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 (Maelestes 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

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Triassic origin of scales, scutes, hair, etc. as biting fly barriers?

During the Middle to Late Triassic

  1. Mammals developed fur/hair.
  2. Aetosaurs developed plates and horns beyond the earlier paired dorsal scutes.
  3. Crocodylomorphs developed large scales beyond the earlier paired dorsal scutes.
  4. Dinosaurs lost those paired scutes and developed placodes and quills. Ultimately these became scales and feathers.
  5. Turtles developed hard scales over a carapace and plastron.
  6. Lepidosaurs developed small scales.
  7. Pterosaurs and their Late Triassic sisters developed pycnofibers

All of these developed on the soft, naked skin
(think of a plucked chicken) that was a universal covering for Carboniferous and Permian tetrapods (Early forms retained large ventral scales inherited from finned ancestors, but these were lost by the Permian).

All of these extradermal structures have one thing in common.
They separated and/or protected the animal’s naked skin from the environment, one way or another. They developed by convergence. Dhouailly 2009 and other workers discussed the chemical similarities of the keratin found in these dermal structures. 

The question is:
What was different about the Triassic environment that was not present in earlier Carboniferous and Permian environments? We can 
eliminate heat, cold, UV rays, rain, aridity, etc. as possible reasons for the development of insulator structures because those factors had always been present. So what was new in the Triassic that affected all terrestrial tetrapods?

Flies and their biting, piercing kin.
“The earliest definitive flies known from the mid-Triassic of France, approximately 230 Ma (Krzemiski and Krzeminska, 2003)” according to Blagoderov, Grimaldi and Fraser 2007. The order Diptera (flies, mosquitos and kin) tend to land on large tetrapods for food, blood, etc. Scales, scutes, hair, feathers, etc. all separate flying insects from the naked skin of Triassic terrestrial tetrapods. Williams et al. 2006 even found mosquito repellents in frog skin. It is notable that, except for armored placodonts and mosasaurus (derived varanid lepidosaurs), aquatic and marine tetrapods also had naked skin with the thalattosaur, Vancleavea, a notable sermi-terrestrial exception. Is that because they had aquatic antecedents in the Triassic that were never affected by flying insects?

It’s not just the insect bite that drives this evolution,
it’s the appearance of new vectors for the rapid spread of disease that drives this evolution.

Interesting coincidence.
If this is not the case, this will take further study.

Figure 1. Lacertulus, a basal squamate from the Late Permian

Figure 1. Lacertulus, a basal squamate from the Late Permian

Carroll and Thompson 1982 report
on the Late Permian lepidosaur, Lacertulus (Fig. 1), “No scales dermal or epidermal are evident in the specimen.”

From the Dhouailly 2009 abstract:
“I suggest that the alpha-keratinized hairs from living synapsids may have evolved from the hypothetical glandular integument of the first amniotes, which may have presented similarities with common day terrestrial amphibians.

Concerning feathers, they may have evolved independently of squamate scales, each originating from the hypothetical roughened beta-keratinized integument of the first sauropsids. The avian overlapping scales, which cover the feet in some bird species, may have developed later in evolution, being secondarily derived from feathers.” Not realized by Dhouailly, the purported clade ‘Sauropsida’ is paraphyletic and a junior synonym for Amniota and Reptilia in the LRT.

Earlier we looked at the first appearances
of hair, quills, pycnofibers and hard scales in a three-part series here, here and here

Exceptionally, humans are terrestrial tetrapods
that have lost most of their hair, more or less returning to the primitive naked state. And yes, flies and mosquitos do bother humans. It is the price we pay for the benefits of naked skin. Clothing helps provide a barrier.

Remember:
Just because an idea is proposed and a hypothesis is advanced doesn’t make it so. In science ideas have to be confirmed or refuted following their first appearance. If anyone has data concerning scales or other dermal structures in Carboniferous or Permian taxa, please make us aware of those.

