Bishops enters the LRT

Figure 1. The dentary of Bishops compared to its Late Cretaceous sister, Asioryctes, which has fewer and larger premolars.

Figure 1. The dentary of Early Cretaceous Bishops (1.5cm long) compared to its Late Cretaceous sister, Asioryctes, which has fewer and larger premolars and one more molar.

The genus Bishops whitmorei
(Rich et al. 2001; Early Cretaceous, Australia; Fig.1) is represented by a small mandible with a high coronoid process, six premolars and only three molars. In the LRT it nests basal to the much larger carnivorous marsupials (= creodonts), starting with the wolf-sized Arctocyon. It is a sister to Asioryctes (Fig. 1) which is basal to the herbivorous marsupials of Australia.

What makes this important?
It is the only tiny creodont known. All others are dog to wolf-sized. Cenozoic descendants of Bishops include the following carnivorous marsupials: Thylacinus, Thylacosmilus, Borhyaena, Hyaenodon and Vincelestes.

References
Rich TH, Flannery TF, Trusler P. Kool L, van Klaveren NA and Vickers-Rich P 2001. A second tribosphenic mammal from the Mesozoic of Australia. Records of the Queen Victoria Museum 110: 1-9.

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Convergent anterior shifts of the zygomatic arch in Glires

Something  a little strange in the course of mammal evolution here.
The temporal region of rodents (Fig. 1) appears to ‘break the rules’, but on further examination merely bends them.

We’re used to seeing
the orbit separate from the temporal fenestra, as in most reptiles, but it is a little disconcerting to see them confluent, as in most mammals, knowing that the eyeball and temporal muscle share the same opening without division.

In some rodents,
like the capybara (genus: Hydrochoerus) (Fig. 1), the lateral temporal arch (aka: zygomatic arch) drifts/shifts so far forward that it moves anterior to the temporal region and just borders the orbit (or so it seems).

Actually
the temporal jaw muscle continues to dive inside the temporal arch to the coronoid process of the dentary (= mandible). The difference is the temporal muscle in capybaras pulls at an angle over the valley created by the squamosal (Fig. 1, lower right).

FIgure 3. Hydrochoerus the capybara. At lower right, large jaw muscles are illustrated.

FIgure1. Hydrochoerus the capybara. At lower right, large jaw muscles are illustrated. The temporalis muscle (light red) anchors on the temple and inserts on the coronoid process as usual, just angled much closer to the eyeball to maintain these contacts. In dorsal view the zygomatic arch is located anterior to the temporal region of the skull here.

In other words,
the zygomatic arch (maxilla + jugal + squamosal bar) in the capybara and several other rodents and their allies (Fig. 3) does not extend to the posterior skull, as it does in basal tetrapods and most mammals, including multituberculates (Fig. 3). Even so, the temporalis muscle (Fig. 1, light red) always anchors on the temple and inserts on the coronoid process. In the capybara the temporal muscle inserts much closer to the eyeball.

Rodents have a loose jaw joint
(Fig. 2) that permits the mandible to move freely (not restricted by an axle and shaft as in Carnivora) within a muscular sling to alternately gnaw with incisors in one position, then grind with the molars in another. Primates, including humans, are similar in this regard. The alignment of the teeth can shift because the axis of rotation is loose.

Figure 3. Xianshou animation showing the loose jaw joint permitting both gnawing and grinding.

Figure 2. Xianshou animation showing the loose jaw joint permitting both gnawing and grinding. The zygomatic arch is shifted slightly anteriorly here, distinct from sister taxa (Fig. 3). If the jugal is still present, it is located as a vestigial patch on the inner rim of the zygomatic arch, as in sister taxa. This is obviously a highly derived skull and it nests at a highly derived node, contra the present paradigm and despite its early Late Jurassic appearance in the fossil record.

Cox et al. 2012 report,
“The masticatory musculature of rodents has evolved to enable both gnawing at the incisors and chewing at the molars. In particular, the masseter muscle is highly specialised, having extended anteriorly to originate from the rostrum. All living rodents have achieved this masseteric expansion in one of three ways, known as the sciuromorph, hystricomorph and myomorph conditions. Our results show that the morphology of the skull and masticatory muscles have allowed squirrels to specialise as gnawers and guinea pigs as chewers, but that rats are high-performance generalists, which helps explain their overwhelming success as a group.”

