Monday, August 30, 2010

Recoding Holtz 1994 Part 2- 36% miscoded and alternate placements

Holtz's analysis was impressive at the time for its size, in both characters and taxa used. The analysis has a refreshingly low Consistancy Index of 0.51, indicating it was not designed with a particular result in mind. Only 7% (9/126) of the characters are composites and only 3% (4/126) are correlated with others, which is quite an improvement over Gauthier's (1986) analysis. Another improvement is the use of lower level taxa for OTUs, with only Megalosaurus being a potential composite due to the inclusion of Duriavenator's states. The use of polymorphies is an improvement over most prior analyses, but the all-zero outgroup is again an issue as it should be coded as apomorphic or polymorphic for at least 24% (30/126) of the characters. While Holtz did not test any alternate topologies, he did illustrate a consensus cladogram showing which avetheropod nodes collapsed with one more step, and noted in three more steps basal tetanurine phylogeny collapsed. The two main issues with the study were both mentioned by Clark et al. (1994). First, many characters are vague (e.g. depression in periotic region; narrow nasals; periotic region highly pneumatized; chevrons attenuated distally) and a much larger number are more precisely stated but still unquantified. As none were described in more detail, and the cited sources like Bakker et al. (1988) were just as vague, it's difficult to code these unambiguously. Yet the greatest problem are the miscodings. Some are simply due to specimens which were unknown at the time, like most Majungasaurus material, Currie and Carpenter's (2000) Acrocanthosaurus skeleton, Sues' (1997) Chirostenotes skeleton and the MOR Troodon material. Many are due to depending on the pre-1994 literature, especially for coelophysids, Dilophosaurus, Megalosaurus, basal ornithomimosaurs, and non-Oviraptor oviraptorids. Yet it seems as if in most cases, taxa were coded based on what they should have instead of what they were known to have. This is most obvious in the many codings based on elements which are not even preserved in the taxa in question. For instance, Ornitholestes is coded for at least nine characters involving the pubic boot, proximal femur, tibia and tarsus though these are not preserved in the only specimen, which Holtz examined personally. Similarly, Torvosaurus is coded for at least ten characters involving the occiput, axis, distal caudals, carpals and femur though none of these are preserved (the axis and femur are preserved in the "Brontoraptor" specimens some authors assign to Torvosaurus, but these were discovered after Holtz's paper was submitted). Another manifestation of this flaw is when the character states are obvious, but don't match the clade the character was meant to diagnose. Examples are the circular orbits and anteromedially projected femoral heads of coelophysoids and the outgroup, or the large laterotemporal fenestrae of oviraptorids. Since Gauthier used circlar orbits as a coelurosaur character, non-coelurosaurs were seemingly just assumed not to have it.  Similarly, the anteromedially projected femoral head and large laterotemporal fenestra were used to diagnose neoceratosaurs by Bonaparte, so any taxon outside that clade was apparently assumed not to have it.  In all, over a third of the entres (36%- 905/2520) were miscoded.

Regarding Elaphrosaurus, Holtz's original matrix moderately supports it as a ceratosaur, and moderately rejects it as a coelophysoid (7 more steps) or basal ceratosaur (6 more steps).  It strongly rejects placing Elaphrosaurus in Ornithomimosauria (13 more steps).  In the recoded matrix, it takes 4 more steps to put it in Ceratosauria or 5 more to put it in Coelophysoidea.  Making it an abelisauroid is 7 steps longer though.  This all indicates its position in Holtz's cladogram is due to miscodings.  Elaphrosaurus does have some coelurosaur-like characters, such as broad anterior articular surfaces on its anterior cervical centra, no obturator foramen in its pubis, and a triangular obturator process on its ischium.

In Holtz's original matrix, Acrocanthosaurus becomes a basal coelurosaur with a single extra step, while in the recoded matrix is shifts from being a basal coelurosaur to being a carnosaur in three steps.  So the interrelationships of Allosaurus, Acrocanthosaurus and Coelurosauria are weakly supported in both versions.  More interestingly, Acrocanthosaurus is a tyrannosauroid with only a single extra step in the recoded matrix (compared to a moderate 5 steps in the original dataset).  For those wondering, making it AND tyrannosaurids carnosaurs adds 6 steps in both versions.

