Recalibrating clade origins, part 4

Earlier
we looked at the first part and second part and third part of Marjanovic’s 2019 chronological recalibration of vertebrate nodes. Today we continue in part 4 of 5.

Mammalia (Prototheria + Theria)
Based on the literature, Marjanovic 2019 considers morganucodontans and tiny Hadrocodium to be mammalomorphs, not mammals. He is unsure about haramiyidans. With regard to the first dichotomy of mammals, he reported, “I recommend a hard minimum age of 179 Ma for this calibration.”

By contrast the large reptile tree (LRT, 1630+ taxa) nests Morganucodon (Late Triassic, 205 mya), Hadrocodium (Early Jurassic) and Henosferus (Middle Jurassic) together in the most basal subclass within Theria, part of the first dichotomy within Mammalia. Marjanovic considered Henosferus one of the oldest uncontroversial mammals at 179 mya. Megazostrodon is a late surviving (early Jurassic) last common ancestor taxon of all mammals in the LRT. It must have appeared prior to Morganucodon in the Late Triassic.

Theria (Metatheria + Eutheria)
Marjanovic reports, “The oldest securely dated eutherian is Ambolestes at 126 Ma.” Then he reports, “Accepting that Juramaia is not from the Lanqi Fm, I propose 160 Ma as the soft maximum age of this calibration.”

By contrast, Morganucodon (Late Triassic, 210mya) nests as the oldest therian. Ambolestes nests with Didelphis, the opossum, within the Theria, not Eutheria.  Thereafter the traditional Metatheria splits in three clades in the LRT, a largely herbivorous branch with Glironia and Marmosops at its base, and a largely carnivorous branch with Monodelphis and Chironectes at the base of one branch and Caluromys + Placentalia at yet another. So, while Caluromys (Fig. 3) retains a pouch, it is also the last common ancestor of all placentals.

Figure 1. Pteropus and Caluromys compared in vivo and three views of their skulls. Caluromys is in the ancestry of bats and shows where they inherited their inverted posture.

Figure 1. Pteropus and Caluromys compared in vivo and three views of their skulls. Caluromys is in the ancestry of bats and shows where they inherited their inverted posture.

Marjanovic often errors by not including extant taxa that are more primitive than extinct taxa that are older. This comes back to bite him several times, especially so when he relies on a single fossil tooth rather than a living animal he can hold. The LRT tests both living and extinct taxa to minimize taxon exclusion.

Marjanovic discusses the possibility that Sinodelphys is the oldest known metatherian, but Sinodelphys nests as one of the most primitive prototherians in the LRT, as we learned earlier here.

Placentalia (Atlantogenata + Boreo(eu)theria)
In the world of gene studies, Atlantogenata include the highly derived elephants and anteaters. The Boreoeutheria include the highly derived whales, humans and hooved mammals. Genomic studies deliver false positives, and these are among the most blatant, so ignore these. They don’t deliver a gradual accumulation of derived traits.

By contrast, in the LRT the first dichotomy in the placentalia splits arboreal Vulpavus from arboreal Nandinia and thereafter arboreal Carnivora (mongooses and raccoons) from arboreal Volantia (bats and colugos) + arboreal Primates and the rest of the Placentalia. All of these civet-like and tree opossum-like taxa look like Caluromys (Fig. 1), as you can see. Elephants and anteaters come later. Adding living taxa to Marjanovic’s search for primitive placentals would have helped clarify his research and conclusions, preventing him form perpetuating old myths.

Carnivora (Feliformia + Caniformia)
Marjanovic errs by reporting the basal dichotomy within Carnivora splits cats from dogs.

By contrast in the LRT cats and dogs are closely related and derived taxa, not basal. As mentioned above, civets, mongooses and raccoons are basal Carnivora.

Euarchontoglires/Supraprimates (Gliriformes + Primatomorpha)
Marjanovic discusses several poorly preserved, sometimes one tooth only, fossil taxa from the early Paleocene (65mya). Some of these are anagalids, which nest at the base of yet another clade in the LRT, the one with tenrecs and odontocetes (toothed whales).

By contrast in the LRT lemur-like adapids appear at the base of the Primates. Tree-shrews appear at the base of the Glires.

