Whatever Happened to Euskelosaurus?

In principle, any dinosaur name considered dubious or a synonym can be brought back into use unless it is an objective junior synonym (based on the same specimen as a previous name) or suppressed by the International Commission on Zoological Nomenclature. In practice, some names are much more likely than others to rise out of the mire. For example, when was the last time you thought about Polyonax? (Is this the first time you've ever had occasion to think about Polyonax?) Then there are historical names based on questionable material that once were widely used but have now fallen completely out of favor. Think Monoclonius, Palaeoscincus, or Trachodon, long-time favorites that have staggered into obsolescence and cheap dinosaur toys.

Prosauropod rehab

I once wrote that "When you get interested in something, what's already there when you arrive will seem like it has always been there, and everything that comes along later will always seem a little new to you, even if it has been decades." Taxonomic fashion is one of those things. When I got into dinosaurs in the mid-1980s, prosauropods had shrunk to something like a half-dozen well-regarded genera plus some dross and a few names that were too new for someone to have gotten around to sinking. One of the survivors was always Ammosaurus, which somehow parlayed a few pelvic differences from Anchisaurus into family-level separation (Ammosaurus the plateosaurid, Anchisaurus the anchisaurid). Meanwhile, all of Africa's prosauropod diversity was being crammed, with varying degrees of success, into Euskelosaurus, Massospondylus, these things called "roccosaurids", and Aristosaurus. (Aristosaurus? You had to be there.) Since I had no personal memory of the work that had gone on in years before, and at five years old wasn't really pouring through the primary literature, it seemed like things had always been that way. This is not the case, though. The vast majority of this synonymization had taken place in the previous 10–15 years and was the work of just a few authors. What had come before that?

Well, obviously, what had come before that were the decades of describing and naming scads of prosauropods; otherwise, there wouldn't have been anything to synonymize. It seems like people loved to name them and then forget about them. The history of their research is littered with misconceptions, some of which seem blindingly obvious in hindsight (big predatory prosauropods? You sure you're not just looking at shed crowns from carnivorous animals?). It's a matter of lack of respect and interest: If dinosaurs were a sideshow to the evolution of important things (i.e., mammals), prosauropods were a sideshow to the sideshow, a couple of card tables featuring a guy with a big unibrow and someone doing "pull my finger" jokes.

Skip ahead a few decades. Some of my readers will have never heard of Aristosaurus (well, okay, nobody living has heard of Aristosaurus except me and maybe 10 other people who have been sworn to secrecy, and now I've lost my membership in the group for a cheap gag) and know only of Euskelosaurus as this thing that nobody talks about except to badmouth in the "Background" section of papers. You wouldn't remember the heady days of the '00s, when that genus collapsed under the weight of its synonyms. Instead, it will have always been a nomen dubium to you, and if it somehow returned (stranger things have happened), it wouldn't be a resumption of your interrupted Euskelosaurus service but something new. (Why, yes, I do have a sense I'm getting older. The only thing that saved me from being older than all MLB players this year was four games and 3 2/3rds innings from Rich Hill.)

Anyway, I've long had a soft spot for prosauropods. They're never been supremely popular and had the poor marketing insight to go extinct before they could be menaced by particularly charismatic theropods, but they have a subtle charm, like small bipedal ornithischians. Over the past year or so, there's been a series of papers providing redescriptions and historical analyses of some of the most venerable genera and species of prosauropods, including comments on some of those 1970s–1980s synonymizations. They include Barrett and Chapelle (2024) on Massospondylus, Barrett and Choiniere (2024) on Melanorosaurus, and Regalado Fernández et al. (2023) and Schaeffer (2024) on Plateosaurus.

Massospondylus

Another instance of perception not matching up with chronology: one of the things that comes out of Barrett and Chapelle (2024) was that before the 1970s, Massospondylus was kind of a dog of a taxon. It achieved its present exalted status mostly by getting there first and being less of a dog than the other choices. If Richard Owen had decided to lead with Leptospondylus or Pachyspondylus in 1854 instead of Massospondylus, it wouldn't have had that first point in its favor, and then where would we be? When a name was needed to absorb the mid-sized prosauropods of the Early Jurassic of southern Africa, M. carinatus was there waiting. Some of the new material was good enough to support a name, and our conception of Massospondylus carinatus drifted from the varied bones named by Owen to that material, which eventually supplied a neotype (a type or name-bearing specimen selected after a species was named, to replace a type that was lost or not specified). The original material was no longer available to query, having been destroyed in World War II, but casts and line drawings of some of it remain, and Barrett and Chapelle (2024) provided redescriptions of as much as they could. In the end, it's just as well that the species is now represented by a neotype, because that original material is not diagnostic beyond the level of Massospondylidae at best. The drift of the conception of Massospondylus, plus the uncritical assignment of a lot of material since the 1970s, leaves the possibility of surprises once "Massospondylus" specimens are thoroughly examined. This also applies to Melanorosaurus and Plateosaurus. (Granted, I'm not convinced that every new genus and species will stand the test of time, but that's for another time, when the pendulum eventually swings back to consolidation.)