Holtz's Oviraptoridae + Arctometatarsalia clade (which he named in his thesis) takes 9 extra steps to recover after recoding, making it moderately rejected.  Arctometatarsalia itself takes 15 more steps after recoding, meaning it is strongly rejected.  It breaks down in the original matrix with only 3 added steps, indicating it was poorly supported from the start.

The ornithomimosaur-troodontid clade Bullatosauria breaks down with a single step in the original matrix, if one groups ornithomimosaurs with tyrannosaurids instead.  In the recoded matrix, it only takes 5 extra steps to get Bullatosauria back, so it's still not strongly rejected.  However, in Holtz's original matrix it took a whopping 20 extra steps to enforce Deinonychosauria.  In the recoded version, it only takes one extra step.  This indicates that far from being caused by an absence of transitional taxa like Sinovenator, Bullatosauria existed due to miscoding the derived taxa that were known at the time.

Saturday, August 28, 2010

Recoding Holtz 1994 Part 1

Ah, Holtz 1994.  The second detailed theropod analysis to be published (after Gauthier, 1986), Holtz's paper was the first to quantitatively support several standard hypotheses such as Elaphrosaurus as a ceratosaur, Dilophosaurus as a coelophysoid, Megalosaurus and Torvosaurus as basal tetanurines, Acrocanthosaurus as a carnosaur, and Compsognathus and Ornitholestes placed outside Maniraptoriformes.  It popularized the idea theropods with arctometatarsi form a monophyletic group, which Holtz named Arctometatarsalia. It also popularized the ornithomimosaur-troodontid pairing which Holtz named Bullatosauria.  The latter two ideas are not currently the consensus, with the usual rationale for Holtz's results being that he didn't include basal troodontids like Sinovenator, or basal tyrannosauroids for that matter.  I decided to test this by recoding Holtz's matrix using only taxa he did (so Troodontidae is only Troodon, Saurornithoides, Zanabazar, Borogovia and IGM 100/44 for instance).  As a reminder, Holtz's original tree was-

   |  |--Coelophysoidea
   |  |  |--Dilophosaurus
   |  |  `--Coelophysidae
   |  `--Neoceratosauria
   |     |--Ceratosaurus
   |     `--Abelisauroidea
   |        |--Elaphrosaurus
   |        `--Abelisauridae
            |  |--Allosaurus
            |  `--Acrocanthosaurus
                     |  `--Archaeopteryx

Note Holtz's use of Maniraptora and Arctometatarsalia differ from current definitions, where only dromaeosaurids and Archaeopteryx would be maniraptorans, while oviraptorids would also be arctometatarsalians.  And what does it look like after the recodings?


I find this interesting because while the basal placement of tyrannosaurids and content of Maniraptora and Paraves is like the modern consensus, so many other things aren't.  In fact, they're amusingly close to what Holtz was refuting- 'ceratosaurian' Torvosaurus of Britt 1991, basal coelurosaurian Acrocanthosaurus of Bakker et al. 1988, ornithomimosaur-like Elaphrosaurus, a rather PDW-like Intertheropoda...

Coming up next- what kind of coding issues are there, and just what percent were miscoded?

Friday, August 20, 2010

My Summary of This Year's Coolest SVP Abstracts

... can't be written.  The abstract volume for SVP 2010 came out today, and contains loads of interesting news for us paleontologists.  But the Society of Vertebrate Paleontology has a ridiculous policy that-

"Unless specified otherwise, coverage of abstracts presented orally at the Annual Meeting is strictly prohibited until the start time of the presentation, and coverage of poster presentations is prohibited until the relevant poster session opens for viewing.  As defined here, “coverage” includes all types of electronic and print media; this includes blogging, tweeting and other intent to communicate or disseminate results or discussion presented at the SVP Annual Meeting. Content that may be pre-published online in advance of print publication is also subject to the SVP embargo policy"

I wish to know- exactly what is the point?  Anybody who cares about the contents has the abstract book anyway.  So disallowing public discussion does absolutely nothing to keep the material secret, and is not going to make the presentations and posters more of a surprise.  It's the Shiny Digital Future- when something's released online, everyone has it.  So you can't claim it's to prevent stolen ideas ala Aetogate, since any paleontologist who cares has the abstract before SVP anyway.  Even assuming they somehow didn't, public discussion is allowed after the meeting (or they could just attend the meeting itself), and it's not like the papers that (some) abstracts lead to are published immediately.  There's plenty of time to get your own knock-off paper published, especially if you're the editor of an in-house journal so that you can speed the process up.