Marsupialia (Didelphimorphia – Paucituberculata + Australidelphia)

  • Didelphimorphia = opossums from North America
  • Paucituberculata = South American marsupials, sans Dromiciops
  • Australidelphia = Australian and Asian marsupials, plus Dromiciops

Marjanovic reports, “I therefore propose 55 Ma as a probably overly strict hard minimum age for this calibration.” He later reports, “Rather than the beginning of the Maastrichtian, I propose the beginning of deposition of the Lance and Hell Creek formations, where Glasbius has been found, as the hard maximum age for this calibration, which I estimate as 68 Ma.”

See Figure 1 for a different three-part marsupial split from the LRT. Dromiciops is only one of many similar herbivorous marsupials. Middle Late Cretaceous Asioryctes is a basal member of the largely herbivorous clade. Early Cretaceous Vincelestes is a basal member of the largely carnivorous clade. So Middle to Late Jurassic (175mya) is a better estimate for the genesis of marsupial diversity. That means marsupials dispersed during the Pangean era without the need of an oceanic dispersal.

Marjanovic mistakenly reports, “Marsupials, other metatherians and indeed other therians are wholly absent from the Late Cretaceous mammaliform record of South America, which consists instead of gondwanatherian haramiyidans and a very wide variety of meridiolestidan stem-theriiforms.”

  • Meridiolestida = non-therian mammals (= Prototheria, Montremata) seems to be based on tooth traits. Cronopio and Necrolestes are among the only tested taxa also  found in the LRT. Cronopio is an omnivorous member of the pre-metatherian Theria in the LRT. Necrolestes is a basal member of the placental clade, Glires, derived from the treeshrew Tupaia in the LRT. So, again, we have a mismatch due to not testing all the mammals against all the mammals. That is what makes the LRT such a powerful tool that should be more widely used to avoid such old school mythology.

More tomorrow as we conclude part 5 of 5.


References
Marjanovic D 2019. Recalibrating the transcriptomic timetree of jawed vertebrates.
bioRxiv 2019.12.19.882829 (preprint)
doi: https://doi.org/10.1101/2019.12.19.882829
https://www.biorxiv.org/content/10.1101/2019.12.19.882829v1

SVP abstracts – Ambolestes and the origin of placentals

Bi S-D et al. 2019 discuss Early Cretaceous Ambolestes
(Figs. 1, 2) and the Early Mesozoic marsupial/placental split.

Figure 1. Ambolestes tracing from Bi et al. 2018.

Figure 1. Ambolestes tracing from Bi et al. 2018.

From the abstract:
“Extant placental and marsupial mammals are the dominant vertebrates in many ecosystems, which makes the placental-marsupial dichotomy a significant event in Earth’s history.”

The large reptile tree (LRT, 1592 taxa) splits placentals from marsupials as shown below (Figs. 3, 4). The Early Cretaceous marsupial Bishops splits from the placental outgroup taxon, the extant marsupial Caluromys (Fig. 6). More timely, derived placental multituberculates, like Megaconus (Fig. 5), have been found in Middle Jurassic strata. That means a long line of undiscovered small, arboreal, placentals extends back to the Late Triassic/Earliest Jurassic.

Figure 3. Ambolestes skull reconstructed. Jaw tips restored.

Figure 2. Ambolestes skull reconstructed. Jaw tips restored.

Bi et al. continue:
“Molecular estimates of the divergence of placentals and marsupials (and their broader clades Eutheria and Metatheria) fall primarily in the Jurassic.”

Since Early Jurassic Megazostrodon is the proximal outgroup for all mammals, and Early Triassic Morganucodon is a marsupial, and Middle Jurassic Megaconus the LRT supports a Late Triassic split for placentals and marsupials.

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

Figure 3. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here. Note the large gaps of time in which fossils are not known.

Bi et al. continue:
“In support, the oldest purported eutherian, Juramaia, is reported to be from the early Late Jurassic (160 million-years ago) of Liaoning Province, northeastern China.”

In the LRT (subset Fig. 1) Juramaia nests as a basal prototherian, an egg laying basal mammal.

“The oldest purported metatherian, Sinodelphys, is 35 million-years younger from the
Early Cretaceous Jehol Biota also in Liaoning Province, northeastern China.”

In the LRT Sinodelphys is another monotreme.