Massospondylus as a flowchart. Figure 4 of Barrett and Chapelle (2024). CC BY 4.0.

Melanorosaurus

Proposed synonymizations don't always pan out, of course (one reason not to go overboard committing to the conclusions of New Paper Of The Week). The proposed sinking of Lufengosaurus and Yunnanosaurus into Massospondylus at the height of prosauropod consolidation did not attract much support, and the proposed sinking of Melanorosaurus into Euskelosaurus fared only marginally better. Although it escaped that fate, Melanorosaurus has never seemed to get much respect. It's almost a reflex for authors to mention its questionable type material, as if apologizing for trotting it out. Barrett and Choiniere (2024) went back to the original syntype series (a group of specimens used to bear a name) to see what could be salvaged.

M. readi, like many a species, was based on your standard pile o' bones of unclear association. Barrett and Choiniere (2024) regarded a subset, including several partial or complete vertebrae, an ulna, a radius, an ilium, a partial pubis, two tibiae, a fibula, and parts of four metatarsals, as representing one skeleton that they designated a lectotype (a name-bearing specimen selected from a syntype series). Furthermore, they were able to find several diagnostic features. It's not the prettiest type, but it *is* useable. The authors compared the lectotype to several other specimens frequently used as Melanorosaurus exemplars in the literature. Unfortunately, this part doesn't go quite so well for M. readi; NMQR 3314, one of the more important specimens (including a skull), is excluded on both anatomic and stratigraphic grounds. Another specimen, NMQR 1551, is considered consistent with M. readi but an assignment is not confirmed. (I personally hope it is, because I don't want an excuse for someone to try to bring back "Roccosaurus" as a name.) The upshot is M. readi is valid but not particularly helpful at the moment, pending reclamation of more complete material.

Plateosaurus

If Massospondylus is what you get when specimen assignment goes to the last species standing, Plateosaurus is what happens when people care too much. For sheer mind-numbing taxonomic complexity in the world of dinosaurs, it's hard to beat the adventures of Upper Triassic European prosauropods. An extensive historical review can be found in Regalado Fernández et al. (2023), along with detailed reports on many of the significant specimens involved in the story. One of the things I find useful is rather than synonymize the many lesser lights with a species of Plateosaurus, the authors treat them as dubious species. Why is this useful? Keeping them separate helps to keep the concept of the host species from drifting. It's not as if there's some prize for having the tidiest faunal list, after all; specimens should only be assigned to the level of confidence.

Stratigraphic distribution of German Plateosaurus and friends. I could quote every figure in this article, but then I might as well reproduce the whole thing. Figure 2 in Regalado Fernández et al. (2023) CC BY 4.0.

Coming at Plateosaurus from a different angle, Schaeffer (2024) redescribed the holotype (singular name-bearing specimen) of P. trossingensis, which became the type species of Plateosaurus a few years back (another long story). This gets right at the heart of the taxonomic drift: you have to establish your basis of comparison before you can compare anything to it. Continuing with this, Schaeffer ran a phylogenetic analysis of sauropodomorphs with Plateosaurus trossingensis based just on the holotype. Fortunately, the species behaved predictably in terms of general location and neighbors.

References

Barrett, P. M., and K. E. J. Chapelle. 2024. A brief history of Massospondylus: its discovery, historical taxonomy and redescription of the original syntype series. Palaeontologia africana 58: 97–131.

Barrett, P. M., and J. N. Choiniere. 2024. Melanorosaurus readi Haughton, 1924 (Dinosauria, Sauropodomorpha) from the Late Triassic of South Africa: osteology and designation of a lectotype. Journal of Vertebrate Paleontology 44(1): e2337802. doi: https://doi.org/10.1080/02724634.2024.2337802

Regalado Fernández, O. R., H. Stöhr, B. Kästle, and I. Werneburg. 2023. Diversity and taxonomy of the Late Triassic sauropodomorphs (Saurischia, Sauropodomorpha) stored in the Palaeontological Collection of Tübingen, Germany, historically referred to Plateosaurus. European Journal of Taxonomy 913(1): 1–88. doi: https://doi.org/10.5852/ejt.2023.913.2375