The only people who don't have easy access to the abstracts are those outside the paleontological community, and the media couldn't care less about basically any of the content.  Even on the incredibly rare occassion the general public might care, like the Triceratops-Torosaurus synonymy announced in an SVP abstract last year, the news outlets mostly got it wrong and most people misunderstood it anyway.  For last year's SVP, Google finds a whopping eight news stories that cover six abstracts, and that's due to only about five reporters.  Does the SVP really think that reporters are going to scour the abstract volumes of scientific meetings or dinosaur blog posts for stories?  Even things like the PepsiCo ScienceBlogs mess that caused Pharyngula to go on strike barely get noticed.

Who does notice and care though?  Us, the amateur and professional paleontologists who host blogs and use mailing lists, message boards, and newsgroups.  We'd gladly write up summaries of all the cool stuff happening at SVP, which would be seen by the portion of the public that is actually interested in paleontology.  That's free publicity for the society, the authors, and their work.  But by the time we're allowed to, there will be new stories to cover, and we won't be nearly as hyped about it.  All this accomplishes is decreasing the excitement and interest in SVP's meeting among the young readers who could make up its membership in the future, and thus be paying to attend, subscribe to the journal, etc..

(Of course there's also the obligatory crowd that claims abstracts are gray literature that isn't properly citable, but they should be on my side too, because what sense does it make to have an embargo on information you consider to be equivalent to hearsay?)

Saturday, August 14, 2010

Is a theropod-ornithischian group that crazy?

While recoding Holtz's (1994) theropod analysis, I've been using Heterodontosaurus as one of the main outgroup taxa, being a basal ornithischian known at that time.  And I've noticed just how many characters it shares with basal avepods that aren't found in basal sauropodomorphs.  For instance-

- Promaxillary recess and fenestra (Butler et al., 2008).
- Subnarial gap.
- Dorsoventrally expanded anterior dentary with enlarged tooth.
- At least five sacral vertebrae.
- Deep extensor pits on metacarpals.
- Manual phalanx III-3 longer than III-1 or III-2.
- Elongate preacetabular process.
- Fully open acetabulum.
- Well developed antitrochanter.
- Narrow pubic apron.
- Proximally projected anterior trochanter (in other ornithischians; it's fused in Heterodontosaurus)
- Fibular crest on tibia (also in Saturnalia; though seemingly absent in Pisanosaurus).
- Fused tibiotarsus in adult.
- Fused tarsometatarsus in adult.
- Metatarsal V reduced in size and without phalanges.
- Feathers? (in Tianyulong- they're not known from sauropodomorphs yet after all)

Also keep in mind that nearly all analyses have used the more derived Lesothosaurus as a proxy for Ornithischia, at least using its codings instead of Heterodontosaurus' when they differ.  Not that I'm saying Saurischia is wrong, I just think we can get so comfortable with an idea that we stop looking at alternatives.

Friday, August 6, 2010

Brief Thoughts on Chromogisaurus and Guaibasauridae

Since SVP 2009 we've been hearing about Ezcurra's idea a broad range of basal saurischians (Guaibasaurus, Saturnalia, Panphagia, Agnosphitys) can be referred to a clade he calls Guaibasauridae, placed in Sauropodomorpha.  Previously Guaibasaurus has emerged as a basal theropod, while Saturnalia and Panphagia were basal sauropodomorphs.  Now thanks to the description of the new guaibasaurid Chromogisaurus by Ezcurra (2010), we have the matrix and character evidence for this arrangement.  His cladogram is-

Strict consensus tree of Ezcurra's (2010) analysis, after Figure 15A in that paper.

My first thought on seeing this is that it's based on Yates' analysis, and indeed it's the version with Glacialisaurus added from Smith and Pol (2007), with 15 added characters and three added taxa (Chromogisaurus, Panphagia and the undescribed herrerasaurid MACN-PV18649a).  Ezcurra also corrected several characters, which was especially useful for Guaibasaurus as it incorporated the referred specimen UFRGS PV0725T.  Yates' analysis is excellent, but the problem here is that it's a sauropodomorph matrix, not a dinosaur matrix.  In other words, it was designed to determine the relationships among sauropodomorphs (which make up at least 37 of the 50 taxa), not the relationships of Herrerasaurus, Eoraptor, Staurikosaurus, Guaibasaurus, theropods, etc..  This is why Neotheropoda is a single OTU, whereas in any analysis of saurischian relationships you'd want it to be split into several taxa.  Similarly, Pisanosaurus would be useful for outgroup comparisons, as would more basal silesaurids like LewisuchusTawa also seems vital for any current current saurischian analysis, though of course it wasn't known when Yates constructed the dataset.  But since this is the matrix we have, let's see what it supports.

