Dinosaur family tree: Langer et al. responds to Baron et al. 2017 in Nature

Earlier
Baron et al. revised the dinosaur family tree by uniting Ornithischia with Theropoda to the exclusion of Herrerasaurus + Sauropodomorpha. Then Baron and Barrett 2017 moved Chilesaurus (Fig. 1) from Theropoda to Ornithischia, confirming the earlier hypothesis advanced here in 2015, but in the context of uniting Ornithischia with Sauropodomorpha (= Phytodinosauria) to the exclusion of Herrerasaurus + Theropoda.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Today
Langer et al. 2017 argue, “we evaluate and reanalyse the morphological dataset underpinning the proposal by Baron et al.5 and provide quantitative biogeographic analyses, which challenge the key results of their study by recovering a classical monophyletic Saurischia and a Gondwanan origin for dinosaurs. Our international consortium of early dinosaur evolution specialists has come together to critically assess the Baron et al.5 dataset.”

The Langer team recovered a traditional Saurischia/Ornithischia tree, but noted it would take only 2-3 additional steps to enforce a Sauorpodomorpha/ Ornithoscelida split, as recovered by the Baron team – this after changing 2,500 scorings (10% of the dataset). The Langer team also confirmed the origin of dinos in southern Pangaea and left with three conclusions (my comments follow):

  1. There is currently great uncertainty about early dinosaur relationships and the basic structure of the dinosaur family tree. Not in the large reptile tree (LRT, 1119 taxa)/
  2. Dataset construction is key. No, taxon inclusion is the key. Neither the Baron team nor the Langer team included the correct outgroup taxa nor a long list of basal dinosaur taxa (see below) that direct the tree topology toward the phytodinosaur clade.
  3. It is important to use appropriate computational analytical tools before making macro-evolutionary claims. No, taxon exclusion will lead to wrong results. Trait selection matters, but not as much. Scoring correctly matters, but not as much. Employing decades old software does not matter because the math and statistics are the same. Remember, only a poor workman blames his tools so don’t  blame the “computational analytical tools” for poor macro-evolutionary claims.

Bottom line:
The Langer team used the same incomplete taxon list as the Baron et al. team did. So they were looking for their ‘keys’ beneath the bright lamp, while the keys were lost in the dark alley they ignored.

This happens so often.

And
Baron et al. 2017 reply. “This  extensive re-scoring results in recovery of the ‘traditional’ topology, although with less resolution and very weak support; their result is statistically indistinguishable from the possibility that our topology provides a better explanation of the data. This weak support, despite these extensive changes, suggests that the ‘traditional’ tree struggles to account for many character distributions.”

And they disagree with many of the re-scorings. Their re-scoring of just Pisanosaurus reproduced the clade Ornithoscelida in their revised tree.

Both presented trees were poorly resolved.
The LRT is fully resolved. Baron et al. defended the possibility of a Northern origin for dinosaurs. That big ‘maybe’ does not follow the data in the LRT.

On a similar, but side note
Biology Letters was kind enough to publish my reply to the Baron and Barret 2017 paper on Chilesaurus, but much of it has bearing for today’s discussion. Here is that letter in its entirety:

Baron and Bennett [1] nest Chilesaurus [2] as the sister group to Ornithischia, rather than a tetaneuran theropod as previously proposed [2]. Unfortunately, the Baron and Bennett [1] taxon list, like the Novas et al. [2] taxon list before it, did not include many of the taxa essential to resolve this issue.

A larger study of over 1060 taxa [3] includes more taxa essential to resolve this issue. The matrix was created using MacClade [4]. Analyses were run in PAUP 4.0b10 [5] using a heuristic search and a Bootstrap/Jackknife search for 100 random addition replicates. Scores are indicated on the webpage.