“In 2018, we reported a new eutherian, Ambolestes zhoui, also from the Jehol Biota. The fossil, a nearly complete skeleton, preserves anatomical detail unknown from contemporaneous eutherians including the hyoid apparatus and ectotympanic. The complete hyoid is the first known for any Mesozoic mammaliaform, and the ectotympanic resembles that in some extant didelphid marsupials.”

In the LRT (Fig. 1) Ambolestes (Figs. 3, 4) is a metathere/marsupial close to the extant Virginia opossum, Didelphis.

Figure 1. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

Figure 4. Subset of the LRT focusing on the Kynodontia and Mammalia. Non-eutherian taxa in red were tested in the LRT but not included because they reduce resolution. Eutherian taxa in red include a basal pangolin and derived xenarthran, clades that extend beyond the bottom of this graphic. The pink clade proximal to mammals was considered mammalian by Lautenschlager et al. due to a convergent mammalian-type jaw joint.

Bi et al. continue:
“In our phylogenetic analysis concentrating on the eutherian-metatherian 
dichotomy, the closest relative of Ambolestes was Sinodelphys, and both fell within Eutheria.”

As shown above, the LRT does not confirm that hypothesis of interrelationships.

Figure 1. Subset of the LRT focusing on Glires and subclades within.

Figure 5. Subset of the LRT focusing on Glires and subclades within.

Bi et al. continue:
“With Sinodelphys as a eutherian, postcranial differences formerly thought to indicate different invasions of a scansorial niche by meta and eutherians in Jehol are only variations among the early members of the placental lineage. Additionally, the earliest known metatherians are approximately 15 million years younger than previously thought and their

fossils, isolated teeth and fragmentary jaws, are from North America. Our tree results in a 50 million-year ghost lineage for Metatheria, accepting the 160 million-years age for Juramaia. 

The LRT confirms a 210 mya origin for Metatheria, starting with Morganucodon, so no ghost is necessary.

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 largest of the mouse opossums, to scale with its LRT sister, Vulpavus, a basal member of Carnivora and Placentalia.

Bi et al. continue:
“A possibility raised elsewhere is that the age of Juramaia is incorrect; rather than Late Jurassic, perhaps it is from the Early Cretaceous Jehol Biota. In our study, Juramaia is in a clade with Albian/Aptian Prokennalestes and Late Cretaceous eutherians by having a more molariform ultimate upper premolar. In contrast, Ambolestes, as in the outgroups, has a non-molariform ultimate upper premolar. Although resolution of this intriguing debate is not currently possible, our understanding of the issues has been furthered by the discovery of Ambolestes.”

As shown above, the LRT does not confirm the Bi et al. hypothesis of interrelationships.


References
Bi S-D et al. 2019. The Early Cretaceous eutherian Ambolestes and its implications for the Eutherian/Metatherian dichotomy. Journal of Vertebrate Paleontology abstracts.

Two new Royal Society papers suffer from taxon exclusion

Gutarra et al. 2019
tested the effects of several body plans on the hydrodynamic drag of simplified 3D digital ichthyosaurs. They reported, “Our results show that morphology did not have a major effect on the drag coefficient or the energy cost of steady swimming through geological time.”

Unfortunately
the Gutarra team included the basal sauropterygian ichthyosaur-mimic Cartorhynchus as their basal taxon, ignoring the following four valid ichthyosaur basal taxa.

  1. Wumengosaurus
  2. any hupehsuchid
  3. Thaisaurus
  4. Xinminosaurus

Given the Gutarra et al. similar results
for all included digitally generated taxa, it would have been instructive to test at least one of these basal taxa or perhaps outgroup taxa from the Mesosauria and/or Thalattosauria in order to set a baseline. Co-author professor MJ Benton has been reprimanded for excluding taxa several times before, and doggone it, he did it again.


Halliday et al. 2019
“supports a Late Cretaceous origin of crown placentals with an ordinal-level adaptive radiation in the early Paleocene, with the high relative rate permitting rapid anatomical change without requiring unreasonably fast molecular evolutionary rates.” 

By contrast
the large reptile tree (LRT, 1413 taxa) nests several placental taxa (like multituberculates) in the Jurassic with placental origins likely in the Late Triassic very soon after the origin of Mammalia.

Halliday’s team differentiates extant placentals from several extinct eutherians,
while the LRT finds only one extant taxa, the arboreal didelphid Caluromys, in the Eutheria outside of the Placentalia.

