The origin and evolution of the Dicynodontia

Figure 1. The origin of the Dicynodontia from basal therapsids. Here Cutleria, Stenocybus, two unnamed taxa, Otsheria, Venjukovia, Eodicynodo and Dicynodon are shown in order along with the major trait that each portrays.

Figure 1. The origin of the Dicynodontia from basal therapsids. Here Cutleria, Stenocybus, two unnamed taxa, Otsheria, Venjukovia, Eodicynodo and Dicynodon are shown in order along with the major trait that each portrays. Click to enlarge. Most basal synapsids are known from skulls. During the evolution of the dicynodonts, the tail became very short and the toes were shorter as well.

Earlier we looked at the origin of dicynodonts and dromasaurs separate from all other synapsids. Here (Fig. 1) are a selection of skulls that demonstrate the evolutionary origin of the very odd skull of Dicynodon, only one of many dicynodonts.

Wikipedia reports
“Dicynodontia is a taxon of anomodont therapsids or synapsids with beginnings in the mid-Permian, which were dominant in the Late Permian and continued throughout the Triassic, with a few possibly surviving into the Early Cretaceous. Dicynodonts were small to large herbivorous animals with two tusks, hence their name, which means ‘two dog tooth’. They are also the most successful and diverse of the non-mammalian therapsids, with over 70 genera known, varying from rat- to ox-sized.”

Wikipedia also reports
that dicynodonts nested between dinocephalians and gorgonopsians. That is not supported in the large reptile tree as both are quite derived and don’t take into account basal pre-dicynodont taxa.

You can see a number of therapsid skulls in evolutionary order here.

 

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Restoring the manus of Lewisuchus

Earlier we looked at several hands (the manus) of basal bipedal crocs, some of which nest basal to dinosaurs. Today we’ll restore the manus of little Lewisuchus (Romer 1972, Fig. 1; note: Bittencourt et al. 2014 considered the digits proterochampsid pedal elements without restoring the pes according to their hypothesis). The picture tells the story.

Figure 1. Restoration of the manus of Lewisuchus based on patterns found in sister taxa.

Figure 1. Restoration of the manus of Lewisuchus based on patterns found in sister taxa. Click to enlarge.

Now, just to be clear,
if it’s difficult to determine whether these are pedal or manual elements, it’s also difficult to determine which metatarsal is which. Phylogenetic bracketing helps, but there is as much art here as science. Note that much of this data is based on line drawings. So, what I’m presenting here is a hypothesis. Not fact. Still, it’s good to see all this data together in one place.

It’s notable that
in Lewisuchus metacarpal 4 is half as long as metacarpals 2 and 3, and metacarpal 5 is a vestige. That leaves only metacarpal 1 absent. In sister taxa the fingers are often missing. And that pattern is what makes me consider this a manus.

Phylogenetically
note the apparent reduction of the metacarpals from Lewisuchus to Pseudheseperosuchus, Trialestes and Herrerasaurus. It’s also interesting to note that Lewisuchus had relatively longer fingers than did Junggarsuchus, but, let’s face it, the data is pretty crappy. Let’s all hope for some better future discoveries.

Still haven’t seen
the transitionally reduced proximal carpals between pseudhesperosuchids and basal dinos, but maybe we’ll find them close to PVL 4597, a taxon closer to dinosaurs, but only known from a pelvis and hind limb.

References
Bittencourt JS, Arcucci AB, Maricano CA and Langer MC 2014. Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chañares Formation, Argentina) its inclusivity, and relationships amongst early dinosauromorphs. Journal of Systematic Palaeontology. Published online: 31 Mar 201. DOI:10.1080/14772019.2013.878758
Nesbitt SJ. et al. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464(7285):95-8
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchusadmixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13

wiki/Lewisuchus

Have you ever seen an Anolis quite like this?

This is a striking variation on the Anolis lizard theme with an extended premaxilla/snout. Meet Anolis proboscis, aka the Pinocchio lizard. This trait is restricted to males and this species was thought to be extinct for several decades. Click here or on the image to learn more.

Figure 1. Long-nosed Anolis from the Luke Mahler lab.

Figure 1. Long-nosed Anolis from the Luke Mahler lab. Only males have this trait.  Click to learn more.

The image comes from the Luke Mahler website. Dr. Mahler is a former student of Jonathan Losos, now at Harvard, formerly of Washington University, here in St. Louis. Jonathan’s work with Anolis has shed new light on the process of evolution.