References
Blagoderov V, Grimaldi D and Fraser NC 2007. How Time Flies for Flies: Diverse Diptera from the Triassic of Virginia and Early Radiation of the Order. American Museum Novitates 3572:1-39. DOI: 10.1206/0003-0082(2007)509[1:HTFFFD]2.0.CO;2
Carroll RL and Thompson P 1982.
A bipedal lizardlike reptile from the Karroo. Journal of Palaeontology 56:1-10.
Dhouailly D 2009.
A new scenario for the evolutionary origin of hair, feather, and avian scales Journal of Anatomy 214(4): 587–606. doi: 10.1111/j.1469-7580.2008.01041.x
Krzeminnski, W., and E. Krzeminska. 2003. Triassic Diptera: descriptions, revisions and phylogenetic relations. Acta Zoologica Cracoviensia (suppl.) 46: 153–184.
Maderson PFA and Alibardi L 2000.
The Development of the Sauropsid Integument: A Contribution to the Problem of the Origin and Evolution of Feathers. American Zoologist 40:513–529.
Rohdendorf BB, Oldroyd H and Ball GE 1974. The Historical Development of Diptera. The University of Alberta Press, Edmonton, Canada. ISBN 0-88864-003-X.
Williams CR, Smith BPC, Best SM and Tyler MJ 2006.
Mosquito repellents in frog skin. Biol Lett. 2006 Jun 22; 2(2): 242–245. doi: 10.1098/rsbl.2006.0448

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.

 

Bird, pterosaur, dinosaur simplified chronology

Following the earlier post on non-arboreal post K-T boundary birds…

…this one pretty much speaks for itself.
Here (Fig. 1) is a chronology, very much simplified, of birds, pterosaurs and dinosaurs according to the LRT.

Figure 1. Mesozoic chronology of bird, dinosaur and pterosaur clades.

Figure 1. Mesozoic chronology of bird, dinosaur and pterosaur clades based on taxa in the LRT.

If you’re curious about any of the taxa,
in the chronology, simply use Keywords to locate them.

What is Dyoplax?

Figure 1. Dyoplax arenaceus Fraas 1867 is a mold fossil recently considered to be a sphenosuchian crocodylomorph. Here it nests as a basal metriorhynchid (sea crocodile) in the Late Triassic.

Figure 1. Dyoplax arenaceus Fraas 1867 is a mold fossil recently considered to be a sphenosuchian crocodylomorph. Here it nests as a basal metriorhynchid (sea crocodile) in the Late Triassic.

Dyoplax arenaceus (Fraas 1867, Lucas, Wild and Hunt 1998) is a unique Late Triassic crocodylomorph, one of the three original crocodylomorphs in the 19th century along with two aetosaurs.

No bones are present.
Like Cosesaurus it is a natural cast mold. Fraas thought it had the head of a lizard, but the armor of a gavial. Lucas et al. nested it as the oldest sphenosuchian crocodylomorph. Maish et al. nested it with Erpetosuchus.

Here
Dyoplax nests as a basal metriorhynchid, those Jurassic sea crocodiles with flippers for fore limbs and a curved fish-like tail (Fig. 2), which was probably too early to be present. Unfortunately the tail tip and limb tips are missing from the fossil (Fig. 1)

The fossil appears to have no arch of bones separating the upper and lateral temporal fenestrae, but the intervening squamosal appears to be flipped and displaced near the neck ribs. (Black arrow in fig. 1)

Figure 2. Several Jurassic sea crocs, apparently derived from Late Triassic Dyoplax.

Figure 2. Several Jurassic sea crocs, apparently derived from Late Triassic Dyoplax.

Boy, it’s been awhile
since I posted anything. Feels good to be back… but for how long?

References
Fraas O 1867. Dyoplax arenaceus, ein neuer Stuttgarter Keuper-Saurier. Jh. Verein vaterländ. Naturk. Württemberg 23:108-112; Stuttgart.
Lucas SG, Wild R, Hunt AP 1998. Dyoplax O. Fraas, a Triassic sphenosuchian from Germany. Stuttgarter Beiträge zur Naturkunde, B. 263: 1–13.
Maisch MW;  Matzke AT; Rathgeber T 2013. Re-evaluation of the enigmatic archosaur Dyoplax arenaceus O. Fraas, 1867 from the Schilfsandstein (Stuttgart Formation, lower Carnian, Upper Triassic) of Stuttgart, Germany. Neues Jahrbuch für Geologie und Paläontologie – Abhandlungen. 267 (3): 353–362.

 

What is the enigmatic Otter Sandstone (Middle Triassic) diapsid?

Coram, Radley and Benton 2017
presented a “small diapsid reptile [BRSUG 29950-12], possibly, pending systematic study, a basal lepidosaur or a protorosaurian.” According to Coram et al. “The Middle Triassic (Anisian) Otter Sandstone was laid down mostly by braided rivers in a desert environment.”

Figure 1. The Middle Triassic Otter Sandstone diapsid BRSUG 29950-12 under DGS nested with basalmost lepidosaurs like Megachirella.

Figure 1. The Middle Triassic Otter Sandstone diapsid BRSUG 29950-12 under DGS nested with basalmost lepidosaurs like Megachirella. Skeleton is exposed in ventral (palatal) view.