Schaeffer, J. 2024. Osteological redescription of the holotype of Plateosaurus trossingensis (Dinosauria: Sauropodomorpha) from the Upper Triassic of SW Germany and its phylogenetic implications. Journal of Systematic Palaeontology 22(1): 2335387. doi: https://doi.org/10.1080/14772019.2024.2335387

Roundup on Thunderstorm Ridge: keeping up with Petrified Forest NP

Last week's publication of the stem-caecilian Funcusvermis reminded me that I really ought to show off Petrified Forest National Park (PEFO) more often. Since the last spotlight, on the "biloph" trilophosaur Trilophosaurus phasmalophus in spring 2020, four more species of Triassic vertebrates have been described from fossils found in the park, to go along with a bushel of reports on anatomy, identifications of rare forms, vertebrate trace fossils, stratigraphy and geochronology, and other topics, to say nothing of conference abstracts and papers that mention PEFO fossils in wider contexts. (SVP conferences are usually good for a handful of PEFO topics.) Because we're talking the Late Triassic, the taxonomic diversity is wide. There's a little bit of almost everything.*

*Interestingly enough, that includes Paleozoic marine invertebrates: reworked fossiliferous cobbles of the Permian Kaibab Formation have been found in the park's Chinle Formation outcrops, particularly the Sonsela Member. There are also limited Neogene deposits with Hemphillian vertebrates.

Classically, as with most places that have produced vertebrate fossils for more than a century, big singular fossils were long the focus (e.g., skulls of phytosaurs). Although there is still interest in those kinds of fossils, increasingly study has focused on bonebeds and microvertebrates, with much more care given to stratigraphic placement. It turns out that PEFO holds a whole weird and wonderful landscape of everything that gave it a go in the Late Triassic, before a couple of groups of archosaurs took over land management for the rest of the Mesozoic. (And if you don't like vertebrates, the plants are just about as wild in their own way, and there are freshwater and terrestrial invertebrates as well.) It practically begs for a book like those on Florissant, Fossil Lake, the Morrison, and the White River Badlands.

Here's a strat column to help keep the geologic units straight, borrowed from the park website.

With all of that in mind, here are a few quick hits from the past few years of research at PEFO:

Mbiresaurus and Tuebingosaurus

Things have picked up since late June. In the past couple of weeks, two new "prosauropods" have been published that cover both ends of the prosauropod spectrum. One, Mbiresaurus raathi, represents the early part of sauropodomorph evolution, while the other, Tuebingosaurus maierfritzorum, is close to the transition from clear-cut "prosauropods" to clear-cut "sauropods". Long-time readers will be familiar with the author's inexplicable fondness for prosauropods, so let's invite them in.

Issi saaneq and Brighstoneus simmondsi

A couple of new non-theropod names have come down the pipe since the beginning of November, and I thought I'd cover them together. Let's tackle the earlier of the two (both geochronologically and publication-wise) first.

Fossil Crocodylomorphs of the National Park Service

For this year's National Park Service fossil group inventory, I've chosen crocodylomorphs, which for convenience I'm going to refer to as "crocs". Crocodylomorpha encompasses the true crocodilians and their closest extinct relatives, which over the years has been defined to exclude major groups of allied Triassic archosaurs (rauisuchids, poposaurs, prestosuchids, etc.). (Technically speaking, traditional Crocodilia is closer to the clade Crocodyliformes, but I have a soft spot for "sphenosuchians" and it's my blog.) Non-crocodilian crocodylomorphs were big players throughout the Mesozoic but came to peter out in the Cenozoic, with holdouts into the Miocene (Sebecosuchia). Some of these non-crocodilian crocodylomorphs looked basically like modern crocodilians and presumably filled very similar niches, but by definition weren't crocodilians*. Others were quite a bit different; for example, small, long-legged terrestrial crocs had a wide distribution from the Late Triassic through the Jurassic, and there were multiple groups of marine forms.

*I have certain misgivings about crown groups, particularly that future stability of usage relies on groups not going extinct (or there would have to be backdating, like radiocarbon dates are pegged to 1950), although at this point I might as well complain about the decline in use of Etruscan.

The NPS record of croc fossils turns out to be sparser than I expected: there are 17 park units with solid records (albeit two of these being reworked or washed up, making them hard to place stratigraphically), and another couple potential records. Here is the requisite map and its accompanying long caption:

Click to embiggen. The sites mentioned in this post are: 1. John Day Fossil Beds National Monument; 2. Bighorn Canyon National Recreation Area; 3. Fossil Butte NM; 4. Dinosaur NM; 5. Colorado NM; 6. Curecanti NRA; 7. Bryce Canyon National Park; 8. Glen Canyon NRA; 9. Petrified Forest NP; 10. Chaco Culture National Historical Park; 11. Theodore Roosevelt NP; 12. Badlands NP; 13. Agate Fossil Beds NM; 14. Niobrara National Scenic River; 15. Big Bend NP; 16. Waco Mammoth NM; 17. Gateway NRA; 18. Fort Washington Park; 19. Cumberland Island National Seashore.