"The basic message is that each new analysis does not make prior analyses wrong. One point to Nesbitt et al.'s credit is that they ran constraint analyses to determine just how unlikely alternate arrangements are. We need to keep these in mind whenever we look at a cladogram. Cladograms always show us the MOST parsimonious arrangement, but perhaps it's more useful to be aware of which nodes are SIGNIFICANTLY MORE parsimonious."

Unfortunately, Ezcurra does not run any constraint analyses, so it's hard to know how well supported his new hypotheses are.  He does use Bootstrap and Bremer support, but these are highly influenced by a taxon's completeness so aren't very useful in analyses like this with many partially coded OTUs.  I tried to run Ezcurra's matrix, but it seems that only Crurotarsi is coded for all 378 characters, while all other taxa are coded for only 377.  Based on the codings for the last character, it doesn't seem to be 378, so I'm assuming that's the one that's missing.  So I deleted Crurotarsi's coding for character 378, leaving 377 characters coded for all taxa.  The resulting tree is identical to that in Ezcurra's figure, but is 1181 steps instead of 1186 steps long.

EDIT: I have now received the actual NEXUS file with proper codings for all 378 characters which finds trees 1186 steps long as in Ezcurra's paper, and the results differ slightly.  Those who read this in the first hour of its posting saw incorrect numbers below, though the conclusions remain the same.

So let's examine how other topologies fare, going from most plausible to least.

Making Chindesaurus a herrerasaurian as suggested by several authors is only one step longer, so basically equivocal. Similarly, forcing it to be a sauropodomorph as originally reported is only one step longer.

Ezcurra found Chromogisaurus to be closer to Saturnalia than to Panphagia or Guaibasaurus, and named this clade Saturnaliinae.  It breaks down in two steps, so is very poorly supported.

Placing Panphagia basal to Saturnalia + other sauropodomorphs as in its original description is only three steps longer. This indicates Guaibasauridae is very poorly supported.

Placing Agnosphitys outside of Dinosauria (as in its original description) is only three steps longer.

Making Chindesaurus closer to Herrerasaurus than Staurikosaurus (as in Nesbitt et al., 2009) is also only three steps longer.

Placing Guaibasaurus closer to Neotheropoda than Saturnalia, sauropodomorphs, herrerasaurids and Eoraptor (as in other published analyses) is only four steps longer.  Panphagia and Chromogisaurus remain in a clade with Saturnalia at the base of Sauropodomorpha, while Agnosphitys could go in any eusaurischian position outside Saturnaliinae and Pantydraco + other sauropodomorphs.  Thus another aspect of Guaibasauridae is poorly supported.

Placing Eoraptor outside Eusaurischia as in many analyses is also only four steps longer. Chindesaurus has an uncertain position in Saurischia but outside Sauropodomorpha, while herrerasaurids are more basal.  Notably, this also results in a lack of support for guaibasaurid monophyly. 

Placing Agnosphitys in Theropoda (as in Yates' analyses) is four steps longer.

Making Chindesaurus closer to Staurikosaurus than Herrerasaurus (as in Sereno, 1999) is only four steps longer, so weakly rejected.

Forcing Eoraptor to be a sauropodomorph is also only four steps longer, so is only weakly rejected.

Interestingly, placing Silesaurus in Saurischia is only four steps longer, which is actually more parsimonious than making it an ornithischian.

Placing Silesaurus in Ornithischia is surprisingly only five steps longer, but as noted above including more basal ornithischians and silesaurids might affect this.

Placing herrerasaurids outside Dinosauria, as was popular before 1993, is also only five steps longer.  Less than I thought it would be.  Chindesaurus follows them, but Eoraptor and the other saurischians remain in their usual positions.

Placing Herrerasaurus in Sauropodomorpha is six steps longer, and thus only moderately rejected.  I find this interesting as it is more parsimonious than placing it in Theropoda, but has not been advocated since 1981.  Staurikosaurus is still a herrerasaurid, while Chindesaurus is a slightly more derived sauropodomorph.