On that cladogram Chilesaurus (Late Jurassic) nests as a basal ornithischian in a clade that also includes Daemonosaurus (Late Triassic) and Jeholosaurus (Early Cretaceous). The latter two taxa were not included in Baron and Bennett [1]. This clade of three taxa nested as a sister to the Sauropodomorpha with Leyesaurus at its base. The analysis recovered the clade Phytodinosauria as the sister taxa to the Theropoda. Herrerasaurus was the outgroup to these two clades as basalmost member of the Dinosauria. Basal phytodinosaurs not nesting within Sauropodomorpha + Ornithischia include Barberenasuchus, Eodromaeus, Eoraptor and Pampadromaeus.

On that cladogram Silesaurus nests within a clade Poposauridae outside the Archosauria. The clade Archosauria includes only the Crocodylomorpha + the Dinosauria. Lagerpeton nests with Tropidosuchus and other proterochampsids. The pterosaur, Dimorphodon, nests with lepidosaurs like Huehuecuetzpalli, Macrocnemus and Cosesaurus (the last of which had an antorbital fenestra by convergence [6, 7]). None of these are archosauriformes nor prolacertiformes [contra 7]. The following pertinent taxa were not included in Baron and Bennett [1]: Daemonosaurus, Jeholosaurus, Haya, Barberenasuchus, Buriolestes, Segisaurus, Procompsognathus, PVL 4597, Junggarsuchus, Pseudhesperosuchus, Carnufex, Trialestes, Gracilisuchus, Scleromochlus, Decuriasuchus, Turfanosuchus, Poposaurus, Lotosaurus, Shuvosaurus, Effigia and Tropidosuchus.

Chilesaurus was first nested as a basal ornithischian in April 2015 [8] in an earlier version of the above analysis, then with fewer taxa. With the addition of more pertinent taxa the position of Chilesaurus is indeed well resolved contra the previous e-letter [9].

1.Baron M.G., Barrett P.M. 2017 A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biology Letters 13, 20170220.
2.Novas F.E., Salgado L., Suárez M., Agnolín F.L., Ezcurra M.D., Chimento N.R., de La Cruz R., Isasi M.P., Vargas A.O., Rubilar-Rogers D. 2015 An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature 522(7556), 331.
3. http://www.ReptileEvolution.com/reptile-tree.htm .nex file link on that webpage
4.Maddison, D,R., & Maddison. W.P. 2001 MacClade 4.08: Analysis of Phylogeny and Character Evolution. Version 4.03. Sinauer Associates.
5. Swofford, D. 2002 PAUP 4.0 b10: Phylogenetic analysis using parsimony. Sinauer Associates.
6. Ellenberger, P. and de Villalta, J.F. 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
7. Peters D. 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
8.https://pterosaurheresies.wordpress.com/2015/04/28/chilesaurus-new-dinos…
9. King B. 2017. Chilesaurus is not a basal ornithischian. http://rsbl.royalsocietypublishing.org/content/13/8/20170220.e-letters

References
Baron M.G., Barrett P.M. 2017 A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biology Letters 13, 20170220.
Baron MG, Norman DB and Barrett PM 2017.
xxxx Nature 543: 501–506;  doi:10.1038/nature21700
Baron MG, Norman DB and Barrett PM 2017. Baron et al. reply. Nature 551: doi:10.1038/nature24012
Langer et al. (8 co-authors) 2017. Untangling the dinosaur family tree. Nature 551: doi:10.1038/nature24011

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Professor TR Holtz on Dinosaur Classification

An Albert Einstein anecdote is appropriate to today’s discussion. 
One of his students staood up 15 minutes into an exam saying, “The questions in this year’s exam are the same as last year’s exam.” Einstein replied, “Don’t worry; the answers are different this year.”

It’s got to be difficult telling students
how basal dinosaurs are related. The answers are different this year. Do they traditionally split into Saurischia and Ornithischia? Or do ornithischians nest with theropods, as Baron, Norman and Barrett 2017 proposed a few months ago. Or do they split into Theropoda and Phytodinosauria, as recovered here in the large reptile tree (LRT)?

