Further evidence for digitigrady in the multituberculate Kryptobaatar

Earlier we looked at
PILs, parallel interphalangeal lines (Peters 2000, 2010), which appear in a wide variety of tetrapod extremities, typically, but not always, aligning phalanges in parallel sets. Note the PILs of the plantigrade pes of Kryptobaatar (from Kielan-Jaworowska and Gambaryan 1994; Fig. 1 left) become better aligned when the pes is elevated to the digitigrade configuration (Fig. 1 right), as illustrated by Kielan-Jaworowska and Gambaryan in their 1994 paper (Fig. 1 middle).

note the PILs of Kryptobaatar become better aligned when the pes is elevated to the digitigrade configuration.

note the PILs of Kryptobaatar become better aligned when the pes is elevated to the digitigrade configuration. Note the angled calcaneum and reduced astragalus, two derived traits.

Living taxa that are both digitigrade and arboreal
include the big cats. The claws are not so sharp and not so curved, but they are long and their keratin sheath may have been much sharper and more curved.

Note the very short astragalus
and the calcaneum positioned at an odd lateral angle. These unique traits suggested to early researchers that Kryptobataar could have rotated its ankle to a great degree, ideal for establishing a good grip from any angle, even inverted, on a tree limb.

References
Kielan-Jaworowska Z and Gambaryan PP 1994. Postcranial anatomy and habits of Asian multituberbulate mammals. Fossils & Strata 36:1-92.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500
Wible JR Rougier GW 2000. Cranial anatomy of Kryptobaatar dashzevegi (Mammalia, Multituberculata), and its bearing on the evolution of mammalian characters. Bulletin of the American Museum of Natural History 247:1–120. doi:10.1206/0003-0090(2000)247<0001:CAOKDM>2.0.CO;2.

wiki/Kryptobaatar

PILs (Parallel Interphalangeal Lines) and Paddles

Paddle PILs
Peters (2000, 2010, 2011) described PILs (Parallel Interphalangeal Lines) that can be drawn through any tetrapod manus or pes. Primitively three sets are present, medial, transverse and lateral. The lines indicate phalanges that act in sets while grasping (flexion) or during locomotion (extension). As digits are reduced, as in theropod or horse feet, the PILs tend to merge.

Figure 1. On left: Tylosaurus pelvis with an anteriorly-leaning ilium. On right: Tylosaurus forelimb paddle. Note the PILs are not continuous but  stop at digit 2, the main spar of this aquatic "wing".

Figure 1. On left: Tylosaurus pelvis with an anteriorly-leaning ilium. Note the acetabulum is not facing the reader. This is the medial view of the pelvis. In the middle, the two sacral vertebrae of Tylosaurus. On right: Tylosaurus forelimb paddle. Note the PILs are not continuous but stop at digit 2, the main spar of this aquatic “wing”.

Tetrapods with flippers or paddles present a special case,
but even then, PILs are present. Recently I took a look at the manus of Tylosaurus and noticed that the PILs were not continuous from side to side, as they are typically (but not universally) in terrestrial tetrapods. With Tylosaurus the transverse set was not apparent. The medial set extended to digit 2. So did the lateral set. Digit 2 in the wing-like paddle of Tylosaurus is analogous to the main wing spar of an airplane wing. And that spar is not supposed to bend. Apparently in this case, the absence of transverse PILs that would have allowed flexion and extension showed that the flipper was most efficient when it did not flex and extend much.

Pelvis
In most tetrapods the ilium extends posteriorly. In many the ilium also extends anteriorly, creating a long lateral plate for the attachment of many large muscles. In aquatic forms the ilium is generally reduced. As you might expect, in some taxa that also reduces the number of sacral vertebrae. In others, oddly, the number of sacrals can double to four. In many aquatic taxa, and a few arboreal forms, the ilium has no posterior process, but extends dorsally. Rarely, as in Tylosaurus (Fig. 1) the ilium tilts anteriorly. Only the presence of the laterally-facing acetabulum assures you that this orientation is correct. I’m not sure why this is so. That ilium angle is 90º from the scapula angle in a bird, bat or pterosaur, animals that fly through the air and employ the scapula to anchor muscles that raise the wing (the details differ between all three flyers, btw, with birds employing a pulley-like bone to bend the action of a pectoral muscle to aid in wing elevation). Tylosaurus may have had the same problem to overcome, paddle elevation, but used a tall narrow anchor, rather than a low, long anchor to do the job.