These 19 units are primarily in the Colorado Plateau and northern Great Plain, and these two areas correlate in large part to temporal distribution: the Colorado Plateau records are mostly Jurassic and Cretaceous, and the Great Plains records are Cenozoic. A couple of compact diagrams will show this:

Part 1 shows the Mesozoic, Paleocene, and Eocene records, which make up the bulk of the reports.

Part 2 shows the few younger records; the two that can't be pinned down are added to keep them company.

You can probably guess a lot of the story if you have some familiarity with the stratigraphy of western North America. As so many other groups of terrestrial vertebrates, the place to go in the NPS for Triassic crocs is Petrified Forest National Park, where "sphenosuchians" have been found in the famous Chinle Formation. (Ignore the phytosaurs; they only look like crocs.) After that, possible early croc tracks have been found in the Navajo Sandstone of Glen Canyon National Recreation Area; with all of the Early Jurassic tracks in the Colorado Plateau parks, there are likely other track records. We have no body fossil records in the parks' rocks yet, though (the facies aren't as forgiving as elsewhere). Four parks have records for the Late Jurassic: Bighorn Canyon National Recreation Area has swim traces attributed to crocs in the Sundance Formation, and no points for guessing what's represented at the other three. (It's the Morrison Formation.) NPS Morrison crocs are best known from Dinosaur National Monument, which primarily has the well-represented Amphicotylus (formerly Goniopholis), but also produced the type specimen of the diminutive Hoplosuchus kayi.

The Cretaceous is more sparsely represented, with nothing confirmed from the Early Cretaceous. Ot the Late Cretaceous records, neither Bryce Canyon NP (Straight Cliffs and Wahweap microvertebrate remains) nor Chaco Culture National Historical Park (Menefee isolated material) have much to speak of. Big Bend NP, on the other hand, has the most impressive croc record in the NPS. Granted, that's an easy call when you can point to the type specimen of the suitably Texas-sized Phobosuchus riograndensis (now a species of Deinosuchus), but the park also has by far the longest record of crocs in the NPS. Five formations are represented: the Aguja Formation and Javelina Formation, both Late Cretaceous; the overlying Black Peaks Formation, which straddles the Cretaceous and Paleocene; the Early Eocene Hannold Hill Formation; and the Middle Eocene Canoe Formation. Recently a second croc species has been named from Big Bend NP fossils: Bottosaurus fustidens, from the Paleocene part of the Black Peaks Formation. Other taxa are present, but have not been studied in as much detail.

Looking elsewhere in the Paleocene, there is a single record of a partial bone from the Aquia Formation at Fort Washington Park, and Theodore Roosevelt NP has crocs in the Bullion Creek and Sentinel Butte Formations, comparable to nearby Wannagan Creek (only not quite so concentrated). The Eocene is fairly good for NPS crocs. Apart from Big Bend, we have croc fossils in: the Wasatch Formation at Fossil Butte NM; the Clarno Formation at John Day Fossil Beds NM; and the Chadron Formation at Badlands NP. The type specimen of Caimanoidea visheri (now considered a synonym of Alligator prenasalis) may have come from Badlands NP.

And that's almost the end. Crocs disappeared from the drying interior of North America during the middle Cenozoic. For the Miocene, we have one *very* sketchy potential record from the early Miocene Anderson Ranch Formation of Agate Fossil Beds NM and better records of crocs from the middle Miocene Valentine Formation of Niobrara National Scenic River (including the type specimen of Nordenosaurus magnus, originally described as a big lizard but now identified as a small crocodilian). At Waco Mammoth NM there is late Pleistocene alligator material, but we are otherwise lacking Pleistocene crocodilian records. Two units have material of uncertain provenance: a scute found in dredge material at Cumberland Island National Seashore and various croc fossils that have washed up at Gateway NRA.

Rugalichnus

Here we have a textbook example of Rugalichnus. This particular example was observed in the Wupatki Member of the Moenkopi Formation at Wupatki National Monument.

Click to embiggen; it's the labyrinthine surficial feature.

That's all well and good, but what is Rugalichnus?

First off, it's not Rivularites. (For some reason Rivularites has attached itself to my mental conception of features like this, but that's not important.)

Rugalichnus is a microbially induced sedimentary structure (MISS), which is a reasonably self-explanatory name: it's a sedimentary structure that resulted from the influence of a microbial mat. This means it has a metaphorical foot both in "trace fossil" and in "sedimentary structure". Another way of thinking about it is as a cousin to stromatolites. It isn't stacked and occurs in clastic rocks rather than carbonates, but it shares the same principle of a sticky microbial mat influencing the preservation of sedimentary features.