Halliday’s team cites the Luo et al. 2011 report
of “a Jurassic eutherian mammal” (= Juramaia) with reservations. In the LRT Juramaia nests with basal prototherians, not eutherians.

None of Halliday’s published work
matches the topology of the LRT. The Halliday team nests highly derived hedgehogs, elephants and armadillos as a closely related clade at the base of their cladogram of extant placentals.

By contrast and employing more taxa
the LRT documents the evolution of three clades of basal placentals, like arboreal civets, bats, dermopterans, pangolins and tree shrews (Primates + Glires), from arboreal marsupials, like Caluromys. 

Evolution: small changes over time.
The editors and referees approved Halliday’s ‘traditional’ topology. Someone should have checked results for relationships that minimize differences between recovered sisters. More taxa and avoiding genetic scoring would have helped.

Halliday’s study supports several invalidated genetic clades,
including Atlantogenata (anteaters + elephants and kin), Boreotheria (mice + whales + humans and kin), and Afrotheria (elephant shrews + elephants and kin). Even so, editors, paleoworkers and referees approved these untenable and refuted relationships.

That’s why the LRT is here,
to lift the covers and show you untenable traditional relationships, then to offer a tree topology in which all included taxa document a gradual accumulation of derived traits.


References
Gutarra S, Moon BC, Rahman IA, Palmer C, Lautenschlager S, Brimacombe AJ, and Benton MJ 2019. Effects of body plan evolution on the hydrodynamic drag and energy requirements of swimming in ichthyosaurs. Proc. R. Soc. B 286: 20182786. http://dx.doi.org/10.1098/rspb.2018.2786
Halliday TJD, dos Reis M, Tamuri AU, Ferguson-Gow H, Yang Z and Goswami A 2019. Rapid morphological evolution in placental mammals post-dates the origin of the crown group. Proc. R. Soc. B 286: 20182418. http://dx.doi.org/10.1098/rspb.2018.2418

Tweaking Palaechthon (basal Volitantia)

Kay and Cartmill 1977 wrote:
“The Middle Paleocene paromomyid Palaechthon nacimienti has the most primitive cranial anatomy known for any plesiadapoid. In relative size and functional morphology, its molars resemble those of primates and tree shrews known to feed largely on insects. Its orbits were small, laterally directed, and widely separated, and the relative size of its infraorbital foramen shows that it had well-developed facial vibrissae resembling those of extant erinaceids. Its anterior dentition was probably also hedgehog-like. These features suggest that it was a predominantly terrestrial insect-eater, guided largely by tactile, auditory and olfactory sensation in its pursuit of prey. Adaptations to living in trees and feeding on plants probably developed in parallel in more than one lineage descended from the ancestral plesiadapoids. A new genus and species of paromomyid, Talpohenach torrejonius, is erected for material originally identified as Palaechthon.”

This was done in the days before software phylogenetic analysis.
In the large reptile tree nests (LRT, 1413 taxa) Palaechthon as a sister to the dermopterans, like Cynocephalus, both derived from basal Carnivora, like Vulpavus (Fig. 1). All of these taxa are basal to Primates in the LRT.

Palaechthon is known from several partial specimens
combined to make the incomplete skull drawing shown here (Fig. 1).

Figure 1. Palaechthon compared to outgroup, Vulpavus, and sister, Cynocephalus using drawings from Kay and Cartmill 1974. Colors added.

Figure 1. Palaechthon compared to outgroup, Vulpavus, and sister, Cynocephalus using drawings from Kay and Cartmill 1974. Colors added.

Palaechthon nacimienti (Wilson and Szalay 1972) ~4cm skull length, middle Palaeocene, was originally and traditionally considered a basal plesiadapiform (traditionally considered a clade of basal primates). Here derived from a sister to the basal placental and carnivoran, Vulpavus, Palaechthon phylogenetically nests with Cynocephalus, the colugo. The premaxilla is missing and may have been nearly toothless, as in the colugo. Distinct from Vulpavus, but as in Cynocephalus, and the basalmost eutherian, Caluromys, four molars are present in Palaechthon.

Figure 3. Cynocephalus, the flying lemur, shares many traits with Ptilocercus and basal bats.