Dr. Thomas R. Holtz (U of Maryland, PhD from Yale U) is often seen on TV and YouTube as a popularizer/explainer of all things dinosaur. Recently he uploaded a web page that showed several options for dinosaur and outgroup relations. This was part of his lecture series.

Holtz reports,
“Dinosauria is comprised of three major clades: Ornithischia, Sauropodomorpha, and Theropoda. Traditionally, sauropodomorphs and theropods were recognized to form a clade Saurischia. However, recent discoveries have reduced the support for this hypothesis, and alternative relationships are possible.”

Things would be easier and more logical
if Holtz knew the precise outgroup for the Dinosauria. Unfortunately he does not. He bought into the Avemetatarsalia hypothesis, when that was invalidated 17 years ago (Peters 2000).

In the LRT
Crocodylomorpha and Poposauridae are successively more distant outgroups to the Dinosauria. In Holtz’s view crocs are distantly related with a common ancestor close to Euparkeria. Pterosaurs, Lagerpeton, Lagosuchus and Silesaurus are closer relatives. In the LRT pterosaurs are lepidosaurs, Lagerpeton is a sister to Tropidosuchus and Lagosuchus is a theropod and Silesaurus is a poposaur.

Holtz also believes in the clade Ornithodira
even though that was also invalidated 17 years ago (Peters 2000). Holtz reports, “Unfortunately, at present we have no pterosaur-lineage animals which are not already highly derived for flight, so we can’t yet trace the transformations from walking to flying in this group.” This is wrong. Peters 2000 listed a half dozen taxa with a gradual accumulation of pterosaur traits, even when tested against archosaurs. The concept of the clade Ornithodira survives to this day due to taxon exclusion. And Peters exclusion, even published work in academic journals. Apparently no one wants to test what happens when various tritosaurs are entered into the taxon list.

Holtz believes that very small dinosaurormphs
left footprints in the Early Triassic. This was invalidated earlier here.

Holtz believes that dinsoauromorphs

  1. had a parasagittal stance with erect hind limbs, but several clades develop this
  2. had simple hinge ankle joints, but both mammals and tritosaur lepidosaurs had this
  3. had a digitigrade posture, but both mammals and tritosaur lepidosaurs had this

See what happens
when Holtz tries to pull a “Larry Martin“? Larry would have given the same answers with a wry smile. Holtz needs to base his conclusions on a large gamut phylogenetic analysis that considers all possible candidates, not a short list of convergent traits.

Holtz mentions Nyasasaurus
an incomplete taxa considered a Middle Triassic dinosaur. Here it compares well with the basal popoaur, Turfanosuchus, only much larger.

Phytodinosauria
Holtz reports, “No recent computer-generated phylogenetic analysis has supported [Phytodinosauria]. This is wrong. The LRT recovered the clade Phytodinosauria six years ago. Holtz also reports, “but possible support for this arrangement may exist in the enigmatic Chilesaurus.” Yes. And you heard that first here two years ago.

Holtz lists
several Late Triassic dinosaurs of uncertain position.

In the LRT
none of the taxa listed by Holtz nests in an uncertain position… and he would discover that, too, if he also ran a large gamut phylogenetic analysis. He has access to all the literature and specimens, more so than I do. Instead of leaving dinosaur origins as a big question for his students, Holtz could find out for himself and provide an unequivocal answer. This is science. Anyone can do it, whether PhD or independent researcher.

References
Baron MG, Barrett PM 2017. A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biol. Lett. 13: 20170220. http://dx.doi.org/10.1098/rsbl.2017.0220 pdf online
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.
Peters D 2000b. A reexamination of four prolacertiforms with implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293–336.

You heard it here first: Chilesaurus is a basal ornithischian confirmed.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Figure 2. Look familiar? Here are the pelves of Jeholosaurus and Chilesaurus compared. As discussed earlier, this is how the ornithischian pelvis evolved from that of Eoraptor and basal saurorpodomorpha.