Lingham-Soliar (1992) described subaqueous flying in a mosasaur, but concentrated on the pectoral area and forelimb, ignoring the pelvis and hind limb.

References
Lingham-Soliar T 1992. A new mode of locomotion in mosasaurs: subaquaeous flying in Plioplatecarpus marshii. Journal of Vertebrate Paleontology 12:405-421. 
Peters D 2000. Description and interpretation of interphalangeal iines in tetrapods
Ichnos, 7:11-41.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605

Synapsid manus and pedes study

A recent online paper by Kümmel and Frey (2014) describes the mobility of the ‘thumb’ and medial toe in non-mammalian Synapsida and one extant mammal.

Figure 1. Manus of Galesaurus, an arboreal dromasaur, anomodont, synapsid.

Figure 1. Manus of Galesaurus, a basal cynodont synapsid. PILs added. This is where grasping first emerged, later dropped by many later mammal clades, but retained by primates and other arboreal forms.

From their abstract
The the reduction of autopodial rotation can be estimated, e.g., from the decrease of lateral rotation and medial abduction of the first phalanx in the metapodiophalangeal joint I. Autopodial rotation was high in Titanophoneus and reduced in derived Cynodontia. In Mammaliamorpha the mobility of the first ray suggests autopodial rolling in an approximately anterior direction. Most non-mammaliamorph Therapsida and probably some Mesozoic Mammaliamorpha had prehensile autopodia with an opposable ray I. In forms with a pronounced relief of the respective joints, ray I could be opposed to 90° against ray III. A strong transverse arch in the row of distalia supported the opposition movement of ray I and resulted in a convergence of the claws of digits II–V just by flexing those digits. A tight articular coherence in the digital joints of digits II–V during strong flexion supported a firm grip capacity.

Figure 2. Pes of Titanophoneus-like synapsid from Kümmel and Frey. PILs added. Approximately middle of the propulsion phase (A), followed by plantar flexion of metatarsalia II–V and distale I (B). C shows the start of the raising phase of the metapodialia and D the start of the raising phase of the digits

Figure 2. Pes of Titanophoneus-like synapsid from Kümmel and Frey. PILs added. Approximately middle of the propulsion phase (A), followed by plantar flexion of metatarsalia II–V and distale I (B). C shows the start of the raising phase of the metapodialia and D the start of the raising phase of the digits

Not mentioned, or referenced, but clearly visible
is the presence of PILs (parallel interphalangeal lines) that enable the phalanges to work in sets.

If you ever wondered where your grasping hand first appeared, it is here (Figure 1) in cynodonts.  No matter that grasping later disappeared in many later mammal clades, it was retained by arboreal and carnivorous clades.

The authors discuss the alignment of the phalanges without discussing the 14-year-old paper on PILs (Peters 2000), which might have been appropriate in this study. So, it’s brought up here.

The arching of the metacarpals an metapodials is also shown here. A similar arching was shown to exist in the pes of Pteranodon (Peters 2000). That arching in the human metacarpals produces a fist clearly aligns the knuckles for branch grabbing. Otherwise, when flattened and useful only for clapping and slapping, the knuckles are not clearly aligned.

References
Kümmell SB and Frey 2014. Range of Movement in Ray I of Manus and Pes and the Prehensility of the Autopodia in the Early Permian to Late Cretaceous Non-Anomodont Synapsida. PLoS ONE 9(12): e113911.doi:10.1371/journal.pone.0113911 http://www.plosone.org/article

Peters D 2000. Description and interpretation of interphalangeal iines in tetrapods
Ichnos, 7:11-41.