References
Poe et al. 2012.
Morphology, Phylogeny, and Behavior of Anolis proboscis. Breviora Number 530 :1-11. 2012 online here and here.

The origin of dinosaur hands

Dinosaurs have a variety of hands.
Everyone knows T-rex had but two fingers. Birds have fused fingers derived from fingers 1-3 that are not fused in the basal bird, Archaeopteryx. Basal sauropodomorphs had five fingers, but later giants reduced this to a U-shaped column of metacarpals.

Figure 1. the manus and carpus of several basal bipedal crocs and basal dinosaurs showing the similarities and differences. Click to enlarge.

Figure 1. the manus and carpus of several basal bipedal crocs and basal dinosaurs showing the similarities and differences. Click to enlarge.

The currently known basalmost dinosaur
Herrrerasaurus, had five metacarpals, but digit 4 was a vestige and digit 5 was absent on a vestigial metacarpal (Fig. 1).

Proximal outgroups to the Dinosauria
among basal bipedal crocs appear to have had four digits and a vestigial metacarpal 5 (Fig. 1), but this may be due to poor preservation and excavation more than in vivo reality.

Among other basal crocs,
digits 1-4 are sometimes known, sometimes partly known (distal phalanges tend to disappear first) and sometimes not known. In all basal crocs, metacarpals 1-3 appear to increase in length laterally and are aligned distally.

The carpus in bipedal basal crocs
includes an elongate ulnare and an even longer radiale (both are proximal wrist bones). No dinosaurs have elongate proximal wrist bones, a key difference. But proto-dinosaurs, like Junggarsuchus and Trialestes, did.

A transitional croc/dino taxon with slightly longer proximal carpals has not been identified.

Like dinosaurs,
most crocs reduce metacarpals 4 and 5, both in length and diameter. But there are certain exceptions (Fig. 1), including Terrestrisuchus and the SMNS 12352 specimen. Junggarsuchus may also have a reduced digit 5, as it appears from the photo, distinct from the Clark et al. 2004 interpretation (Fig. 1) which includes a large metacarpal 5 and lacks a metacarpal 1, distinct from all known sister taxa.

Compared to dinos
the basal bipedal crocs appear to have shorter distal phalanges and smaller hands, as if they were more often in contact with the ground and not used to snare prey.

By contrast, in basal dinos
like Herrerasaurus and Tawa the distal phalanges were trenchant claws and the penultimate phalanges were likewise elongated. Among dino and croc ancestors only tiny Lewisuchus has such long dangerous claws and fingers, perhaps developed by convergence.

The hands of more ancient rauisuchians
are not often found and what is known suggests the hands were small with short distal phalanges. The hands of Eoraptor have fingers longer than metacarpals and relatively longer lateral metacarpals, but the fingers are not so long as in therpods. In Massospondylus there’s a combination of short fingers and large trenchant claws, probably for grabbing tree trunks for feeding high in the boughs.

The difference in manual claw size between basal dinos and basal bipedal crocs may be one of the key differences that enabled dinosaurs to rise to greatness in the Mesozoic.

References
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Clark JM et al. 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20 (4): 683–704.
doi:10.1671/0272-4634(2000)020[0683:ANSOHA]2.0.CO;2.
Clark JM, Xu X, Forster CA and Wang Y 2004. A Middle Jurassic ‘sphenosuchian’ from China and the origin of the crocodilian skull. Nature 430:1021-1024.
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.
Sereno PC, Forster CA, Rogers RR and Moneta AM 1993. Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
Sereno PC, Martínez RN and Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12: 83-179 DOI:10.1080/02724634.2013.820113

wiki/Eoraptor
wiki/Herrerasaurus
wiki/Sanjuansaurus
wiki/Pseudhesperosuchus

Herrerasaurus coracoid: maybe it is not your typical dinosaurian coracoid

Figure 1. Herrerasaurus and Sanjuansaurus scapulocoracoids. The coracoid was largely restored by Sereno 1993, but maybe its not such a disc (in green). It could be more of a transitional strust/disc (in blue), like Pseudhesperosuchus.

Figure 1. Herrerasaurus and Sanjuansaurus scapulocoracoids. The coracoid was largely restored by Sereno 1993, but maybe its not such a disc (in green). It could be more of a transitional strust/disc (in blue), like Pseudhesperosuchus (Fig. 2). In like fashion, Alcober and Martinez 2010 added plaster to their Sanjuansaurus coracoids, both of them. Take away most of the plaster and things become more interesting. Click to enlarge.