Placing Herrerasaurus in Theropoda is seven steps longer, so moderately rejected.  When this happens, there is no resolution in the clade containing Herrerasauridae, Chindesaurus, Eoraptor and Theropoda.

Forcing a polyphyletic Herrerasauridae with Herrerasaurus closer to (at least) theropods than Staurikosaurus results in the latter being an ornithischian, and is only seven steps longer. 

Placing Panphagia or Saturnalia in Theropoda is nine steps longer. Placing Chromogisaurus there is ten steps longer.  Thus they are probably sauropodomorphs.

Forcing a monophyletic Phytodinosauria of Sauropodomorpha+Ornithischia is thirteen steps longer, so strongly rejected, but not by as much as I'd think.  Herrerasaurids, Chindesaurus and Eoraptor form successively closer sister groups to Avepoda.

Then again, enforcing a completely heterodox Theropoda+Ornithischia clade is only fourteen steps longer.

Making Staurikosaurus a sauropodomorph but leaving Herrerasaurus outside the clade (as in Kischlat's papers where he calls the Staurikosaurus+Sauropodomorpha clade Pachypodosauria), is sixteen steps longer, so is strongly rejected.

Placing Marasuchus in Theropoda, as suggested by Olshevsky (they both lack phalanges on metatarsal V- character 352 in this analysis), is 28 steps longer, so is strongly rejected.  Amusingly, Marasuchus drags Herrerasauridae into Theropoda with it and becomes their sister group.  Since Herrerasaurus has a phalanx on metatarsal V, Olshevsky's character doesn't even work in this topology.  In case you're cusrious, it takes 14 steps to make Marasuchus a dinosaur and 8 stepts to make it and Silesaurus switch positions.

What conclusions can we draw from this study?

First, note that some very good recent analyses have recovered relationships that are rejected by as much as seven extra steps in this one.  The Tawa paper's placement of Herrerasauridae inside Theropoda for instance.  This indicates that just about any suggested arrangement of basal dinosauriforms is plausible, except that Saurischia is real, Marasuchus isn't a dinosaur, and Staurikosaurus is not a 'pachypodosaur' to the exclusion of Herrerasaurus.  It also suggests some current ideas (Silesaurus is an ornithischian, herrerasaurids and/or Eoraptor are theropods) are actually less parsimonious than some heterodox alternatives that have not been suggested before in a modern context (Silesaurus is a saurischian, herrerasaurids and/or Eoraptor are sauropodomorphs).  These could be some interesting hypotheses to explore.

But back to the idea at the start of the post- how well supported is Guaibasauridae?  Very poorly.  Agnosphitys can be placed pretty much anywhere, Guaibasaurus goes back to Theropoda with only four steps, and Panphagia is a more basal sauropodomorph with only three steps.  Even confining ourselves to trees less than five steps longer than the most parsimonious one, an approximation of what this dataset indicates would be-

      |  `--Staurikosaurus
            `--other sauropodomorphs

Ezcurra, 2010. A new early dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: A reassessment of dinosaur origin and phylogeny. Journal of Systematic Palaeontology. 8(3), 371-425.

Thursday, August 5, 2010

Shenshiornis is *gasp* another young Sapeornis

Speaking of synonyms, I was hardly surprised to learn that yet again, another supposed new "sapeornithid" is just plain old Sapeornis chaoyangensis (like Omnivoropteryx, Didactylornis and Sapeornis angustis).  First, I want to implore everyone to use Omnivoropterygidae instead of Sapeornithidae as it was published four years prior.  Yes I hate Czerkas' paper as much as anyone, but dislike of a work is no reason to ignore the principle of priority.  Of course that would require acknowledging Omnivoropteryx even exists, which almost no one seems willing to do either.  What makes it especially deplorable is that both Shenshiornis and Sapeornis angustis were diagnosed partly by characters that were also supposed to distinguish Omnivoropteryx from Sapeornis.  It's almost like Hu et al. and Provini et al. were describing Omnivoropteryx under new names.  Not that I know anyone's motives, but the authors must be aware of Omnivoropteryx (Xu and Zhou are coauthors, and Czerkas' volume could not have skipped by them) but neither paper even cites Czerkas and Ji, 2002.  Am I the only one who finds Omnivoropteryx's near absence from the literature suspicious?