 

The foot of Archicebus – an early primate

We haven’t looked at mammals or synapsids for awhile.
If you want to check the large reptile tree, they’re still reptiles — just hairier. Todays let’s look at Archicebus (Early Eocene, 55 mya, Ni et al. 2013),  the oldest primate known from a skeleton. Notharctus is 5 million years younger, but more primitive, just as living lemurs are more primitive than modern apes and humans are. Ipso facto, the discovery of Archicebus pushes the origin of lemurs back even further. The younger lemuroid, Smilodectes, had a similar short-snout skull.

Figure 1. Archicebus is close the ancestry of tarsiers and monkeys. It retained a lemur-like foot,

Figure 1. Archicebus is close the ancestry of tarsiers and monkeys. It retained a lemur-like foot, but with distinctly tarsier-like proportions starting to show. Here in a published illustration pedal digit 1 is not illustrated as more robust and the metatarsals are inaccurately similar in length. The size was about that of the smallest living lemur, the pygmy mouse lemur. Moderate size eye sockets are distinct from the giant eye sockets of living nocturnal tarsiers.

My interest in feet
and PILs (parallel interphalangeal lines) goes back a long way (Peters 2000a, 2010, 2011). Pterosaur PILs are instructive, helping flatten or elevate plantigrade and digitigrade pedes. Cats put an interesting twist on PILs due to their retractable claws. Primates do too, because they are adapted to cylindrical substrates (branches), not flat (the ground).

Figure 2. Click to enlarge. The two pedes of Archicebus, bottom sides flipped to match top sides. Toes colorized for reconstruction (Fig. 3).

Figure 2. Click to enlarge. The two pedes of Archicebus, bottom sides flipped to match top sides. Toes colorized for reconstruction (Fig. 3). The talus/astragalus is missing from both pedes.  Note the massive proximal articulation of the big toe.

Nature reports, “By analyzing almost 1,200 morphological aspects of the fossil and comparing them to those of 156 other extant and extinct mammals, the team put the ancient primate near the base of the tarsier family tree.” I haven’t repeated that experiment, buy it looks to me that pedal characters alone would tell the tale Figs. 3,4).

Figure 3. The reconstructed foot of Archicebus alongside that of the basal lemur, Notharctus. Note the gathering of metatarsals 2-4, as in tarsiers.

Figure 3. The reconstructed foot of Archicebus alongside that of the basal lemur, Notharctus. Note the gathering of metatarsals 2-4, as in tarsiers. The missing astragalus/talus sits on top of the calcaneum, a trait first appearing on cynodonts like Probelesodon. I suppose this is an example of modular evolution: first the toes, then the ankle.

Nature reports, “The mammal sports an odd blend of features, with its skull, teeth and limb bones having proportions resembling those of tarsiers, but its heel and foot bones more like anthropoids.” Actually the Archicebus foot is also an “odd blend.” The ankle is short, like that of most other primates (Fig. 3, not just anthropoids). But digit 2 is short and digit 4 is long, like those of tarsiers (Fig. 4). Really it comes down to just these two traits for an accurate nesting of Archicebus. Perhaps an accurate reconstruction would have helped. I took my data (Fig. 1) from online photos of Ni et al. 2014, but I have not seen the paper.

Figure 4. Archicebus pes compared to a living tarsier  pes. Note the elongated proximal tarsals in the tarsier. Archicebus has the elongate digit 3 retained by tarsiers.

Figure 4. Archicebus pes compared to a living tarsier pes. Note the elongated proximal tarsals and shorter metatarsals in the tarsier. Archicebus has the elongate digit 4 and short digit 2 retained by tarsiers. Both of these reconstructions are flattened, which is not the way tarsiers hold their toes (Fig. 5). The lengthening of the ankle makes tarsiers excellent leapers.

PIL continuity
The foot of Archicebus appears to lose the continuity of many PILs (Fig. 4) when laid flat. But that’s not the way tarsiers hold their toes in vivo (Fig. 5). Similarly in the human hand the PILs become more continuous in use, like when you grasp a golf club, hold a baseball bat or make a fist. And, of course, the opposable thumb does not work as part of the lateral four digit sets. It goes its own way.