Features similar to Rugalichnus have been reported for more than a century. Charles Doolittle Walcott (1914) named the ur-example Kinneyia simulans from Precambrian rocks in Montana, thinking it was an algal fossil. It became a topic of controversy for decades, culminating in the "genus" and "species" turning out to be an inorganic dud (Davies et al. 2016; Stimson et al. 2017). If you look at the figure below, you might think you're seeing a bedding plane, like the object in my photo. However, it isn't; it's actually an artifact of weathering. Stimson et al. (2017) opted to coin the new name Rugalichnus matthewi for the MISS features that people had been calling "Kinneyia" (and yes, the naming of MISS is a can of worms unto itself).

Kinneyia simulans. Plate II, Figure 3 in Walcott (1914).

There are many varieties of MISS, reflecting the many ways a microbial mat can interact with its sedimentary substrate. Rugalichnus is known from storm wave deposits, indicating it formed after storms (Herminghaus et al. 2016). When Rugalichnus (as Kinneyia) was first interpreted as a MISS, it was thought to have formed beneath an active microbial mat (see for example Porada et al. 2008 and Thomas et al. 2013). Mariotti et al. (2014) proposed instead that the ridges and troughs resulted from rolling mat fragments and loose mat ends. Herminghaus et al. (2016) conceded that there were circumstances that could produce the wrinkle structures using the Mariotti et al. model, but preferred their own hydrodynamic instability model from Thomas et al. (2013). Given that there are abiotic features that look very similar (Davies et al. 2016), there's certainly no reason to assume that all Rugalichnus-like wrinkles formed the same way. Me, I don't have a horse in this race; I just find the structure interesting.

References

Davies, N. S., A. G. Liu, M. R. Gibling, and R. F. Miller. 2016. Resolving MISS conceptions and misconceptions: A geological approach to sedimentary surface textures by microbial and abiotic processes. Earth-Science Reviews 150:210–246.

Herminghaus, S., K. R. Thomas, S. Aliaskarisohi, H. Porada, and L. Goehring. 2016. Kinneyia: a flow-induced anisotropic fossil pattern from ancient microbial mats. Frontiers in Materials 3. doi:https://doi.org/10.3389/fmats.2016.00030.

Mariotti, G., S. B. Pruss, J. T. Perron, and T. Bosak. 2014. Microbial shaping of sedimentary wrinkle structures. Nature Geoscience 7:736–740.

Porada, H., J. Ghergut, and E. H. Bouougri. 2008. Kinneyia-type wrinkle structures—critical review and model of formation. PALAIOS 23:65–77.

Stimson, M. R., R. F. Miller, R. A. MacRae, and S. J. Hinds. 2017. An ichnotaxonomic approach to microbially induced sedimentary structures from the Saint John Group of New Brunswick: why comparison to Kinneyia Walcott 1914 must be abandoned. Ichnos 24(4):291–316. doi:10.1080/10420940.2017.1294590.

Thomas, K., S. Herminghaus, H. Porada, and L. Goehring. 2013. Formation of Kinneyia via shear-induced instabilities in microbial mats. Philosophical Transactions of the Royal Society A 371(2004):20120362.

Walcott, C. D. 1914. Cambrian geology and paleontology III, no. 2. Pre-Precambrian Algonkian algal flora. Smithsonian Miscellaneous Collections 64:77–156.

Kholumolumo ellenbergerorum and "Thotobolosaurus"

With the twin unofficial missions of Equatorial Minnesota to cover Elliot Formation "prosauropods" and find out what happened to unpublished dinosaurs of the past, it was imperative that Kholumolumo ellenbergerorum receive a post. Before we get into the actual science of K. ellenbergerorum, though, let's step back through the mists of time. First we'll stop off in the far-off world of 1985.

You've just gotten the Normanpedia (Norman 1985) and are busy committing it to memory. On page 97, in the "Dubious" section of the list of prosauropod genera, is Thotobolosaurus. It doesn't ring a bell, so you pull out "A Field Guide to Dinosaurs" (Lambert 1983). It shows up on page 104, as a "roccosaurid" with Riojasaurus and something called Roccosaurus, which wasn't in the Normanpedia. "Roccosaurids" apparently have "unusually sharp fangs and an especially strong joint linking hips with the backbone", but might actually be melanorosaurids. This last bit just ends up being confusing because (stepping outside of 1985 for a moment) "A Field Guide to Dinosaurs" was caught up in the contemporary fad for lumping all kinds of prosauropods, so Melanorosaurus is sunk into Euskelosaurus. Anyway, Thotobolosaurus is "a big prosauropod that lived in early Late Triassic Lesotho, southern Africa." Okay, sure. Does "The New Dinosaur Dictionary" (Glut 1982) have anything more, and can it explain what exactly is a Roccosaurus?