Other groups not traditionally associated with rodents,
but nest with rodents in the large reptile tree (LRT, 1281 taxa) include:

  1. The aye-aye (genus: Daubentonia) possesses large, ever-growing incisors, which it uses to gnaw wood and to access the subsurface larvae it locates through tapping. This feature of ever-growing teeth was once considered unique among primates (Simons 1995), but the aye-aye is not a primate in the LRT. Daubentonia also uses its rodent-like teeth to gnaw at nuts and hard-shelled fruits (Sterling et al. 1994, Sterling 1994b). In the LRT, Daubentonia is not a primate, but a rodent, explaining all of the above issues.
  2. The Multituberculata and Haramiyida, also posses large presumably ever-growing incisors, which they presumably use as rodents use these teeth.
  3. Maiopatagium was originally considered a haramiyid, but here nests with porcupines.

Typically the auditory bulla becomes larger
as the zygomatic arch advances forward, thus filling the vacated space below the temples. This happened several times by convergence. I wonder if that was the driving force: improved hearing for predator avoidance and/or prey detection, that made this happen in the following taxa and their kin.

  1. Scutisorex (shrews)
  2. Chrysochloris (golden moles)
  3. Macroscelides (one type of elephant shrew not related to the other type.)
  4. Solenodon 
  5. Gomphos (rabbits)
  6. Coendou (porcupines) and maybe Maiopatagium (back of skull missing)
  7. Heterocephalus (naked mole rats)
Figure 2. A selection of taxa from figure 1 more or less to scale and in phylogenetic order (pink arrows). Hope this helps with the concept of a gradual accumulation of traits. The hedgehogs Erinaceus and Echinops are transitional to the higher taxa with teeth and without.

Figure 3. A selection of taxa from figure 1 more or less to scale and in phylogenetic order (pink arrows). The hedgehogs Erinaceus and Echinops are transitional to the higher taxa with a complete arcade of teeth and without. Today, please note the posterior anchor of the squamosal (zygomatic arch).

I wonder if multituberculates are no longer with us
because they could not hear as well, based on their smaller auditory apparatus? Good question…

References
Cox et al. 2012. Functional evolution of the feeding system in rodents. PloS One 7(4): e36299. Online here.
Simons EL 1995. History, anatomy, subfossil record and management of Daubentonia Madagascariensis. In: Alterman L, Doyle GA, Izard MK. Creatures of the dark: the nocturnal prosimians. New York: Plenum Pr. p133-140.
Sterling EJ, Dierenfeld ES, Ashbourne CJ, Feistner ATC 1994. Dietary intake, food composition and nutrient intake in wild and captive populations of Daubentonia madagascariensis. Folia Primatol 62(1-3):115-24.
Sterling E 1994b. Ayes-ayes: Specialists on structurally defended resources. Folia Primatologica 62:142-154.

http://pin.primate.wisc.edu/factsheets/entry/aye-aye

Asioryctes: Re-restoring a pes, re-nesting a taxon

I should have noticed this pairing earlier.
Evidently it escaped everyone else’s notice, too. Asioryctes nemegetensis (Kielan-Jaworowska 1975, 1984; Figs. 1,2; middle Late Cretaceous, Djadokhta Formation, ~85 mya) is a good match for the living bandicoot, Perameles. Maga and Beck 2017 nested Asioryctes with the coeval Ukhaatherium, and the extant Perameles with another bandicoot, Echymipera.

FIgure 1. Skulls of Asioryctes, Perameles and Macrotis compared.

FIgure 1. Skulls of Asioryctes, Perameles and Macrotis compared. The overall shapes are similar, and so are the teeth, and other details. Historically the feet have been different, and that’s our starting point. 

Figure 2. Left: original restoration of Asioryctes pes. Colors added. Right: New restoration based on phylogenetic proximity to Perameles and other marsupial taxa with vestigial digit 1 and gracile digits 2 and 3 (grooming claws).

Figure 2. Left: original restoration of Asioryctes pes. Colors added. Right: New restoration based on phylogenetic proximity to Perameles and other marsupial taxa with vestigial digit 1 and gracile digits 2 and 3 (grooming claws). 

The first three taxa
are members of the large reptile tree (LRT, 1272 taxa), but the first two don’t nest together. The LRT now nests Asioryctes with Perameles and Macrotis, two extant bandicoots. Ukhaatherium nests with the basalmost members of Theria several nodes earlier.