In any case, on to Shenshiornis.  The paper itself is typical of Mesozoic bird descriptions, with a brief osteology and empty discussion.  The high resolution color photos are outstanding though, and the skull deserves a closer look, as (besides Omnivoropteryx which only has X-rays published) it is by far the best preserved of any published omnivoropterygid.  The adjacent illustration does a poor job of showing the antorbital morphology, missing the antorbital fossa and maxillary fenestra for instance.

Skull of Sapeornis chaoyangensis (Shenshiornis primita holotype LPM B00018) after Hu et al. (2010).

Sapeornis Zhou and Zhang, 2002
= Shenshiornis Hu, Li, Hou and Xu, 2010
S. chaoyangensis Zhou and Zhang, 2002
= Shenshiornis primita Hu, Li, Hou and Xu, 2010
Early Albian, Early Cretaceous
Jiufotang Formation, Liaoning, China
- (LPM B00018; holotype of Shenshiornis primita) (subadult) skull (40 mm), mandibles (40 mm), eleven cervical vertebrae, cervical ribs, eleven dorsal vertebrae, five dorsal ribs, gastralia, synsacrum, more than ten caudal vertebrae, few chevrons, ilia (44 mm), pubes (57 mm), ischium, femora (62 mm), tibiotarsi (64 mm), fibulae, distal tarsal, metatarsals I (8.9 mm), phalanges I-1, pedal ungual I, metatarsals II, phalanx II-1, phalanx II-2, pedal unguals II, metatarsals III (32 mm), phalanges III-1, phalanx III-3, pedal unguals III, metatarsals IV, phalanges IV-1, phalanges IV-2, phalanges IV-3, phalanges IV-4, pedal unguals IV, metatarsal V (11.3 mm) (Hu et al., 2010)

Comments- Discovered in 2005, the partial skeleton LPM B00018 was described as Shenshiornis primita by Hu et al. (2010). They recognized it as a subadult specimen based on cervical ribs unfused to vertebrae, intercentral sutures visible on sacrum, and unfused metatarsals. This is further indicated by the small size, short pubis and perhaps unfused distal caudals. They distinguished it from Sapeornis based on several characters. The prenarial portion of the premaxilla is supposedly shorter, but it is identical to the situation Omnivoropteryx and longer than JZPM-LSV-130. The apparently different proportions in IVPP V13275 and V13276 are due to them being in dorsal view. The left premaxilla of IVPP V13276 also has an elongate subnarial process, suggesting the seemingly short process on the right side and in JZPM-LSV-30 is due to breakage. Premaxillary and maxillary teeth are present in all omnivoropterygids, just as dentary teeth are absent. Didactylornis, IVPP V13275 and JZPM-LSV-30 all have subtriangular tooth crowns wider than their roots as well. The cervical centra are described as amphicoelous or amphiplatyan, while those of Sapeornis (Zhou and Zhang, 2003; Provini et al., 2009) and Didactylornis have been described as heterocoelous. However, basal avialans generally have variation within the neck, with posterior cervicals being amphicoelous while anterior cervicals are only semi-heterocoelous (heterocoelous anterior articular surface but primitively concave posterior one). This intermediate state has led to taxa like Confuciusornis being described as both amphicoelous and heterocoelous by different authors. In fact, both Confuiciusornis and Shenzhouraptor are coded as amphicoelous like Shenshiornis in Hu et al.'s matrix, although descriptions of both genera have indicated they have the semiheterocoelous state. The photographed cervicals certainly don't rule out a semiheterocoelous condition in Shenshiornis, and considering the issues above, I don't view this as a definitive or distinct character of the taxon. Though most specimens are not preserved and/or illustrated in sufficient detail, at least CAGS-03-07-08 shares the elongate posterior cervical postzygapophyses of Shenshiornis. The new genus was said to differ from Sapeornis angustis and Didactylornis in having seven sacrals, as in S. chaoyangensis. Yet the posterior sacrum of S. angustis is covered by pelvic elements and has enough space for two other vertebrae. In Didactylornis, the sacrum is stated to be poorly preserved and the figure suggests it is broken and partially covered by other elements. Notably Yuan (2005) coded Didactylornis as having the same number of sacrals as Sapeornis, even though he wrote only five could be observed. Hu et al. state more than ten free caudal vertebrae are preserved in Shenshiornis. IVPP V13275 has six or seven, S. angustis has at least seven, and Didactylornis' number is estimated at less than eight. However, the caudal series in Shenshiornis is poorly preserved and only loosely articulated. Even if true, the lack of distal fusion may be ontogenetic as in Zhongornis and some enantiornithines, as this specimen is smaller than any Sapeornis with a preserved pygostyle. The anteriorly pointed ilium is not necessarily different from other specimens (which are hidden or broken), and indeed seems to be the case in the fragmentary right ilium of CAGS-03-07-08. While the S. chaoyangensis holotype was schematically illustrated as having shorter rounded preacetabular processes on both ilia, the photo of the right ilium shows it to be fragmented and partly covered anteriorly by elements not indicated in the original figure. Thus the shape and completeness of the left ilium is also called into question. The short pubis is expected in a subadult and is longer (~92%) than in Omnivoropteryx (84%). The supposedly long metatarsal I (28% of metatarsal III) is indeed a bit longer than in IVPP V13275 (26%), but is the same as in CAGS-03-07-08 and actually shorter than in Didactylornis (33%). Metatarsal V (35% of metatarsal III, not ~40% as stated in the diagnosis) is also supposedly long, and is actually longer than the available measurements for CAGS-03-07-08 (15%) and IVPP V13275 (23%). In CAGS-03-07-08 at least, the elements may only be shaft fragments as one doesn't extend to the proximal tarsometatarsal edge while the other lies diagonally across metatarsals III and IV. Note too that most specimens (including the Sapeornis holotype) cannot be measured for this ratio, which has yet to be studied ontogenetically. Thus of the characters listed in Shenshiornis' diagnosis, only the metatarsal V 12% longer than IVPP V13275 is even possibly valid and could not be used to divide most omnivoropterygid specimens in any case. Like Omnivoropteryx and Sapeornis angustis, there is no reason to consider Shenshiornis anything other than a young Sapeornis chaoyangensis.