Trilophosaurus and Ophthalmothule

For this post, we highlight two extinct reptiles that have been the subjects of publications in the past few days. Other than that, they don't have much in common. One is a short-necked terrestrial herbivore less than three meters long that lived during the Late Triassic in what is now Arizona, the other is a long-necked marine carnivore between five and six meters long that lived at about the Jurassic–Cretaceous boundary at what is now an Arctic island (not quite so arctic at the time).

What I Did While I Was Out

I made no post last week because I was traveling for work. Generally, I don't have the time to work on a post while traveling, and this was no exception. In order to get the most bang for our buck on work travel, we try to schedule as many projects as possible, and this trip was no exception. I had four separate projects scheduled over eleven days.

Tiny frogs of the Chinle

I am embarrassed to realize that after five full years of doing this, with a header mentioning "National Park Service paleontology" and "the Mesozoic", I had not done anything with the Chinle Formation or Petrified Forest National Park. This week offers a fine opportunity to correct this oversight, with the publication of the first frog fossils from the Chinle Formation.

The Chinle Formation is a terrestrial unit, with its colorful rocks deposited in various floodplain, river, lake, and other settings during the Late Triassic. The mosaic of settings led to a variety of rock types, from conglomerates to mudstones, with a healthy supply of volcanic ash from eruption centers to the west. Because we're talking about the Late Triassic, before the end-Triassic extinctions gave dinosaurs the opportunity to fill most of the empty terrestrial niches, the Chinle is full of fossils from all sorts of unusual and obscure animals. (And also petrified wood and freshwater mussels. Lots and lots of petrified wood and mussels!) It and its correlatives are usually good for at least one surprise every few years. Lurking among all of the phytosaurs and aetosaurs and so forth were representatives of lineages that would blossom later. Some of them we know from bones (dinosaurs, early croc relatives, pterosaurs, etc.), others can be suspected based on time and place but haven't yet been found. With this week's announcement, frogs move from the potential to the confirmed (Stocker et al. 2019).

The Chinle Formation is divisible into a number of members, depending on where you are. In ascending order, the units in the Petrified Forest NP area are the Mesa Redondo, Blue Mesa, Sonsela, Petrified Forest, and Owl Rock members. For the frogs, we're dealing with the Blue Mesa and Sonsela members, representing approximately 223 to 213 million years ago. (Following the stratigraphy of the Chinle Formation requires a certain amount of effort and dedication. Not only are different members found in different places, but usage has varied over time.) The three frog-producing localities are spread from the park to the famous Placerias Quarry near St. Johns, Arizona (Stocker et al. 2019).

An outcrop of the Sonsela Member, from the park website (NPS/Andrew V. Kearns).

Frogs, being small and delicate, are not exactly heavily represented in the fossil record. The frog lineage, Salientia, is known back to the Early Triassic, but if you were doing a Compact Thescelosaurus-like project on Mesozoic salientians you'd be finished pretty quickly. Salientia includes early stem-frogs plus the crown group Anura, which is the group consisting of all living frogs, their most recent common ancestor, and everything else that falls within that group. There are to date two Early Triassic stem-frogs, the well-known Triadobatrachus of Madagascar and the somewhat less famous Czatkobatrachus of Poland, with the next named salientian being a respectable frog's jump all the way into the Early Jurassic of Arizona, Prosalirus of the Kayenta Formation. The Kayenta has historically proven stubborn about this whole "absolute dating" thing, but an age in the first half of the Early Jurassic is the consensus, so that's a gap of something approaching 60 million years (approximately 250 to 190 million years). The unnamed Chinle salientian practically splits the difference (Stocker et al. 2019).

The Chinle salientian is known from five tiny fossils. Fortunately, four of them are ilia, which are very distinctive bones in frogs: there is a cup-like socket for the femur and a long thin blade directed anteriorly, lengthened by cartilage, part of the mechanism that gives frogs their spring. In the case of the Chinle form, they are also well into the microvert range: approximately 6.2 mm long, or about a quarter of an inch, the kind of stuff you find when you run sediment through screens as opposed to spotting while hiking around. The whole animal would have been less than about 25 mm long, or about an inch, in the realm of modern miniature frogs (Stocker et al. 2019). So, if you were thinking that ur-frogs might have been prehistoric giants, well... not so much in the Chinle. There *were* enormous amphibians in the Chinle, specifically the metoposaur Koskinonodon and its equally enormous taxonomic history, but it isn't closely related to anything living and it didn't look much like a frog.