One of the problems with this
is the original restoration of the Asioryctes pes, based on disarticulated parts (Kielan-Jaworowska 1975; Fig. 2). The REAL problem is no other mammal has gracile lateral metatarsals. Sans the pes, the skull nests with Perameles and Macrotis (Fig. 1), taxa with only a vestige pedal digit 1 and reduced digits 2 and 3.

Hmmm.
That opens up a possibility not foreseen by Kielan-Jaworowska.

A new restoration
of the illustrated elements (Fig. 2) identifies the slender metatarsals as 2 and 3. The tarsal elements are all present (contra Kielan-Jaworowska 1975) just reidentified here in accord with a standard bandicoot foot.

And… so… for the first time
we can see a predecessor taxon demonstrating a transitional morphology to the reduced pedal digits 1–3 seen in bandicoots and kangaroos.

References
Geoffrey Saint-Hilaire E 1803. Note sur les genres Phascolomis et Perameles, nouveaux genres d’animaux à bourse. Bulletin des Sciences par la Société Philomathique de Paris 80, 49–150.
Kielan-Jaworowska Z 1975. 
Preliminary description of two new eutherian genera from the Late Cretaceous of Mongolia. Palaeontologia Polonica 33:5-15.
Kielan-Jaworowska, Z 1984. Evolution of the therian mammals in the Late Cretaceous of Asia. Part VII. Synopsis. Palaeontologia Polonica 4:173-183. online pdf
Maga AM and Beck RMD 2017. Skeleton of an unusual, cat-sized marsupial relative (Metatheria: Marsupialiformes) from the middle Eocene (Lutetian: 44-43 million years ago) of Turkey. http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0181712

wiki/Asioryctes
wiki/Perameles
wiki/Macrotis

Reassessing Maiopatagium: now it’s a Jurassic porcupine!

Modified August 21, 2018 with the note that a procoracoid and coracoid were likely present in Jurassic rodents. These traits appear to be atavisms since taxa between prototheres and Jurassic rodents do not have these bones. 

Another case of taxon exclusion…remedied.
Earlier we looked at the Jurassic mammal, Maiopatagium, a putative glider, surrounded by a deep halo of long, straight hair. Meng et al. 2017 nested Maiopatagium between Sinoconodon and Haldanodon, taxa more primitive than mammals.

By contrast
the large reptile tree (LRT, 1235 taxa) nested Maiopatagium with Vilevolodon and Shenshou, two Jurassic arboreal rodents.

Now with 24 more taxa,
and several new ones from the rodent clade, the LRT nests Maiopatagium with the only tested porcupine, the small arboreal Coendou.

Figure 1. Subset of the LRT focusing on Scandentia + Glires. Yellow-green taxa are Jurassic in age.

Figure 1. Subset of the LRT focusing on Scandentia + Glires. Yellow-green taxa are Jurassic in age.

With this nesting
that halo of long straight hair on Maiopatagium
(Fig. 4) takes on a new identity as a pelage of still soft pre-quills, similar to a closely related taxon, Chinchillanesting with a former enigma taxon, Neoreomys

To no one’s surprise,
the guinea pig (genus: Cavia) nests with the pig-sized capybara (genus: Hydrochoerus). All but Maiopatagium are widely recognized members of the Hystricomorpha clade of rodents. The presence of Maiopatagium in this rodent clade supports the previously reported Jurassic radiation and dispersal of rodents (Fig. 1) currently represented by  a few specimens not widely recognized as rodents, nor tested against rodents. Porcupines and chinchillas were not in the Meng et al. taxon list.

Shifting Maiopatagium in the LRT to Sinoconodon adds 49 steps. Shifting to the more primitive Haldanodon adds 58 steps.

Distinct from all extant and extinct rodents Maiopatagium was reported to have a small coracoid and pro-coracoid, traits that disappear in therian mammals. This could be an atavism (= reversal) or it could be a misinterpretation of a crushed process of the scapula that appears in other hystricomorphs (Fig. 2). Vilevolodon has a protothere-like pro-coracoid and coracoid and it is medial to the scapula, not lateral as shown below. Interesting that similar structures appeared medial and lateral to the shoulder joint by convergence.

Figure 2. Possible source for the coracoid and procoracoid in Maiopatagium as crushed parts of the acromion process on other hystricomorphs.

Figure 2. Possible source for the coracoid and procoracoid in Maiopatagium as crushed parts of the acromion process on other hystricomorphs. At left is Hydrochoerus, the capybara. Above right is Cavia, the guinea pig. Lower right is Maiopatagium from Meng et al. 2017. Crushing would tend to break this fragile process. 