Figure 2. Look familiar? Here are the pelves of Jeholosaurus and Chilesaurus compared. As discussed earlier, this is how the ornithischian pelvis evolved from that of Eoraptor and basal saurorpodomorpha.

A new paper by Baron and Barrett 2017 confirms Chilesaurus (Fig. 1) as a basal member of the Ornithischia, not a bizarre theropod. As long time readers know, this was put online two years ago (other links below) in this blog.

Unfortunately, the authors don’t have an understanding of the interrelationships of phytodinosaurs, even though they report, For example, Chilesauruspossesses features that appear ‘classically’ theropod-like, sauropodomorph-like and ornithischian-like…” Nor did they mention the sister taxon, Jeholosaurus (Fig. 2).

Remember,
discovery only happens once.
More on this topic later.

This note went out this morning:
Thank you, Matthew,
for the confirmation on Chilesaurus.
In this case, it would have been appropriate to include me as a co-author since I put this online two years ago.

https://pterosaurheresies.wordpress.com/2015/04/28/chilesaurus-new-dinosaur-not-so-enigmatic-after-all/
http://www.reptileevolution.com/reptile-tree.htm
http://www.reptileevolution.com/chilesaurus.htm

References
Baron MG, Barrett PM 2017. A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biol. Lett. 13: 20170220. http://dx.doi.org/10.1098/rsbl.2017.0220 pdf online

Best regards,

What traits separate phytodinosaurs from theropods?

Yesterday we looked at the origin of dinosaurs in the context of and contra the recent Baron et al. 2017 paper. Today we’ll look at the basal split between basal phytodinosaurs, like Eodromaeus (Figs. 1, 2), with the closely related basal theropods, like Tawa (Fig. 1).

Figure 1. The theropod Tawa compared to the closely related phytodinosaur, Eodromaeus.

Figure 1. The theropod Tawa compared to the closely related smaller phytodinosaur, Eodromaeus.

Placed side-by-side to scale
Tawa and Eodromaeus are similar overall, though the plant-eaters were initially smaller. The details (below) demonstrate the initial steps toward herbivory that characterize the Phytodinosauria, distinct from the Theropoda and basal Dinosauria from which they evolved (contra Baron et al. 2017).

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

How do basal phytodinosaurs differ from the basal theropods?
Here’s the LRT list:

  1. lateral rostral shape: convex and smoothly curved (also in ancestral Herrerasaurus and Gracilisuchus);
  2. premaxilla/maxilla angle 25–45º;
  3. naris shape in lateral view almost round (not longer than tall or taller than long);
  4. postfrontal has no contact with the upper temporal fenestra;
  5. opisthotic oriented laterally without posttemporal fenestrae;
  6. palatal teeth (only on basalmost taxa);
  7. maxillary tooth depth ≤ 2x width in lateral view;
  8. last maxillary tooth at mid orbit (also in Herrerasaurus);
  9. olecranon process present (convergent in Buriolestes);
  10. metacarpals 2 and 3 align with m1.1 (except Eodromaeus);
  11. acetabulum laterally oriented (no ventral deflection, as in basal theropods);
  12. femoral head with neck and offset (appears later in theropods);
  13. penultimate manual phalanges not the longest in each series;
  14. loss of pubic boot (likely plesiomorphic because outgroups to Herrerasaurus do not have a pubic boot).

Summary and significance
Compared to the closely related theropod Tawa, the overall similar phytodinosaur Eodromaeus had a taller rounder rostrum, shorter teeth, a higher coronoid process, a longer dorsal region with more robust dorsal vertebrae, reduced gastralia, a more robust pectoral girdle and forelimb with shorter, less raptorial fingers, a deeper pubis and ischium with more robust hind limbs. The shorter teeth and larger belly together with the more robust limbs and back are traits seen in a wide variety of herbivores, even if only transitional at this early stage.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.