The form of the hip joint and shaft suggest that the Chinle form was well on the way to modern frog jumping mechanics (Stocker et al. 2019). It may have been a true anuran, but only time and more fossils will tell. The presence of a stem-frog or true anuran in the Chinle also has some implications for frog distribution and paleoecology. The two Early Triassic stem-frogs lived outside of the tropics, whereas the Chinle form was more or less at the equator in western Pangea, showing that the frog lineage had spread across the supercontinent within the Triassic. Also of interest is the persistence of salientians from the Chinle into the Kayenta; over the length of the Chinle, the local climate became more and more arid (Stocker et al. 2019).

A Triassic frog clings to the snout of a phytosaur, used with permission by Andrey Atuchin (supplied by Adam Marsh/PEFO).

References

Stocker, M. R., S. J. Nesbitt, B. T. Kligman, D. J. Paluh, A. D. Marsh, D. C. Blackburn, and W. G. Parker. 2019. The earliest equatorial record of frogs from the Late Triassic of Arizona. Biology Letters 15:20180922. doi:10.1098/rsbl.2018.0922.

Photos from Albuquerque

Back in October I attended the annual meeting of the Society of Vertebrate Paleontology, held at Albuquerque, New Mexico. The meeting was co-hosted by the New Mexico Museum of Natural History and Science. Part of the program of the annual meeting is a reception at a co-hosting institution, where the attendees can mingle among the institution's exhibits. I was fortunate enough to be in the company of JP and Sarah Hodnett, who are very familiar with the museum. (In fact, we'd actually spent most of that day there already, working the National Fossil Day event.) Thanks also to JP and Sarah for helping me get around Albuquerque!

We also had the able assistance of ceratopsids for National Fossil Day.

Bagualosaurus agudoensis: another sauropodomorph ahead of its time

Our subject today is the newly described Bagualosaurus agudoensis from lower Upper Triassic rocks of southern Brazil. If I'd known back in March 2016 that I'd have the opportunity in a couple of years to write about another "prosauropod" that was "ahead of its time", and that it would include a partial skull justifying a terrible "head" pun, maybe I'd have come up with another title then. Oh, well; spilled milk and all that.

Near-dinosaurs

The Triassic was an experimental time for large tetrapods. The Permian–Triassic extinction event had eviscerated the prevailing communities of diverse therapsids (relatives of mammals), various early reptiles, and large temnospondyl amphibians (for more on them, Tetrapod Zoology has made them a cottage industry since 2007). Into that vacuum the survivors seem to have gone with the strategy of "throw everything at the wall and see what sticks". Some lineages stuck quite well. Early turtles and sphenodonts (tuatara) showed up during the Triassic, as well as forerunners of crocodilians and mammals. Lizards are probably in there too somewhere. Frogs may predate the Triassic, but the first good fossils are Triassic. Famously, the dinosaur line, which eventually produced birds, appeared in the Triassic as well. Long-lived but now extinct groups that got their start in the Triassic include the ichthyosaurs, plesiosaurs, pterosaurs, and our friends the choristoderes. There was also a veritable heap of briefly successful groups that for whatever reason went extinct by the close of the Triassic. Among them: aetosaurs (armored herbivores that look vaguely like ankylosaurs), drepanosaurs, nothosaurs and other plesiosaur cousins, placodonts, phytosaurs (crocodiles before there were crocodiles), rauisuchians (carnivores with theropod-like skulls), rhynchosaurs (big beaked herbivores), tanystropheids (necks plus neck delivery systems), thallatosaurs (lanky marine reptiles), and all kinds of other strange one-hit wonders.

Until the 1960s, dinosaurs seemed to have more or less just appeared in the Late Triassic, with theropods and prosauropods (in the sense of "all them sauropodomorphs what ain't sauropods")  recognized as present. What came before those theropods and prosauropods wasn't known, although there were a few guesses and extrapolations. One popular option of the time was that, technically speaking, there weren't really any basal dinosaurs because "dinosaurs" themselves were an artificial group "united" by some coincidental bits of anatomy related to being large land-living animals. This view is practically extinct, although I cannot say completely extinct. One of the great truths of humanity is that there is someone who will believe any proposition. One of the great truths of the Internet is that now you can find that person (or oftentimes, they will find you, if you are holding an opposing position). Other researchers drafted in various poorly known Triassic reptiles. The most enduring may have been Teratosaurus, which people who got into dinosaurs before the late 1980s will probably remember as a sort of megalosaur-like thing stalking the wilds of Late Triassic Europe. It was actually based on jaw material from a rauisuchian, with prosauropod skeletal remains misattributed to it. The misidentification of Teratosaurus, though illustrative, serves mostly as a lesson in the honesty of bonebeds. More recent work with Reveultosaurus, Shuvosaurus, and others shows that it can be darn hard to separate true early dinosaurs from the various wacky archosaurs of the Triassic if you've only got a few remains. The terrestrial Triassic still has more fools to make.