Figure 2. Cynocephalus, the flying lemur, shares many traits with Ptilocercus and basal bats.

Phylogenetic bracketing
makes Palaechthon an arboreal taxon, possibly with a prehensile tail. Kay and Cartmill 1974 imagined large, rodent-like teeth emerging from the missing premaxilla and missing anterior dentary (Fig. 1). Phylogenetic bracketing indicates just the opposite—tiny anterior teeth, as shown in Vulpavus and Cynocephalus. The auditory bulla was probably small, as indicated by phylogenetic bracketing. The post-dentary part of the skull was probably short, as in Cynocephalus.

Figure 1. Ignacius and Plesiadapis nest basal to Daubentonia in the LRT.

Figure 3. Ignacius and Plesiadapis nest basal to Daubentonia in the LRT.

The present ‘tweaking’
benefits from more recent additions to the LRT that provided more clues to the closest relatives of Palaechthon, cementing relationships recovered years earlier. First hand access did not give Kay and Cartmill more insight into the relationships of Palaechthon, a basal member of the clade Volitantia.They presumed from the start that it was a primate ancestor, close to Plesiadapis. Both presumptions have been refuted by the LRT, which tests both basal primates and plesiadapiformes, now nesting within Glires. Based on the appearance of descendant taxa in the Middle Jurassic, Palaechthon had its genesis in the Early Jurassic.


References
Fleagle JG 1988. Primate Adaptation and Evolution. Academic Press: New York
Kaplan M 2012. Primates were always tree-dwellers. Nature. doi:10.1038/nature.2012.11423
Kay RF and Cartmill M 1974. Skull of Palaechthon nacimienti. Nature 252:37–38.
Kay RF and Cartmill M 1977. Cranial Morphology and Adaptation of Palaechthon nacimienti and Other Paromomyidae (Plesiadapoidea, Primates), with a Description of a New Genus and Species. Journal of Human Evolution, Vol. 6, 19-53.
Sloan RE and Van Valen L 1965. Cretaceous mammals from Montana. Science 148:220–227
Van Valen L and Sloan R 1965. The earliest primates. Science. 150(3697): 743–745.
Wible JR, Rougier GW, Novacek MJ and Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary.” Nature volume 447: 1003-1006
Wilson RW and Szalay FS 1972. American Museum Novitates 2499:1.

 

Hey! Some of those Miocene ‘ungulates’ are marsupials!

More heresy today
courtesy of taxon inclusion.

Cassini 2013
looked at several traditional Miocene South American ‘ungulates’ (Fig. 1) unaware that these taxa do not nest in monophyletic clade any more specific than Theria in the large reptile tree (LRT, 1401 taxa). Cassini was reporting the results of “an ecomorphological study based on geometric morphometrics of the masticatory apparatus.” So he was working from prior cladograms and focusing on the mechanics of eating.

Figure 1. Image from Cassini 2013. Pink taxa are marsupials. Others are placentals.

Figure 1. Image from Cassini 2013. Pink taxa are marsupials. Others are placentals.

Earlier
here and here the traditional clade Notoungulata was splintered by the LRT into several clades, some among the marsupials, other among the placentals.

Traditional ‘Litopterna’
Diadiaphorus (Fig. 1) nests at the base of this clade. Theosodon (Fig. 1) nests as a derived taxon. Also included, but not listed: Chalicotherium and other chalicotheres.

Considering its member taxa,
the clade Litopterna (Ameghino 1889) is a junior synonym for Chalicotheridae (Gill 1872) in the LRT.

Considering its member taxa,
the clade Astrapotheria (Lydekker 1894) is a junior synonym for Meniscotheriinae (Cope 1882) and both nest within Phenacodontidae (Cope 1881).

Interatheriidae (Ameghino 1887) traditionally includes Interatherium, Protypotherium, Miocochilius and other taxa listed here. In the LRT Interatherium nests close to the ancestry of the Toxodon clade + the kangaroo clade within Metatheria. By contrast, Protypotherium and Miocochillus nest with Homalodotherium deep within the Eutheria. Homalodotherium traditionally nests with the the metatherian Toxodon. According to the LRT, all the above taxa developed similar enough traits by convergence that all were mistakenly lumped together in the invalid placental clade ‘Notoungulata’.