As all paleontologists know, crocs have a strut-like coracoid and dinos have a disc-like coracoid (untill it is modified back into a strut in flapping pre-birds). Herrerasaurus and Sanjuansaurus (Fig. 1) are very basal dinosaurs. In 1993 Sereno illustrated what was known of the Herrerasaurus scapulocoracoid. The incomplete coracoid was restored in typical dinosaur fashion as a disc, but maybe it wasn’t quite there yet. Alcober and Martinez 2010 did the same with their Sanjuansaurus, to both of their coracoids.

Maybe all that plaster wasn’t necessary.

Figure 2. Pseudhesperosuchus in various views. Note the scapulocoracoid. This taxon is a proximal outgroup to the Dinosauria and Herrerasaurus (Fig. 1).

Figure 2. Pseudhesperosuchus in various views. Note the scapulocoracoid. This taxon is a proximal outgroup to the Dinosauria and Herrerasaurus (Fig. 1).

One of the proximal outgroups
to dinosaurs includes the clade of Pseudhesperosuchus (Fig. 2) and Junggarsuchus. Both have strut-like coracoids, typical of crocs. So does Trialestes, a taxon even closer to dinos. I haven’t seen that material yet, just going by the Clark and Berman 2000 description.

Put the scapulocoracoid of Pseudhesperosuchus next to those of Herrerasaurus and Sanjuansaurus and you’ll see something on the origin of dinosaurs there. It’s heresy, and I realize that. But it’s worth testing.

Food for thought
Yes, crocs and dinos are distinct from each other (just look at those proximal carpals!), but among the 475 tested taxa, no clade nest closer to dinosaurs than crocs (together, of course, creating the clade Archosauria). Lagerpeton and pterosaurs are not even in the running.

More on the evolution of dinosaurs from basal bipedal crocs coming soon.

References
Clark JM, Sues H-D and Berman DS 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20(4):683-704.
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.

wiki/Herrerasaurus
wiki/Sanjuansaurus
wiki/Trialestes

Nundasuchus: a new archosaur (Nesbitt et al. 2014)

Updated May 25, 2015, with a new image of Nundasuchus featuring more elements .

A new paper
by Nesbitt et al. 2014 describes a Middle Triassic archosaur, Nundasuchus, they nested close to Ticinosuchus using Nesbitt (2011). Nundasuchus is also basal to all “pseudosuchia” (phytosaurs, aetosaurs, crocs, rauisiuchians, etc., but not Euparkeria or Proterochampsidae using Brusatte et al. (2010).

Unfortunately,
both studies have serious flaws and red flags we looked at earlier here and here.

Figure 1. from Nesbitt et al. 2014. Plus foot reconstructed here and closeups of the mandible and tooth.

Figure 1. from Nesbitt et al. 2014. Plus foot reconstructed here and closeups of the mandible and tooth.

In the large reptile tree Nundasuchus also nests with Ticinosuchus, but not basal to all pseudosuchians.  Nundasuchus is also a sister to the clade Arizonasaurus + Qianosuchus + Yarasuchus and to Decuriasuchus.

Although represented by only a few bits and pieces (Fig. 1), enough is known to firmly nest Nandusuchus. 

References
Nesbitt SJ, Sidor  CA, Angielczyk KD, Smith RMH and Tsuji LA 2014. A new archosaur from the Manda beds (Anisian, Middle Triassic) of southern Tanzania and its implications for character state optimizations at Archosauria and Pseudosuchia, Journal of Vertebrate Paleontology, 34:6, 1357-1382.

Eoraptor, Panphagia and Pampadromaeus: how closely are they related?

Lumping and splitting
is something paleontologists do with the various specimens they find as they assign them names and nodes in the family tree of life. Jack Horner recently made news for lumping several pachycephalosaur genera together as distinct ontogenetic growth stages of the same genus and species. He did the same with Triceratops, which changed its appearance rather drastically while reaching maturity.

Today
let’s look at three closely related specimens, Panphagia protos Martinez and Alcobar (2009) , Eoraptor lunensis (Sereno et al. 1993, 2014) and Pampadromaeus barberenai (Cabriera et al. 2011, Fig. 1). These three nest between basalmost theropods and basalmost phytodinosaurs (sauropodomorphs + ornithischians) in the large reptile tree. Others consider them basal or stem sauropodomorphs, but only because basal ornithischians are not included in their analyses.