Of all the various bits and pieces put forward as early dinosaurs in the days before the 1960s, the only one that actually is both vaguely useful and does not easily slot into any of the known clades of true dinosaurs is Saltopus elginensis, described in 1910 by von Huene. It got to be in all the best dinosaur books as an archetype, overcoming the significant handicap of being a terrible specimen, which just goes to show that sometimes all you have to do to succeed is show up. The first useful basal dinosaurs to be described, Herrerasaurus and Ischisaurus, were described in 1963, followed by Staurikosaurus in 1970. They were followed by Lagosuchus, Lagerpeton, and Lewisuchus in 1971 and 1972, which were underappreciated at the time but eventually were shown to be dinosaurian cousins once we got that whole "unnatural Dinosauria" thing worked out of the collective scientific system. At the present, there are around 20 species of near-dinosaurs, from Dinosauromorpha to Dinosauria. This doesn't compete with, say, Titanosauria, but it's not bad for about 45 years of serious work. There's a little wiggle room built in depending on how charitable you feel toward Pseudolagosuchus, how you handle "Thecodontosaurus" alophos and borderline cases (e.g. Agnosphitys, Alwalkeria, Teyuwasu), and the occasional analysis that pulls Herrerasauridae or Eoraptor out of Saurischia. Some of them can be grouped as lagerpetids, diminutive bipeds, or as silesaurids, larger animals which could reach roughly the size and shape of Fred Flintstone's pet Dino. At least one of these, the namesake Silesaurus, was equipped with a little can-opener of bone at the tip of its lower jaw, perfect for being confused with the ornithischian predentary. A few others either don't slot comfortably into either group, or are poorly known (and I tend to the conservative when it comes to where I slot, which in this case mostly affects Lewisuchus/Pseudolagosuchus). The chart below lays them out with age and continent denoted. One thing to note is the abundance of species for South America and Africa, which not only suggests a Gondwanan origin but also provides a partial explanation for why these animals have only come to light in the past few decades: there haven't been a lot of paleontologists in those areas until recently.

Click for further enlightenment

Another important thing to keep in mind is to avoid the trap of turning extinction and evolution into a morality play. "Near-dinosaurs" were not merely a sideline, or waiting hopefully to eventually evolve into dinosaurs, or a bunch of saps that got pushed out of the way by their cousins. They were their own creatures, diversifing into several lineages and living alongside their more famous cousins for twenty or so million years, at least. We currently have named examples from four continental landmasses, and it would hardly be surprising to add a few more landmasses, five to ten million years, or additional lineages.

Dromomerom romeri, by Nobu Tamura (from https://commons.wikimedia.org/wiki/File:Dromomeron_BW.jpg). For whatever reason I've always found this restoration charming.

Meroktenos thabanensis: not Early Jurassic, just ahead of its time

It's been about a month since Meroktenos thabanensis made its (re)appearance, but I'm just now getting around to it. A bunch of other topics muscled ahead of it, which is fitting, I suppose. Prosauropods (by which the reader should understand "sauropodomorphs what ain't sauropods"; if you should want me to write "basal sauropodomorph" over and over again instead, I accept large cash bribes) are among those dinosaurs which never seem to get much respect. They have been fertile subjects for arm-waving, though, one result of which being that a survey of popular-audience dinosaur books will turn up wildly differing depictions over the years. Bipedal or quadrupedal? Herbivorous, omnivorous, predatory, or scavenging? A stem leading into sauropods, a bunch of rungs, or a distinct empire of prosauropods? Gleefully extreme lumping or splitting? What about herrerasaurids? What about Teratosaurus? (if that last one means anything to you, congratulations, you're in your mid-30s or older, or you study rauisuchids.) Prosauropods have also not been helped by a certain vague uniformity of body shape, nor have they been done any favors by geography. North America has an unimpressive record for these dinosaurs, so far including Anchisaurus (let's not kid ourselves about Ammosaurus being distinct), Sarahsaurus, Seitaad, the unnamed Nova Scotia form, and odds and ends. For a group that did not have horns, frills, spikes, bony armor, crests, ridge-backs, spectacular size, or big pointy teeth, and had the poor sense to go extinct before the appearance of tyrannosaurs or dromies to prey upon them, not being well-represented in North America has not helped their exposure. But I digress.