Nanosaurus and Small-Bodied Morrison "Ornithopods" review

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Free pdf:

Paul M. Barrett and Susannah C.R. Maidment (2025)
A Review of Nanosaurus agilis Marsh and Other Small-Bodied Morrison Formation “Ornithopods"
Bulletin of the Peabody Museum of Natural History 66(1): 25-50
doi: https://doi.org/10.3374/014.066.0102
https://bioone.org/journals/bulletin-of-the-peabody-museum-of-natural-history/volume-66/issue-1/014.066.0102/A-Review-of-Nanosaurus-agilis-Marsh-and-Other-Small-Bodied/10.3374/014.066.0102.short


A variety of small ornithischian dinosaur specimens collected from the Upper Jurassic Morrison Formation of Colorado and Wyoming were described by O.C. Marsh in the late 19th century. These include Nanosaurus agilis, N. rex, Laosaurus celer, L. gracilis, and L. consors. Another taxon from Wyoming, Drinker nisti, was added to this list more recently. The taxonomy of these species has undergone numerous revisions, with differing opinions regarding their validity and potential synonymies and in terms of the specimens referred to each of them. Their systematics have also been debated, with some taxa regarded as ornithopods and others as earlier diverging ornithischians. Here, we provide a critical reappraisal of these species based on first-hand examination of their type specimens. None of the relevant holotypes or syntype series exhibit either autapomorphies or unique character combinations that could be used to support their validity, and we regard all of these taxa as nomina dubia. Most of these specimens can be identified only as indeterminate neornithischians, but some include juvenile iguanodontian remains. “Drinker” possesses some dental and skull features that are known only in pachycephalosaurs, raising the possibility that this material represents an early diverging member of this clade. Other specimens formerly referred to these taxa should be regarded as indeterminate until they have been restudied in detail in order to establish their affinities rigorously and determine the true species richness of the Morrison Formation ornithischian fauna.

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[Tim Williams]

Wow, this study is almost forensic in its detail.  The following genera are all regarded as nomina dubia: _Nanosaurus_, _Laosaurus_, _Othnielia_, _Othnielosaurus_, and _Drinker_.  In most cases this comes as no surprise; but the taxonomic scheme previously proposed by Galton & Carpenter (2018) is completely overturned.  _Othnielosaurus_, the most recently proposed name (Galton, 2007), follows _Othnelia_ to the chopping block.  Charles Marsh would be happy: he hated the given name Othniel.

There is the potential for a new taxa to be described, including based on the headless partial skeleton BYU ESM 163R (originally described by Galton & Jensen, 1973, as "_Nanosaurus (?) rex_").  Note that Boyd (2015) erroneously regarded BYU ESM 163R as the holotype of _Othnielosaurus consors_; this specimen, and another headless partial skeleton UW 24823, were regarded as representing a single taxon in Boyd's comprehensive phylogenetic analysis. If these specimens, as well as NMZ 1000010 ("Barbara", the most complete), pertain to the same genus and species, there is the promise of a very well-known new Morrison neornithischian.

The proposed pachycephalosaurian identification of _Drinker_ is based not only on the striking resemblance of the teeth to _Dracorex_, but also the presence of an ornamented jugal.  As the authors note in a very appropriate way, the totality of _Drinker_ material is in desperate need of further study.  

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[Gregory Paul]

Nanosaurus is from the very tippy top of the Morrison, while Othnielosaurus is from the middle, so they being the same taxon was not viable. 

GSPaul

[Mickey Mortimer]

I'm inherently untrusting of Maidment's claims of nomen dubium, as she is quick to point out prior suggested diagnostic characters are primitive or shared with other taxa, while never saying e.g. "Nanosaurus can't be distinguished from valid taxa X and Y", and much like Nesbitt just relies on published character lists. As an obvious example here, Barrett and Maidment note that Galton (1978) diagnosed Nanosaurus in part by "(G5) fourth trochanter index of femur about 0.43; and (G6) tibia about 115% of femur length." Their reply is-

"The fourth trochanter index (G5) appears to relate to how proximally the fourth trochanter is positioned. The “index” given by Galton (1978) indicates the fourth trochanter is on the proximal part of the femoral shaft, a feature that is plesiomorphic for Ornithischia (Butler et al. 2008; Boyd 2015). Similarly, a tibia that is longer than the femur (G6) is a feature shared by all bipedal ornithischians and is also an ornithischian plesiomorphy (Maidment and Barrett 2012)."