Basal dinosaurs: what makes them dinosaurs?

Earlier we looked at Baron et al. 2017, who heretically allied ornithischians with theropods to create a new clade ‘Ornithoscelida.’ After a week of study and several tests, I came away with the realization that the Baron et al. tree topology did not depend on taxon exclusion, based on comparable results from subsets of the large reptile tree (LRT). Reducing the list of LRT taxa to match the Baron et al. taxon list did not change the dinosaur tree topology. Rather scoring inaccuracies were the problem, and that goes back to the original matrix presented by Nesbitt 2011, which Baron et al. continued using. You might remember, that’s the study that nests pterosaurs with parasuchians with no intervening taxa, among many other odd nestings. We’ll detail those inaccuracies later this week.

One of the reasons
Baron et al. allied theropods with ornithisichians is because they nested two phytodinosaurs, Eodromaeus (Fig. 1) and Eoraptor, as basalmost theropods. So when they thought they were comparing the ‘theropod’ Eoraptor with the ornithischian, Heterodontosaurus (their figure 2), they were actually comparing two phytodinosaurs. So, of course they shared a long list of characters. To really test their assertion, Baron et al. should have tested Heterodontosaurus against a real basal theropod, like Tawa (Fig. 2). (Tomorrow we’ll compare Tawa with Eodromaeus.)

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Along the way, 
I was able to better understand what was going on at the  base of the Dinosauria and at the basal Theropod/Phytodinosaur split. For instance, as noted earlier, Herrerasaurus (Fig. 3) has a smaller number of cervicals than other dinos do, matching the number found in ancestral basal crocodylomorpha, like Lewisuchus, not silesaurids, like Silesaurus.

Figure 2. Tawa, a basal theropod. Note how closely it resembles the phytodinosaur, Eodromaeus. That's because they are closely related as basalmost dinosaurs.

Figure 2. Tawa, a basal theropod. Note how closely it resembles the phytodinosaur, Eodromaeus. That’s because they are closely related as basalmost dinosaurs.

How do basal dinosaurs differ from the basal croc ancestors?
Here’s the latest list:

  1. skull shorter than cervical series (not Herrerasaurus):
  2. skull width less than 1.2x height;
  3. posterolateral premaxilla narrower than naris (not Herrerasaurus);
  4. quadrate curls posterodorsally (not Herrerasaurus);
  5. dentary tip straight or descends (does not rise);
  6. deep canine maxillary teeth absent (not Herrerasaurus);
  7. nine or more cervicals (not Herrerasaurus);
  8. cervical ribs slender, parallel centra (Herrerasaurus unknown);
  9. 3–4 sacral ribs (not Herrerasaurus, which has 2);
  10. mid-caudal centra 3x longer than tall where known (not Herrerasaurus);
  11. clavicles poorly ossified or absent (reappear in later taxa);
  12. interclavicle poorly ossified or absent;
  13. manual unguals long with penultimate phalanges longer than proximal phalanges;
  14. tibia not shorter than femur (not Herrerasaurus);
  15. fourth trochanter sharp (also in the proximal dinosaur outgroup taxon PVL 4597);
  16. tibia not < 2x ilium length (not Herrerasaurus);
  17. advanced and simple metatarsal joint without a calcaneal tuber (also in Lewisuchus, convergent in Silesaurus);
  18. compact metatarsus;
  19. longest metatarsal: #3;
  20. metatarsal #1 < 75% of metatarsal #3 length;
  21. metatarsals 2–4 ≥ tibia (not Herrerasaurus);
  22. proximal metatarsals 1 and/or 5 reduced in diameter.