References- Czerkas and Ji, 2002. A preliminary report on an omnivorous volant bird from Northeast China. Feathered Dinosaurs and the Origin of Flight. The Dinosaur Museum Journal. 1, 127-135.

Zhou and Zhang, 2002a. Largest bird from the Early Cretaceous and its implications for the earliest avian ecological diversification. Naturwissenschaften. 89, 34-38.

Zhou and Zhang, 2003. Anatomy of the primitive bird Sapeornis chaoyangensis from the Early Cretaceous of Liaoning, China. Canadian Journal of Earth Sciences. 40, 731-747.

Yuan, 2005. Restudy on sapeornithids from the Lower Cretaceous of Yixian County, Liaoning. PhD Thesis. China University of Geosciences. 157 pp.

Yuan, 2008. A new genus and species of Sapeornithidae from Lower Cretaceous in Western Liaoning, China. Acta Geologica Sinica. 82(1), 48-55.

Provini, Zhou and Zhang, 2009. A new species of the basal bird Sapeornis from the Early Cretaceous of Liaoning, China. Vertebrata PalAsiatica. 47(3), 194-207.

Hu, Li, Hou and Xu, 2010. A new sapeornithid bird from China and its implication for early avian evolution. Acta Geologica Sinica. 84(3), 472-482.

Tuesday, August 3, 2010

GSP's new taxon combinations from his dinosaur field guide

Taking a break from non-theropods, I checked out the Google Books preview for Gregory S. Paul's new The Princeton Field Guide to Dinosaurs.  While I'm still disappointed in the basic layout of the book, I'm impressed by just how many new taxa Paul's managed to include, and the new skeletal reconstructions are awesome.  As anyone who's read PDW should expect, Paul creates a ton of new genus-species combinations in this volume.  Someone more well versed with sauropodomorphs and ornithischians will have to handle those clades, but what follows is a discussion of his new theropod names.

Abelisaurus garridoi- Instead of Aucasaurus.  This could very well be true, but since Abelisaurus' skull was only described briefly and before other abelisaurids were recognized, and Aucasaurus' skull remains undescribed and schematically illustrated, it seems premature.

Allosaurus antunesi- Instead of Lourinhanosaurus.  Now this one's problematic.  Lourinhanosaurus has never even been hypothesized to be an allosaurid, and phylogenetic analyses suggest it is a more basal carnosaur or even a megalosauroid.  Most recently, Benson et al. (2010) found it to be sister to Streptospondylus.