This is not the first time
metatherians were split from convergent eutherians. Most creodonts are marsupial predators, phylogenetically distinct from their traditional sisters in the clade Carnivora, within the clade Eutheria (Placentalia).

New taxa added to the LRT:

  1. Hegetotherium (Fig. 1) nests with Mesotherium between Interatherium and the Toxodon clade in the Metatheria. 
  2. Diadiaphorus (Fig. 1), the horse-mimic, nests at the base of the Litopterna/Chalicotheriidae, just basal to Litolophus.

I did not know these two, so I added these two to better understand them.

References
Cassini G 2013. Skull Geometric Morphometrics and Paleoecology of Santacrucian (Late Early Miocene; Patagonia) Native Ungulates (Astrapotheria, Litopterna, and Notoungulata). Ameghiniana 50 (2):193–216. DOI: 10.5710/AMGH.7.04.2013.606

The prehensile hand and foot of Caluromys

Figure 1. Pteropus and Caluromys compared in vivo and three views of their skulls. Caluromys is in the ancestry of bats and shows where they inherited their inverted posture.

Figure 1. Pteropus and Caluromys compared in vivo and three views of their skulls. Caluromys is in the ancestry of bats and shows where they inherited their inverted posture.

I could not find
and still cannot find a complete skeleton for Caluromys (Fig. 1), the transitional marsupial leading to placentals. Argot 2001 published images of the hand in vivo. Argot 2002 published images of the foot in vivo and as an incomplete set of bones (Fig. 2). I matched those bones to the foot, still wishing I had all the bones, as in an X-ray.

Figure 1. Caluromys hand and foot from Argot 2002 compared to Didelphis and repaired here to match.

Figure 2. Caluromys hand and foot from Argot 2001, 2002 compared to the pes of Didelphis. The manus and pes of primates, tree shrews (in Glires) and basal arboreal Carnivorans all arise from Caluromys. These demonstrate the early appearance of the prehensile/opposable big toe and thumb, derived from the semi-opposable big toe of Didelphis, the Virginia opossum and even more so in Caluromys. 

The prehensile manus and pes of Caluromys
is primitive for the Eutheria (= Placentalia). From these arise the wings of bats, the flippers of whales, the hooves of horses as well as the fingers I just used to type this sentence.

References
Argot C 2001. Functional-Adaptive Anatomy of the Forelimb in the Didelphidae, and the Paleobiology of the Paleocene Marsupials Mayulestes ferox and Pucadelphys andinus. Journal of Morphology 247:51–79.
Argot C 2002. Functional-adaptive analysis of the hindlimb anatomy of extant marsupials and the paleobiology of the Paleocene marsupials Mayulestes ferox and
Pucadelphys andinus. Journal of Morphology 253:76–108.

Basal placentals illustrated in phylogenetic order

Eutherian (= placental) mammals
are divided into clades like Primates, Ungulata, Carnivora, etc. Known basal taxa for each of these clades are related to one another in a ladder-like fashion, each one nesting at the base of a bushy clade in the large reptile tree (LRT, 1366 taxa).

Today,
an illustration of skeletons (Fig. 1) and skulls (Fig. 2) in phylogenetic order documents the minor changes (microevolution) between basal taxa that nest at the bases of several increasingly derived placental clades.

FIgure 1. Skeletons of taxa that nest at the bases of several major placental clades, divided between Cretaceous and Paleocene taxa.

FIgure 1. Skeletons of taxa that nest at the bases of several major placental clades, divided between Cretaceous and Paleocene taxa, divided by four different scales. Basal taxa are several degrees of magnitude smaller.

The following placental skulls are not to scale
yet continue to demonstrate the minor changes (microevolution) that occur at the bases of several major placental clades. For instance, Chriacus is basal to bats, while Anagale is basal to odontocete whales. It is difficult, if not impossible, to determine such future developments in these basal taxa without the benefit of a wide gamut analysis, like the LRT.

Figure 2. A selection of placental skulls in phylogenetic order and divided into Cretaceous and Paleocene taxa.

Figure 2. A selection of placental skulls in phylogenetic order and divided into Cretaceous and Paleocene taxa.