Summary and significance
Arising from the morphology of basal bipedal crocodylomorphs, many of these basal dinosaur traits document a longer, leaner bipedal morphology with an increasingly robust fulcrum (pelvis and sacrum), along with longer distal hind limb elements with more reliance on the middle digits for faster locomotion. By contrast the proximal outgroup clade, Junggarsuchus and kin, documents a return to quadrupedalism terminating with Trialestes and its long forelimbs. The increasingly elongate radiale and ulnare of the Junggarsuchus clade turn out to be convergent, rather than homologous, with those of other bipedal crocodylomorphs, like Terrestrisuchus and Hesperosuchus, a clade that also transitioned toward a quadrupedal configuration, as seen in extant crocodylomorphs. We’ll take a closer look at this clade and this transition later this week.

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined.

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.

 

Estimating dino/croc divergence times: Turner et al. 2017

This might have been yet another case
of scientists TRUSTING authority (= the work of others) rather than TESTING competing phylogenetic analyses. In this case, however, two of the three authors in Turner, Pritchard and Matze 2017 relied on their own flawed (= serious taxon exclusion problems) phylogenetic analyses and for good measure they threw in a third flawed (= more taxon exclusion problems) analysis (Nesbitt 2011) that we examined and reexamined in an 11-part series.

In any case, since none of the trees
in the new Turner et al. study  stand up to scrutiny (= do not agree with one another, do not produce gradual accumulations of traits in derived taxa and depend on long ghost lineaages), everything Turner et al. (2017) did afterwards has no credibility and no utility. So sadly, the entire paper is a waste of their time. Metaphorically, they built their house on sand.

On the other hand,
when you start with a study that provides a gradual accumulation of derived traits in all derived taxa, and minimizes the effect of taxon exclusion, like the large reptile tree (LRT (949 taxa) then you’ve metaphorically built your house on solid ground. And it’s much simpler to pinpoint the dino/croc divergence time because you are provided with a last common ancestor for these sister clades: Gracilisuchus (Figs. 1, 2). Crocs and dinos are sister taxa. None of the studies used by Turner et al. (Pritchard et al. 2015, Nesbit 2011, Turner 2015) recovered that tested relationship.

Figure 1. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Herrerasaurus, Tawa and Eoraptor.

Figure 1. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Herrerasaurus, Tawa and Eoraptor.

So when did dinos and crocs diverge?
Let’s look a the three most recent taxa both clades share in common in reverse chronological and phylogenetic order:

  1. Gracilisuchus = 230 mya.
  2. Turfanosuchus = 235 mya.
  3. Decuriasuchus = 240 mya.

So that narrows the divergence time pretty well…

And how did Turner et al. do?
They report,“The average ghost lineage for the group as sampled is 31 million years.” Their conclusion states no firm date or date range. Rather, their whole paper appears to be a long story on how they tested this that and the other without getting around to their headline topic. And without nailing down a last common ancestor or a croc/dino divergence time.

Figure 2. Basal crocs. Decuriasuchus and Gracilisuchus are found in both croc and dino lineages.

Figure 2. Basal crocs. Decuriasuchus and Gracilisuchus are found in both croc and dino lineages.

All the other taxa
and all the other testing performed by Turner et al. were for nought.

For more information
on any of the taxa employed by Turner et al, just look them up at ReptileEvolution.com.

References
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History;352:1–292.
Pritchard AC, Turner AH, Nesbitt SJ, Irmis RB and Smith ND 2015. Late Triassic tanystropheids (Reptilia, Archosauromorpha) from Northern New Mexico (Petrified Forest Member, Chinle Formation) and the biogeography, functional morphology, and evolution of Tanystropheidae. Journal of Vertebrate Paleontology. ;e911186.
Turner AH 2015. A Review of Shamosuchus and Paralligator (Crocodyliformes, Neosuchia) from the Cretaceous of Asia. PLoS ONE. 2015;10(2):e0118116. doi: 10.1371/journal.pone.0118116. pmid:25714338
Turner AH, Pritchard AC and Matzke NJ 2017. Empirical and Bayesian approaches to fossil-only divergence times: A study across three reptile clades. PLoS ONE 12(2): e0169885. doi:10.1371/journal.pone.0169885

