.pdf
protungulatum-donnae website