When the skull of Maiopatagium
nests with rodents we should consider the possibility that it may have included a large braincase (Fig. 3) not figured or restored by Meng et al. 2017 (Fig. 4).

Figure 3. Maiopatagium skull revised with extended, rodent-like cranium. Compared to figure 4. The anterodorsal naris is a hystricomorph trait. So is the premaxilla-frontal contact overlooked by Meng et al. 

Figure 3. Maiopatagium skull revised with extended, rodent-like cranium. Compared to figure 4. The anterodorsal naris is a hystricomorph trait. So is the premaxilla-frontal contact overlooked by Meng et al.

I was never able to see the gliding membrane
distinct from the halo of long hairs on Maiopatagium (Fig. 4) as described by Meng et al. 2017. No related taxa in the LRT are gliders.

Figure 2. Maiopatagium images from Meng et al. with the addition of a braincase restored here.

Figure 4. Maiopatagium images from Meng et al. with the addition of a braincase restored here. The pes has a new reconstruction (Fig. 5) than shown here.

The porcupine Coendou prehensilis
(Fig. 5) is the closest living relative to Maiopatagium in the LRT. Yes, the tooth shapes are distinctly different, but tooth shapes are highly variable and these taxa are separated by 160 million years. The limbs are longer in the Jurassic taxon and the hair has not yet turned into quills. The LRT does not test every trait. However, traits in the LRT nest Maiopatagium as a primitive porcupine and less likely to glide than originally figured.

Figure 4. Coendou, the extant prehensile-tailed porcupine, nests with the Jurassic Maiopatagium in the LRT. No other taxon nests closer among the 1268 tested.

Figure 5. Coendou, the extant prehensile-tailed porcupine, nests with the Jurassic Maiopatagium in the LRT. No other taxon nests closer among the 1268 tested.

The side-by-side alignment of the calcaneum and astragalus
figured by Meng et al. (Fig. 4) is yet another pre-therian trait (see Eomaia for the first shift to the therian state). Rodents don’t have this type of ankle (Fig. 5), so when you see it in the rodent clade we might count this as an atavism… possibly because Maiopatagium could have been hanging from branches or descending tree trunks head first and rotating the ankle, as squirrels do. The other possibility is a misinterpretation of the tarsals by Meng et al. An alternate reconstruction is shown here (Fig. 6).

In the porcupine pes,
please note the large flat bone arising from the medial tarsals (Fig. 5). The chinchilla does not have this disk, but Maiopatagium does (Fig. 6). It is an atavism arising from digit zero on the pes. Atavisms like this form the spur on the screamers.

Figure 6. The pes and tarsus of Maiopatagium traced and reconstructed with DGS methods compared to original art by Meng et al. 2017 (drawing).

Figure 6. The pes and tarsus of Maiopatagium traced and reconstructed with DGS methods compared to original art by Meng et al. 2017 (drawing). The porcupine, Coendou, also has a small digit 1 and a medial disk (tarsal zero) arising from the tarsus. The calcaneum appears to be crushed into several pieces, so the ‘calcar’ may be a broken artifact. No sister taxa have the Meng et al. ankle. Tarsal 5 and the lateral centrale (cuboid) are also separate.

Added almost a day later:
the pes of another specimen, BMNH1133 (from Meng et al. 2017, Fig. 7) compared to Rattus the rat. Pretty similar when reconstructed, aren’t they?

Figure 7. Another pes from Meng et al. 2017, this time reconstructed and compared to Rattus the rat. All the bones are there in just about the same shape and interrelation.

Figure 7. Another pes from Meng et al. 2017, this time reconstructed and compared to Rattus the rat. All the bones of the tarsus are there in just about the same shape and interrelation. The digits differ in proportion. Note the matching of the tibia-fibula width to a typical narrowly stacked astragalus and calcaneum.

 

References
Kermack KA, Kermack DM, Lees PM and Mills JRE 1998. New multituberculate-like teeth from the Middle Jurassic of England. Acta Palaeontologica Polonica 43(4):581-606.
Meng Q-J, Grossnickle DM, Liu D, Zhang Y-G, Neander AI, Ji Q and Luo Z-X 2017.
New gliding mammaliaforms from the Jurassic. Nature (advance online publication)
doi:10.1038/nature23476

wiki/Hydrochoerus
wiki/Maiopatagium
wiki/Coendou
raftingmonkey.com/Neoreomys
wiki/Brazilian_guinea_pig
wiki/Chinchilla

More evidence that euharamyidans are mislabeled Jurassic rodents

Figure 1. The Jurassic mammal Shenshou, which nests within Allotheria (Haramiyida + Mutituberculata) within the Mammalia, as I proposed based on the LRT without knowledge of this paper.