Figure 1. Eoraptor, Pampadromaeus and Panphagia. Three coeval South American dinosaurs between Theropoda and Phytodinosauria. Are they congeneric?

Figure 1. Eoraptor, Pampadromaeus and Panphagia. Three coeval South American dinosaurs between Theropoda and Phytodinosauria. Are they congeneric? Or are they distinct enough to be considered separate genera? To my eye, they appear to be morphs of a single genus.

The number of bones preserved
with each specimen varies, so all the bones cannot be compared with one another. What is preserved, however, appears to be more closely matched than many other specimens sharing the same generic name, like Pteranodon and Rhamphorhynchus. (Evidently dino-workers are splitters and ptero-workers are lumpers as, until recently, they preferred not to provide new names for distinct specimens, some of which were improperly considered juveniles of distinctly different larger specimens).

These three proto-phytodinosaurs (Fig. 1) are obviously similar.  Sereno, et al. (2014) often refers to “the closely related Pampadromaeus and Panphagia” when writing about Eoraptor.

What are the differences? 

Using the 228 characters of the large reptile tree is not enough to split and lump these three specimens. Only these three traits split them and cause loss of resolution.

  1. The anterior nasal is wider in Panphagia.
  2. The mandible tip does not descend in Pampadromaeus.
  3. The tibia is shorter than 2x the ilium length in Pampadromaeus

More resolution might come from adding taxa and more complete taxa, unless these three are indeed congeneric. Almost certainly there are obscure, but important differences not covered by the list of 228 rather obvious traits.

Enter Martinez et al. (2012).
Their strict consensus tree (51 taxa, 378 characters) was also unable to resolve relationships among these three and several other taxa. Their reduced consensus tree (eliminating poor specimens) nested them in ascending order: Phanphagia > Eoraptor > Pampadromaeus separated by single decay indices.

The position of Panphagia as basal to other sauropodomorphs is supported by nine unambiguous synapomorphies:

  1. pterygoid wing of the quadrate extending for more than 70% of the total quadrate length;
  2. presence of postparietal fenestra between supra occipital and parietals;
  3. supraoccipital wider than high;
  4. coarse serrations of the teeth angled upwards at 45◦;
  5. absence of a  postzygodiapophyseal lamina in cervical vertebrae 4–8;
  6. weakly developed laminae in the neural arches of cervical vertebrae 4–8;
  7. minimum width of the scapula less than 20% of its length;
  8. posterior end of the fibular condyle of the tibia anterior to the posterior margin of proximal articular surface;
  9. and strongly laterally curved iliac blade in dorsal view.

The more derived position of Eoraptor is supported by three unambiguous  synapomorphies:

  1. subtriangular cross-section of the ischial midshaft;
  2. supraacetabular crest of the ilium contacting the distal end of pubic peduncle;
  3. and sub triangular distal end of the ischium.

Pampadromaeus and more derived sauropodomorphs share four unambiguous synapomorphies:

  1. squamosal bordering the laterotemporal fenestra for more than 50% of its depth (62:0)
  2. length of the base of proximal caudal neural spines greater than half the length of the neural arch;
  3. transverse width of the distal humerus greater than 33 of its length;
  4. and length of the pubic peduncle of the ilium greater than twice the anteroposterior width of its distal end.

So the question remains,
are these several distinctions sufficient to split these three specimens? Are they indeed distinct genera? Or are they all three species of Eoraptor?  (Eoraptor is the earliest of these three to be named.)

References
Cabreira SF, Schultz CL, Bittencourt JS, Soares MB, Fortier DC, Silva LR and Langer MC 2011. New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil. Naturwissenschaften (advance online publication) DOI: 10.1007/s00114-011-0858-0
Martínez RN and Alcober OA 2009. A basal sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the early evolution of Sauropodomorpha (pdf). PLoS ONE 4 (2): 1–12. doi:10.1371/journal.pone.0004397. PMC 2635939. PMID 19209223. online article
Martínez RN , Apaldeti C and Abelin  D 2012. Basal sauropodomorphs from the Ischigualasto Format ion, Journal of Vertebrate Paleontology, 32:sup1, 51-69.
Sereno PC, Forster CA, Rogers RR and Moneta AM 1993. Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
Sereno PC, Martínez RN and Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12: 83-179 DOI:10.1080/02724634.2013.820113

wiki/Eoraptor
wiki/Panphagia
wiki/Pampadromaeus