The lesson for today:
Sometimes quantity, without firsthand observation, is needed to put together the ‘Big Picture’ before one is able to pick apart the details that each specific specimen reveals during firsthand study. Traditionally paleontologists have been putting the latter ahead of the former by (too often) excluding pertinent taxa revealed and documented by the more generalized and wide gamut phylogenetic analysis provided by the LRT. Like Yin and Yang, both must be considered. ‘Avoid taxon omission‘ is the single most important rule when constructing a cladogram of interrelationships.

References
See ReptileEvolution.com and links therein.

Mesozoic mammals: Two views

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

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

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

  1. All known and tested Mesozoic mammals (Fig. 1) are either small arboreal taxa or small burrowing taxa (out of sight of marauding theropods).
  2. All Mesozoic monotremes are more primitive than Ornithorhynchus and Tachyglossus (both extant).
  3. All Mesozoic marsupials are more primitive than or include Vintana (Late Cretaceous).
  4. All Mesozoic placentals are more primitive than Onychodectes (Paleocene).
Figure 1. Select basal cynodonts and mammals set chronologically. The divergence times for placentals (Eutheria), marsupials (Metatheria) and monotremes (Mammalia) are estimated here.

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

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

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

Mesozoic Prototherians

  1. All included fossil taxa are Mesozoic. Two others are extant (Fig. 2).
Figure 2. Mesozoic prototherians + Megazostrodon, the last common ancestor of all mammals. Only two taxa (gray) are post-Cretaceous.

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

Mesozoic Metatherians (Marsupials)

  1. Derived Vincelestes is Early Cretaceous, which means Monodelphis and Chironectes were present in the Jurassic.
  2. Derived Didelphodon is Late Cretaceous, which means sisters to Thylacinus through Borhyaena were also present in the Mesozoic.
  3. Derived Vintana is Late Cretaceous, which means sisters to herbivorous marsupials were also present in the Mesozoic.
Figure 3. Mesozoic metatherians (in black), later taxa in gray. Whenever derived taxa are present in the Mesozoic (up to the Late Cretaceous) then ancestral taxa, or their sisters, were also present in the Mesozoic. Didelphis is extant, but probably unchanged since the Late Jurassic/Early Cretaceous.

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

Mesozoic Eutherians (= Placentals)

  1. Rarely are placental mammals identified from the Mesozoic, because many are not considered placentals.
  2. Placentals (in the LRT) are remarkably rare in the Mesozoic, but sprinkled throughout the cladogram, such that all taxa more primitive than the most derived Mesozoic taxon (Anagale and derived members of the clade Glires, Fig. 4, at present a number of multituberculates) must have had Mesozoic sisters (Carnivora, Volitantia, basal Glires). 
Figure 4. Mesozoic euthrerians (placentals, in black). Later taxa in light gray. All taxa more primitive than Mesozoic taxa were likely also present in the Jurassic and Cretaceous. None appear after Onychodectes. Madagascar separated from Africa 165-135 mya, deep into the Cretaceous with a population of tenrecs attached. No rafting was necessary. 

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

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

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

SVP 2018: Placentals in the Cretaceous

Halliday et al. 2018
wonder about “the traditional lack of Cretaceous placental fossils when results from diverse dating analyses favor a Cretaceous origin of Placentalia.”

Unfortunately
they use an outdated cladogram that includes the following invalid clades (superorders) that Halliday et al. surmise should be present in Cretaceous sediments:

  1. Atlantogenata = Afrotheria + Xenarthra (elephants and anteaters in one clade?)
  2. Laurasiatheria = shrews, pangolins, bats, whales, carnivorans and ungulates (whales and bats in the same clade?)
  3. Euarchontoglires = rodents, lagomorphs, tree shrews, colugos and primates (lacking only carnivores, these are basal eutherians

Together these three clades
comprise the entirety of extant Eutheria (placental mammals). All of the above clades are extant. Where are the extinct clades, like Multituberculata?

By contrast,
the large reptile tree  (LRT, 1313 taxa) recovers Middle and Late Jurassic placentals (multituberculate rodents) along with several Early Cretaceous taxa, like the pangolin ancestor, Zhangheotherium (Fig. 1). So “the traditional lack of Cretaceous placental fossils” has been updated in the LRT.

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

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

Halliday et al. conclude: “The lack of definitive Cretaceous placental mammals may therefore be explained by high predicted morphological similarity among stem and basal crown eutherians, providing an avenue for partially reconciling the fossil record and molecular divergence estimates in Placentalia.”