The elongation of the proximal ankle elements in tarsiers enables them to leap tremendous distances. Archicebus did not have that ability. I suppose Archicebus is an example of modular evolution: first the toes, then the ankle, but, of course, it’s never as simple as that.

Figure 6. The tarsier foot and PILs are shown in action at right angles to the tree cylinder and parallel to the long axis. The use of pads appears to change the way the foot operates, without the strong PILs a grasping or walking foot has.

Figure 5. The tarsier foot and PILs are shown in action at right angles to the tree cylinder and parallel to the long axis. The use of pads appears to change the way the foot operates, without the strong PILs a grasping or walking foot has. The fingers and toes don’t lie flat, but strongly flex at the interphalangeal joints. This messes with PILs that are applied to flat reconstructions (Fig. 3, 4)

Archicebus is well-deserving of its celebrity.
According to Nature, “Because A. achilles sits near the base of the tarsier family tree, scientists say it probably resembles the yet-to-be-discovered creatures that lie at the base of most primate groups — including the anthropoid lineage that ultimately gave rise to humans. “If you retrace primate evolution to its beginning, [A. achilles] is what our ancestors most likely looked like,” says Luo.”

References
Ni X, Gebo DL, Dagosto M, Meng J, Tafforeau P, Flynn JJ, Beard KC 2013. The oldest known primate skeleton and early haplorhine evolution. Nature 498 (7452):60–64.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605

wiki/Archicebus

Let’s add PILs to the Poposaurus foot

and see what happens…

The question posed by Farlow et al (2014) is were the toes of Poposaurus (Figs. 1-3) splayed or nearly parallel? Farlow (Fig. 1) showed both possibilities in a digitigrade fashion. Here (Fig. 1) I added PILs (parallel interphalangeal lines, (Peters 2000, 2011) to see which possibility produced the simplest set of PILs.

Figure 1. From Farlow et al. 2014) showing the Poposaurus foot in plantigrade and digitigrade poses. In the ghosted addition I added a digitigrade configuration, but so high as in the Farlow examples. In any case, digit 1 impresses, but shares no PILs, so it acts as a vestige, no longer part of the phalangeal sets.

Figure 1. From Farlow et al. 2014) showing the Poposaurus foot in plantigrade and digitigrade poses. In the ghosted addition I added a digitigrade configuration, but so high as in the Farlow examples. In any case, digit 1 impresses, but shares no PILs, so it acts as a vestige, no longer part of the phalangeal sets. The metatarsals in ventral view are also ghosted to better show the bones that would have contributed to making a footprint. Note: the medial and lateral PILs are complete, but the transverse set is not, but becomes more so with the spreading toes.

Farlow et al. created their splayed foot by spreading the digits as far as they could go on the distal metatarsals. Another way to do this would be to rotate the medial and lateral metatarsals, creating a metatarsal arc, but this was not attempted by Farlow et al. Even a slight axial rotation of these metatarsals would have splayed the digits just a little bit more.

And that’s really all you need.

Here (Fig. 2) we look at an even more splayed foot and now we have complete PILs even in the transverse set, which is the one Poposaurus would have used for locomotion, as in birds and theropods.

Figure 2. When you splay the digits of Poposaurus just a little bit more, the transverse PILs become complete and uninterrupted. This, then, is the most likely configuration of the pes.

Figure 2. When you splay the digits of Poposaurus just a little bit more, the transverse PILs become complete and uninterrupted. This, then, is the most likely configuration of the pes. PILs work!

Now all the PIL sets (except, again, digit 1, which just had to get out of the way) are able to operate at maximum efficiency. They are complete and uninterrupted, as in all other tetrapods.

BTW, Poposaurus is basal to Silesaurus in the large reptile tree, and Silesaurus does not preserve digit 1.

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Three days ago we took our first look at the Farlow et al. 2014 paper.

References
Farlow JO, Schachner ER, Sarrazin JC, Klein H and Currie PJ 2014. Pedal Proportions of Poposaurus gracilis: Convergence and Divergence in the Feet of Archosaurs. The Anatomical Record. DOI 10.1002/ar.22863
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605