 

Go back far enough in dinosaur ancestry and you come to: Heleosaurus

With our never-ending fascination with dinosaurs
it’s interesting to list some of the taxa in their deep, deep!, deep!! ancestry. One such ancestor is Heleosaurus (Fig. 1; Broom 1907; Middle Permian ~270 mya, ~30 cm snout to vent length), the first known basal prodiapsid, the clade the includes diapsids (sans lepidosaurs, which are unrelated but share the same skull topology by convergence).

Figure 1. Heleosaurus is closer to the main lineage of dinosaurs. It retained canine fangs.

Figure 1. Heleosaurus is close to the main lineage of dinosaurs. It retained canine fangs. Note the squamosal distinct from the quadratojugal, as in Nikkasaurus. Also note the continuing lacrimal contact with the naris, as in Protorothyris.

But first
I want to discuss a derived Heleosaurus cousin, Nikkasaurus (Ivahnenko 2000; Fig. 2), also one of the most basal prodiapsids.

It is only by coincidence
that Ivahnenko labeled Nikkasaurus one of his ‘Dinomorpha,’ a clade name ignored by other authors. Wikipedia considers Nikkasaurus one of the Therapsida and possibly a relic of a more ancient stage of therapsid development. Like Heleosaurus, Nikkasaurus had a single synapsid-like lateral temporal fenestra. Only their nesting outside of that clade and basal to the clade Diapsida in the LRT tell us what they really are. Most of the time, as you know, we can tell what a taxon is simply by looking at it. In this case, as in only a few others, we cannot do so readily.

Figure 1. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Figure 2. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Nikkasaurus tatarinovi (Ivahnenko 2000) Middle Permian was a tiny basal prodiapsid with a large orbit. It retained a large quadratojugal. The fossil is missing the squamosal. Others mistakenly considered the quadratojugal the squamosal, as in therapsids. That’s an easy mistake to make. Compare this bone to the QJ in Heleosaurus (Fig. 1), another prodiapsid. Nikkasaurus has small sharp teeth and no canine fang. Nikkasaurus is a sister to Mycterosaurus. They both share a large orbit and fairly long snout. What appears to be a retroarticular process may be something else awaiting inspection in the actual fossil. Based on all other data points, I don’t trust that post-dentary data. It doesn’t match the in situ figure.

Distinct from other prodiapsids,
the Nikkasaurus, Mycerosaurus and Mesenosaurus maxilla extended dorsally, overlapping the lacrimal and contacting the nasal, as it does in Dimetrodon and basal therapsids like Hipposaurus and Stenocybus. This trait tends to be homoplastic / convergent in all derived taxa, but the timing differs in separate clades.

Figure 1. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it.

Figure 2. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it. Note the posterior mandible mismatch in the purported retroarticular process. I suspect the process is not there.

And finally we come back to Heleosaurus.
Slightly closer to the lineage of dinosaurs is the slightly more basal prodiapsid, Heleosaurus (Fig. 2), which retained canine fangs, had a more typical posterior mandible and retained a lacrimal / naris contact. This naris trait was retained by Petrolacosaurus, Eudibamus, Spinoaequalis and other basal diapsids (archosauromorpha with both upper and lateral temporal fenestra ). The maxilla did not rise again to cut off lacrimal contact with the naris in the ancestry of dinosaurs until the small Youngina specimens huddled together, SAM K 7710 and every more derived taxon thereafter, up to and including dinos.

References
Broom R 1907. On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. Transactions of the South African Philosophical Society 18:31–42.
Ivahnenko MF 2000. 
Cranial morphology and evolution of Permian Dinomorpha (Eotherapsida) of eastern Europe. Paleontological Journal 42(9):859-995. DOI: 10.1134/S0031030108090013