After the dinos, tree shrews came down and got big!

During the reign of the dinosaurs
tree shrews, like Ptilocercus (Fig. 1) and Tupaia (Fig. 1), stayed in the trees, evolving into tree-dwelling members of the Carnivora (Genetta, Fig. 1), Volitantia (bats, pangolins and dermopterans), Glires (including multituberculates led by Tupaia) and Primates (Microcebus, Fig. 1) in the large reptile tree (LRT, 1818+ taxa) distinct from all gene studies and all other prior trait studies (due to taxon exclusion). The LRT is the first study that found tree-dwelling Caluromys (Fig. 1), an extant tree shrew-like marsupial, as the proximal outgroup to the Placentalia. Based on chronological bracketing, Caluromys relatives lived in the Early Jurassic.

Figure 1. Mammals at the base of the Placentalia include the outgroup taxon: Caluromys, a basal placental: Genetta, a basal Carnivora: Eupleres, a basal Volitantia: Ptilocercus, a basal Primates: Microcebus, and basal Glires: Tupaia.

Figure 1. Mammals at the base of the Placentalia include the outgroup taxon: Caluromys, a basal placental: Genetta, a basal Carnivora: Eupleres, a basal Volitantia: Ptilocercus, a basal Primates: Microcebus, and basal Glires: Tupaia.

After the Cretaceous some tree shrews became terrestrial.
Leptictids, elephant shrews (Rhynchocyon, Fig. 2 and tenrecs (Tenrec) were phylogenetically among the first of the former tree shrews to become fully terrestrial. They were all small. After the Cretaceous some terrestrial tree shrew descendants began to increase in size. Some became elephants, others horses, still others baleen whales, all following Cope’s Rule.

Figure 7. Rhynchocyon, a living elephant shrew, is a living leptictid.

Figure 2. Rhynchocyon, a living elephant shrew, is a living leptictid and a former tree shrew.

Once established on the ground
and spreading beyond the jungles, the following Early Paleocene terrestrial placentals became cat to tiger size: Onychodectes (Fig. 3), Alcidedorbignya (Fig. 3) and Pantolambda (Fig. 3).

Figure 3. Onychodectes, Alcidedorbignya and Pantolambda are former tree shrews now terrestrial of increasing size in the Early Paleocene.

Figure 3. Onychodectes, Alcidedorbignya and Pantolambda are former tree shrews now terrestrial of increasing size in the Early Paleocene. Note the lost of sharp claws replaced by pre-hooves.

By the late Paleocene
taxa like massive Barylambda showed further increases in size. This taxon was basal to giant glyptodonts and ground sloths, some of which ultimately became smaller and returned to the trees as tree sloths.

Figure 1. Barylambda looks like a large ground sloth for good reason. It is a sister to the direct ancestor and nests at the base of the Xenarthra along with Orycteropus, the aardvark.

Figure 4. Late Paleocene Barylambda looks like a large ground sloth for good reason. It is a sister to the direct ancestor and nests at the base of the Xenarthra along with Orycteropus, the aardvark.

PS… saving the best for last.
Writing this blogpost inevitably brought my gaze back to Fruitafossor (Luo and Wible 2005), a small, Late Jurassic digging mammal with four robust fingers, xenarthran lumbars and single cusp, tubular teeth. When first encountered and based on these traits the LRT mistakenly nested Fruitafossor with edentates for the last four years. That Late Jurassic temporal discontinuity in an otherwise Tertiary clade of edentates required a review and revision of taxon scores for Fuitafossor. That review ultimately re-nested Fruitafossor more plausibly and parimoniously basal to echidnas in the LRT. Fruitafossor is a basal echidna from Colorado. That story comes to you tomorrow.


References
Luo Z-X and Wible JR 2005. A late Jurassic digging mammal and early mammal diversification. Science 308:103–107.

 

 

Cope’s Rule vs. Phylogenetic Miniaturization

Getting bigger
Wikipedia reports, Cope’s rule, named after American paleontologist Edward Drinker Cope, postulates that population lineages tend to increase in body size over evolutionary time. It was never actually stated by Cope, although he favoured the occurrence of linear evolutionary trends.”

Getting smaller
On the other hand, there is no Wikipedia category for Phylogenetic Miniaturization, which postulates that population lineages tend to decrease in body size over evolutionary time.

Both have their time and place.
For instance, if you take a look at the large reptile tree (LRT, 1026 taxa) in the mammal subset you’ll find the following taxon pairs representing Cope’s Rule:

  1. Toxodon is a giant Dromiciops
  2. Panthera (lion) is a giant Procyon (raccoon)
  3. Homo (human) is a giant Ptilocercus (tree shrew)
  4. Physeter (sperm whale) is a giant Tenrec (tenrec)
  5. Balaenoptera (blue whale) is a giant Ocepeia
  6. Giraffa (giraffe) is a giant Cainotherium
  7. Elephas (elephant) is a giant Procavia (hyrax)
  8. Ceratotherium (rhino) is a giant Hyracodon
  9. Paraceratherium is a giant Mesohippus.
  10. And in pterosaurs…Pteranodon is a giant Germanodactylus
  11. Quetzalcoatlus is a giant TM10341

Note:
these are general trends, not always direct lineages. We’ll never find the exact ancestors of living or fossil taxa, though we can get very close! Employed taxa represent evolutionary stages, sets of derived characters mixed with some small or large number of autapomorphic traits not shared by the unknown common ancestor of the small and large taxon pairs.

Likewise
you’ll also find in the LRT the following taxon pairs representing examples of phylogenetic miniaturization, some from the large pterosaur tree:

  1. Gephyrostegus is a tiny Proterogyrinus
  2. Terrapene (box turtle) is a tiny Elginia
  3. Cosesaurus is a tiny Macrocnemus
  4. Hypuronector is a tiny Jesairosaurus
  5. Bellubrunnus is a tiny Campylognathoides
  6. TM 13104 is a tiny Scaphognathus
  7. Tetrapodophis is a tiny Adriosaurus
  8. Hadrocodium is a tiny Haldanodon.
  9. Elaschistosuchus is a tiny Proterosuchus
  10. Gracilisuchus is a tiny Vjushkovia.
  11. Archaeopteryx is a tiny Sinornithoides.

What goes down (gets smaller), usually goes up (gets bigger)
And sometimes what gets bigger gets smaller. Case in point: the Pygmy or Channel Islands mammoth.

Anything can happen at any time in evolution
given enough time. As noted earlier, phylogenetic miniaturization was present at the origin of several major clades of tetrapods and in clades of pterosaurs in particular. And this appears to occur during times of survival stress for several reasons. On the other hand, apparently it takes an epoch filled with plenty of food and other resources to produce giant animals. As you know, various parts of the Earth have created stress and bounty throughout its long prehistory.