I have zero opinion on whether Nanosaurus is valid or not, but you can't just take "about 0.43" and "about 115%" and change that to <0.5 and >100% respectively to dismiss their value as characters.

Mickey Mortimer

[Andrew Rossi]

Can anyone share the PDF?

To view this discussion visit https://groups.google.com/d/msgid/DinosaurMailingGroup/2290b330-770f-4580-a58a-3ec02845887fn%40googlegroups.com.

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Regards,

A. Rossi 

[Tim Williams]

Mickey Mortimer wrote:

> I'm inherently untrusting of Maidment's claims of nomen dubium, as she is quick to point out prior suggested diagnostic characters are primitive or shared with other taxa, while never saying

> e.g. "Nanosaurus can't be distinguished from valid taxa X and Y", and much like Nesbitt just relies on published character lists.

Barrett & Maidment do set the bar very high.  In general, that's not a bad thing - Marsh's original descriptions were often really bad, and many of his taxa not much more than nuisance value. 

But your comment above brought _Drinker_ to mind.  Notwithstanding that the holotype and referred material for _Drinker_ are insufficiently figured/described, there might be enough wiggle-room to regard it as a provisionally valid genus.  Although Barrett & Maidment found its dental and jugal characters to be indistinguishable from pachycephalosaurs like _Dracorex_, there are astragalar characters that might differentiate _Drinker_ from known pachycephalosaurs.  It's slim pickings; but perhaps enough to uphold the validity of _Drinker_.  This assumes that the holotype (CPS 106) craniodental and postcranial material come from the same individual, as stated by Bakker &c (1990).  

Gregory Paul wrote:

> Nanosaurus is from the very tippy top of the Morrison, while Othnielosaurus is from the middle, so they being the same taxon was not viable.

Yes, Carpenter & Galton (2018) assign _Nanosaurus_ to a small hill called 'Cope's Nipple' ("very high in the Morrison Formation").  I note in passing that the _Drinker_ material also comes from the very top of the Morrison Formation ('Breakfast Bench').  

[Jerry Harris]

Tim Williams wrote: " Yes, Carpenter & Galton (2018) assign _Nanosaurus_ to a small hill called 'Cope's Nipple' ("very high in the Morrison Formation").  I note in passing that the _Drinker_ material also comes from the very top of the Morrison Formation ('Breakfast Bench'). "

This is true, but "top" of Morrison in Colorado and Wyoming may not be time equivalent. The Morrison Formation may have at least some degree of time-transgressive-ness, with deposition "following" the retreating Sundance Sea northward through the Late Jurassic. It could be somewhat younger in the north than in the south, so "top" of the Morrison in Colorado might correlate with more middle-y Morrison in Wyoming. Granted, the distance between Cañon City and Como Bluff isn't huge, so equivalence might not be too far off, but for as vast a formation as the Morrison, universal stratigraphic equivalence isn't a given.

[Justin Tweet]

Hi, everyone;

I'd like to see this, too. The pdf is apparently not free for me.-Justin

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Justin Tweet

Equatorial Minnesota, home of The Compact Thescelosaurus

[Gunnar Bivens]

Hi all, here's the PDF

https://drive.google.com/open?id=1cVHyeTr9f64gjAKEIrZ3HlqTPDNclyZm&usp=drive_fs

To view this discussion visit https://groups.google.com/d/msgid/DinosaurMailingGroup/CADGyndiY0X7T%3D0Oe7QzV_ziNF%2BevGnbk8HZAPLfvtSgOZ9mANQ%40mail.gmail.com.