Figure 1. The Jurassic mammal Shenshou, which nests within Allotheria (Haramiyida + Mutituberculata) within the Mammalia, as I proposed based on the LRT without knowledge of this paper.

Euharamyidans include the squirrel-like Jurassic gliders
Shenshou (Figs. 1,2 ), Vilevolodon and Maiopatagium in the large reptile tree (LRT, 1265 taxa). These are sisters to the squirrel, Ratufa, the squirrel-like Paramys and two living rodents, Rattus and Mus (rat and mouse).

Mao et al. 2018 report, “The new evidence suggests presence of diphyodonty in euharamiyidans. While it will take time to amass data to resolve the discrepancy between competing phylogenetic hypotheses about ‘haramiyidans’, multituberculates, and/or allotherians, it is helpful to continue deepening our knowledge about the morphology of euharamiyidans. Our finding of potential diphyodonty in euharamiyidans provides an additional piece of evidence for mammalness of the peculiar group.”

Figure 2. Shenshou skull traced in colors.

Figure 2. Shenshou skull traced in colors.

Above:
The skull of Shenshou (Fig. 2), close to living squirrels. Evidently the molar cusps are convergent with those of Haramiyavia, but there are few other similarities.

Below:
Haramiyavia (Fig. 3), a pre-mammal cynodont with a small canine and large incisors not related to Shenshou. Note the dual articular/dentary jaw joint in Haramiyavia, missing (actually evolved into ear bones) in Shenshou. Such a jaw joint marks this taxon as a pre-mammal synapsid.

Figure 1. Haramiyavia reconstructed and restored. Missing parts are ghosted. Three slightly different originals are used for the base here. The last appears to be the least manipulated and it appears to fit the premaxilla better.  The fourth maxillary tooth appears to be a small canine. The groove on the dorsal premaxillary appears to be for the nasal, not the septomaxilla. Parts are taken from both mandibles

Figure 3. Haramiyavia reconstructed and restored. Missing parts are ghosted. Three slightly different originals are used for the base here. The last appears to be the least manipulated and it appears to fit the premaxilla better.  The fourth maxillary tooth appears to be a small canine. The groove on the dorsal premaxillary appears to be for the nasal, not the septomaxilla. Parts are taken from both mandibles

In the LRT, Haramyavia, a basal member of the Haramiyida
nests with other pre-mammals like Brasiliodon and Sinoconodon, hence: not related to euharamiyidans. Determining the clade based on traits (no matter what these traits may be) is the cause of the phylogenetic confusion based on tooth shape and replacement patterns, which can converge. Only a taxon’s placement on a cladogram can tell you what an animal really is. Sadly, that’s a current heresy, not widely appreciated.

According to Wikipedia
(ref below): Haramiyidans are a long lived lineage of mammaliaform cynodonts. Their teeth, which are by far the most common remains, resemble those of the multituberculates. However, based on Haramiyavia, the jaw is less derived; and at the level of evolution of earlier basal mammals like Morganucodon and Kuehneotherium, with a groove for ear ossicles on the dentary.[1] They are the longest lived mammalian clade of all time.”

As the LRT showed several years ago
the rodent-like Euharamiyidans (Fig. 1) nest with placental rodents in the clade Glires, not with the much more primitive pre-mammals like Haramiyavia (Fig. 3).

Mao et al. 2018 report, “presence of the diphyodont dentition alone is not diagnostic for mammals. This is because a diphyodont dentition exists not only in mammals but also in stem mammaliaforms, such as Morganucodon and docodonts, although there may be more than one replacement for the upper canine of Haldanodon (Martin et al., 2010b).”

By contrast, in the LRT
Morganucodon is a basal metatherian, not a stem mammaliaform. Which is one more reason why it has diphyodont dentition (milk teeth + permanent teeth). The late-surviving docodonts, Haldanodon and Castorocauda nest between the synapsids, Probainognathus and Pachygenelus in the LRT. Those four should be replacing all their teeth all the time. All four had a dual jaw joint that was not quite mammalian, but getting there!

Diphyodont dentition alone is diagnostic for mammals
because it implies toothless, milk-lapping/sucking hatchlings, (but be careful not to pull a Larry Martin here, because the LRT uses 231 traits and diphyodont dentition is not among them).