Streptospondylus nethercombensis- Instead of Magnosaurus.  I should note that Paul also sinks Eustreptospondylus oxoniensis into Streptospondylus altdorfensis.  The latter two species were placed as sister taxa by Allain (2002) and Smith et al. (2007) based on the presence of carotid processes in their anterior dorsal vertebrae, but it seems the feature is barely developed in Eustreptospondylus (Sadlier et al., 2008).  The superior analysis of Benson et al. (2010) found Streptospondylus to be closer to Lourinhanosaurus, and a carnosaur instead of a megalosauroid.  Sadlier et al.'s and Benson's recent papers have shown Rauhut's (2003) proposed relationship of Eustreptospondylus to Magnosaurus isn't as close as he thought, so sinking Eustreptospondylus without sinking Duriavenator and such seems unwise.

Carcharodontosaurus carolinii- Instead of Giganotosaurus.  It turns out Paul was not the first to publish this combination, that honor going to Figueiredo, 1998, but the latter is so muddled that Paul can be said to be the first to competently publish it. ;) In any case, this is one of those subjective decisions.  The problem is that most analyses agree Mapusaurus is closer to Giganotosaurus than to Carcharodontosaurus. So to sink Giganotosaurus you'd need to sink Mapusaurus too.

Monolophosaurus wucaii- Instead of Guanlong.  Er, this I seriously doubt.  I know Carr (2006) proposed Guanlong is a juvenile Monolophosaurus, but Paul doesn't think this is true.  They don't emerge as closely related in any analysis except for Carr's (e.g. Rauhut's, Benson's).

Struthiomimus edmontonicus- Instead of Ornithomimus.  Paul's given up his PDW synonymization of all ornithomimid genera, but edmontonicus is a junior synonym of brevitertius, and there is no evidence this species is closer to Struthiomimus than to Ornithomimus, Anserimimus or Gallimimus for instance.

Caudipteryx yixianensis- Instead of Similicaudipteryx.  The problem here is that all the proposed similarities with Caudipteryx are symplesiomorphies.  Similicaudipteryx shares some synapomorphies with caenagnathids in my analysis, though the new specimens could affect this.

Citipati barsboldi, gracilis, huangi and mongoliensis- Instead of Nemegtomaia, Conchoraptor, Heyuannia and Rinchenia.  Synonymize derived oviraptorid genera if you like, but Conchoraptor has priority over Citipati.  Paul seems to think Conchoraptor gracilis and "Ingenia" yanshini are synonyms, so yanshini should be the name he uses for that species.  Where's Khaan you ask?  A juvenile of Citipati osmolskae according to Paul.  I bet Jaime will have some harsh words for these synonymies.

Sinornithosaurus ashile, lujiatunensis and zhaoianus- Instead of Shanag, Graciliraptor and Microraptor.  Much like the previous example, making all microraptorians one genus is a subjective choice.  Yet Hesperonychus was not sunk, and there is no evidence it's outside the clade formed by the above species.  Shanag's placement as a microraptorian is also questionable.  Another issue is that Richardoestesia is extremely similar to Sinornithosaurus and Shanag, but would have priority if all of these genera were synonymized.  Better to keep them separate in my opinion.

Velociraptor mangas- Instead of Tsaagan.  This is problematic since Tsaagan was most recently placed as sister to Adasaurus (Longrich and Currie, 2009) or as basal to Eudromaeosauria (Senter, 2007). 

These examples indicate the general problems with synonymizing Mesozoic dinosaur genera.  I agree Mesozoic dinosaurs are oversplit compared to recent taxa, and I've been a big proponent of synonymizing species (Alioramus altai, Didactylornis jii, etc.).  But when it comes to genera, you first have to be sure they form a monophyletic group.  I'd say most of Paul's new combinations fail at this.  Even if they do form a clade, you run the risk of having an earlier-named genus ending up in it.  Richardoestesia as a microraptorine was an example above, and Suchosaurus is another if you want to sink Cristatusaurus into Baryonyx.  This makes our current monospecific dinosaur genus trend more stable.  And even if you have a perfectly stable clade of species, you have to ask yourself what the point of synonymization is.  Paul's Carcharodontosaurus is already Carcharodontosaurinae, his Sinornithosaurus is already Microraptoria.  So it doesn't increase the ease of communication, and it misleadingly implies a similarity greater than that between other sister genera.  Are Microraptor and Sinornithosaurus more similar to each other than Achillobator is to Utahraptor, for instance?  Who can say?  All you end up doing is forming instability compared to past publications and sinking a name someone else published.

Paul, 2010. The Princeton Field Guide to Dinosaurs. Princeton University Press. 320 pp.