The LRT is here to nest and identify published enigmas
The large reptile tree (LRT 1041 taxa) nests BRSUG 29950-12 with the basalmost lepidosaur Megachirella. They are a close match and preserve nearly identical portions of their skeletons (Fig. 2). Megachirella was originally considered a sister to Marmoretta, another basal sphenodontian from the much later Middle/Late Jurassic.

FIgure 2. Megachirella (Renesto and Posenato 2003) is a sister to the BSRUG diapsid.

FIgure 2. Megachirella (Renesto and Posenato 2003), also from Middle Triassic desposits, is a sister to the BSRUG diapsid and provides a good guide for its eventual reconstruction.

At the base of the Lepidosauria
in the LRT nests Megachirella, derived from a sister to Sophineta (Early Triassic) and Saurosternon + Palaegama (Latest Permian) and kin. Sisters to Megachirella within the Lepidosauria include the tritosaurs Tijubina + Huehuecuetzpalli (Early Cretaceous), Macrocnemus (Middle Triassic) and the prosquamate Lacertulus (Late Permian). Also similar and related to Palaegama is Jesairosaurus (Middle Triassic). So the genesis of the Lepidosauria is Late Permian. The initial radiation produced taxa that continued into the Early Cretaceous. The radiation of derived taxa continued with three major clades, only one of which, the Tritosauria, is now completely extinct.

Note
It is important to remember that lepdiosaurs and protorosaurs are not closely related, but arrived at similar bauplans by convergence, according to the LRT. The former is a member of the new Lepidosauromorpha. The latter is a member of the new Archosauromorpha. Last common ancestor: Gephyrostegus and kin.

Nesting at the base of the Lepidosauria
in the Sphenodontia clade makes the BSRUG specimen an important taxon. Let’s see if and when this taxon is nested by academic workers that they include all of the pertinent taxa and confirm or re-discover the Tritosauria. The LRT provides a good list of nearly all of the pertinent taxa that should be included in that future study, many of which are listed above. Based on that list, the BSRUG specimen is a late-survivor of a perhaps Middle Permian radiation of basal lepidosaurs.

References
Coram RA, Radley JD and Benton MJ 2017. The Middle Triassic (Anisian) Otter Sandstone biota (Devon, UK): review, recent discoveries and ways ahead. Proceedings of the Geologists’ Association in press. http://dx.doi.org/10.1016/j.pgeola.2017.06.007

Arcticodactylus a tiny Greenland Triassic pterosaur

Arcticodactylus cromptonellus (Kellner 2015, originally Eudimorphodon cromptonellus Jenkins et al. 1999, 1999; MGUH VP 3393) Late Triassic ~210mya ~8 cm snout to vent length was a tiny pterosaur derived from a sister to Eudimorphodon ranzii and phylogenetically preceded Campylognathoides and BSp 1994 specimen attributed to Eudimorphodon. Whether it was a juvenile or a tiny adult cannot be determined because juveniles and even embryos are identical to adults in pterosaurs. Note that that rostrum was not shorter and the orbit was not larger than in sister taxa. This specimen is one of the smallest known pterosaurs., but not THE smallest (Fig. 1) contra the Wikipedia article. That honor goes to B St 1967 I 276.

Figure 1. Articodactylus is evidently NOT the smallest pterosaur. That honor still goes to an unnamed specimen (not a Pterodactylus kochi juvenile) B St 1967 I 276.

Figure 1. Articodactylus is evidently NOT the smallest pterosaur. That honor still goes to an unnamed specimen (not a Pterodactylus kochi juvenile) B St 1967 I 276.

Distinct from E. ranzii,
the skull of Arctiodactylus had a rounder, less triangular orbit. The jugal was not as deep. The sternal complex did not have small lateral processes. The humerus was not as robust. The fingers were longer an more gracile. The prepubis was distinctly shaped.

Distinct from
Bergamodactylus the femur and tibia were smaller but the metatarsals were longer, compact and nearly subequal in length with IV smaller than III.

References
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A primitive pterosaur of Late Triassic age from Greenland. Journal of the Society of Vertebrate Paleontology 19(3): 56A.
Jenkins FA Jr, Shubin NH, Gatesy SM and Padian K 1999. A diminutive pterosaur (Pterosauria: Eudimorphodontidae) from the Greenlandic Triassic. Bulletin of the Museum of Comparative Zoology, Harvard University 155(9): 487-506.
Kellner AWA 2015. Comments on Triassic pterosaurs with discussion about ontogeny and description of new taxa. Anais da Academia Brasileira de Ciências 87(2): 669–689

wiki/Eudimorphodon
wiki/Arcticodactylus