No.
Taxon exclusion has given Halliday et al. an outdated tree topology. There is plenty of evidence for Mesozoic placentals in the LRT. Adding taxa provides every included taxon new opportunities to nest more parsimoniously. A good starter list can be found here (LRT subset Fig. 2). Many taxa from this list are candidates for discovery in the Mesozoic based on the discovery of multituberculates in the Mesozoic.

Figure 3. Subset of the LRT, focusing on basal Eutheria and Heterocephalus. Aqua taxa are arboreal. Tan taxa are terrestrial. Blue taxa are aquatic.

Figure 3. Subset of the LRT, focusing on basal Eutheria and Heterocephalus. Aqua taxa are arboreal. Tan taxa are terrestrial. Blue taxa are aquatic.

References
Halliday TJ et al. (5 co-authors) 2018. Delayed increase in morphological rates of evolution after the origin of the placental mammal crown group. SVP abstracts.

Caluromys vs Vulpavus vs Ptilocercus

At the base of the Eutheria (placental mammals)
nests the small, extant, didelphid marsupial Caluromys. So it’s worthwhile to put the skulls of a few basal placentals next to Caluromys to see what the similarities and differences are.

Vulpavus comparison
In the large reptile tree (LRT, 1272 taxa) Caluromys nests with Vulpavus (Fig. 1, Eocene; Marsh 1871), a basalmost member of the placental clade, Carnivora. Two molars characterize this clade. Juvenile Caluromys (Fig. 2; Flores, Abdala  and Giannini 2010) also have two molars.

Figure 1. Vulpavus compared to Caluromys skulls in lateral view.

Figure 1. Vulpavus compared to adult Caluromys skull in lateral view. These two taxa nest together in the LRT.

Ptilocercus comparison
(pen-tailed tree shrew, extant, Le Gros-Clark 1926) is best compared, both in size and morphology to the juvenile Caluromys (Fig. 2). Though not a permanent member of the LRT, a test nested the juvenile Caluromys with Ptilocercus.

Figure 2. Ptilocercus (pen-tailed tree shrew) compared to Caluromys (wooly-opossum) young juvenile from Flores, Abdala and Giannini 2010.

Figure 2. Ptilocercus (pen-tailed tree shrew) compared to Caluromys (wooly-opossum) young juvenile from Flores, Abdala and Giannini 2010.

Caluromys derbianus (Allen 1904; Flores, Abdala and Giannini N 2010, Fonseca and Astúa 2018. ) is the living ‘wooly opossum’, native to Central America. This taxon nests just inside of the first placental clade, Carnivora, despite retaining a marsupium (pouch). It is an omnivore, as in related basal Carnivora, like Nandinia.

Juvenile skulls have only two molars, the same as those found in Vulpavus (below) and other Carnivora, so this trait is neotonous in Carnivora.

That Caluromys is closely related to basal placental taxa is strong… and heretical.
Earlier we looked at skull similarities between Caluromys and the fruit bat, Pteropus.

References
Allen JA 1904. Mammals from southern Mexico and Central and South America. Bulletin American Museum of Natural History 20(4): 29-80.
Flores DA, Abdala F and Giannini N 2010. Cranial ontogeny of Caluromys philander (Didelphidae: Caluromyinae): a qualitative and quantitative approach. Journal of Mammalogy 91(3):539–550.
Fonseca R and Astúa D 2018. Geographic variation in Caluromys derbianus and Caluromys lanatus (Didelphimorphia: Didelphdiae) Zoologica 32(2):109–122.
Heinrich RE and Rose KD 1997. Postcranial morphology and locomotor behavior of two early Eocene miacoid carnivorans, Vulpavus and Didymictis. Palaeontology 40:279-305
Le Gros-Clark WE 1926. On the Anatomy of the Pen-tailed Tree-Shrew (Ptilocercus lowii.) Proceedings of the Zoological Society of London 96: 1179-1309.
DOI – 10.1111/j.1096-3642.1926.tb02241.x
Marsh 0C 1871. Notice of some new fossil mammals and birds from the Tertiary formations of the West. American Journal of Science, Series 3, 2: 120-127

wiki/Vulpavus
wiki/Water_opossum_Caluromys
wiki/Ptilocercus