Among mammals
Mao et al. 2018 report, “tooth replacement is also complex among mammals. For instance, the molariform teeth of eutriconodonts show replacement and some species have the entire dentition replaced and show at least three tooth generations. Cheek tooth replacement is uncertain in “symmetrodontans”. In North American spalacotheriids deciduous canine and premolars were retained late in life and may never have been replaced; thus, their dentitions perhaps were monophyodont. This has been supported by the spalacolestine Lactodon from the Early Cretaceous Jehol Biota, in which there is no sign of cheek tooth replacement even though this taxon possesses deciduous-like antemolars. New CT scan data (unpublished) further confirmed that there is no tooth germ at any tooth locus, including incisors and canines, of Lactodon [= Lactodens”?]. Thus, presence of the diphyodonty in euharamiyidans, does not constitute a sufficient evidence for the group’s mammalian affinity.”

Let’s examine those arguments
in new light shed by the LRT.

  1. Eutriconodonts (Spinolestes, Gobiconodon and kin): These taxa do not nest within Mammalia in the LRT (contra Martin et al. 2015).
  2. Symmetrodontans (Zhangheotherium and kin): Zhangheotherium is a basal pangolin, hence the atavistic teeth, as in another placental clade, the archaeocete ‘whales’.
  3. Spalacotherids (Lactodon = Lactodens): Taxa like Lactodens nest within the prototheria in the LRT.

It always comes back down to phylogenetic analysis.
And the LRT answesr all such problems within its ken. The radiation of placental mammals was in the Early Jurassic based on the appearance of derived placental mammals in the Late Jurassic. Non-mammalian synapsids survived into the Middle Jurassic, so there was plenty of overlap.

Figure 4. Lactodens in situ. This Early Cretaceous protothere has tooth-lined jaws. At 72 dpi this is about 3x larger than life size.

Figure 4. Lactodens in situ. This Early Cretaceous protothere has tooth-lined jaws. At 72 dpi this is about 3x larger than life size.

PS. If you’re wondering about
Lactodens (= Lactodon; Fig. 4; Han and Meng 2018; Early Cretaceous) here it nests at the base of the echidna + platypus clade, two toothless (as adults) taxa. Perhaps that’s why the diphyodont dental rules start breaking down with this taxon, as described by Mao et al.

References
Mao F-Y et al. (5 co-authors) 2018. Evidence of diphyodonty and heterochrony for dental development in euharamiyidan mammals from Jurassic Yanliao Biota. Vertebrata PalAsiatica DOI: 10.19615/j.cnki.1000-3118.180803

https://en.wikipedia.org/wiki/Haramiyida

Mammal taxa: size categories

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). Yesterday we looked at the deep time chronology of mammals. Today we add size categories to the cladogram to indicate Cope’s Rule (size increase over time) and phylogenetic miniaturization (size decrease over time, Fig. 1).

Looking at various mammal taxa size categories:

Figure 2. Subset of the LRT focusing on mammals. Color bars indicate size categories.

Figure 2. Subset of the LRT focusing on mammals. Color bars indicate size categories. The general trend is toward larger taxa with only a few phylogenetic miniaturization reversals.

Some notes:

  1. Mouse-sized taxa are typical at the origin of the Mammalia and the Metatheria (Marsupialia) with a few taxa growing to cat-sized. The few human-sized taxa are wolf-like or kangaroos. The two cow-sized metatherians are giant wombats.
  2. Cat-sized taxa are typical at the origin of the Eutheria (placentals). Larger taxa do not appear until after the large dinosaurs became extinct. Note: during the Mesozoic some large pre-mammals, like Repenomamus, remained.
  3. There are no elephant-sized prototheres or metatheres.
  4. There are no mouse-sized taxa more derived than Maelestes and close kin.
  5. Phylogenetic miniaturization attends the origin of mammals, the origin of the Hadrocodium clade, and after the glyptodonts. Little to no evidence of miniaturization appears at the origin of metatherians and eutherians. Slight evidence of miniaturization also appears at Ocepeia (pre mysticetes), Cainotherium (pre-artiodactyls) and Ectocion (pre-hyrax/elephant/siren).

Much earlier we looked at birth types (helpless vs. able) in a previous cladogram of the Mammalia that is as up-to-date as this one, but the point is made. We also briefly looked at the flexible spinal column of basal mammals vs. the less flexible spine of derived mammals.

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.