Monday, March 20, 2023

Brooksella: what are star cobbles?

Back in the far-off year of 2012, when I was helping to compile instances of paleontological type specimens found in National Park Service units, we had to make decisions about various edge cases. One of these was how to handle names for what later turned out to be pseudofossils. We decided to record the information as historically relevant but did not include the "taxa" in any counts. On this blog we've actually covered a couple of them already, "Lingula calumet" and "Paradoxoides barberi" from within or very near Pipestone National Monument. Another is "Brooksella canyonensis", a putative jellyfish from the Proterozoic Nankoweap Formation of Grand Canyon National Park. It was first reported as such in Van Gundy (1937) and then named, not entirely enthusiastically, in Bassler (1941). "B. canyonensis" has fared poorly as a jellyfish, but has had its supporters as an organic feature (e.g., Glaessner 1969; Kauffman and Steidtmann 1981; Kauffman and Fursich 1983; tentatively Ciampaglio et al. 2006). However, I favor an inorganic interpretation. Admittedly, there are several to choose from: gas-escape structures or compaction (Cloud 1968), "sand-volcano"-type fluid escape (Ford and Breed 1977; Ford 1990), and mud rolls (Fedonkin and Runnegar 1992).

"B. canyonensis" was not the first species in the genus Brooksella, though. Brooksella was named by Charles Walcott for "star cobbles" from the Coosa Valley of Alabama (Walcott 1896), now attributed to the middle Cambrian-age Conasauga Formation (Nolan et al. 2023). In fact, he named three taxa for different forms of cobbles: B. alternata, B. confusa, and Laotira cambria (Walcott 1896). Star cobbles got their name because at their best they look like the stereotypical twinkly pointed things you might doodle. Some of them even have five rays, although six is more typical and they are more lobed than pointed, so it's not a perfect match.

Brooksella (A–D, K) and Laotira (E–H, J) as illustrated by Walcott (1898) and reproduced as Figure 1 in Nolan et al. (2023) (which see for full caption). CC BY 4.0.

Walcott interpreted the objects as representing jellyfish, which are probably not the first thing you think of when fossils come to mind, but jellyfish fossils are in fact known elsewhere. In this case, though, the interpretation hasn't proved tremendously popular over time, and numerous alternatives have been proposed. These alternatives, though, generally involve some kind of organic origin, either as a true body fossils or a trace fossil of some sort. It's not hard to see why: they look like something that *ought* to be organic, even if the identity of that something is unclear. (Anyone who has gone out fossil hunting will probably recognize this feeling. Sometimes you're right, sometimes you're wrong.)

Nolan et al. (2023) have published a detailed reassessment of Alabama Brooksella. As part of it, they prepared a lovely supplemental figure of various hypotheses, with thumbnail evaluations (discussed at greater length in the text). (*Warning*: Hold off on clicking the link if you'd rather not get their solution immediately.) Studies of Brooksella from the past couple of decades have interpreted it as a trace fossil (either a feeding burrow or a coprolite) or a glass sponge (hexactinellid). Nolan et al. subjected star cobbles to about as many tests as can legally be done to rocks in their analysis of the various possibilities, and came to several conclusions, including:

  • Brooksella specimens do not have a sponge's anatomy. There aren't spicules, features previously interpreted as ostia (pores) bear a strong resemblance to pitting left behind when lichen are cleaned off, and lobes do not feature opening at their ends for radial canals (which were also not found).
  • The orientation of the specimens when found in situ was with the putative central osculum (excurrent vent) down in the sediment, which is an inconvenient place for an osculum. Furthermore, many examples did not even have an "osculum".
  • The specimens include internal voids and tubes, but these spaces do not correspond to the external form, unlike primary burrows (although this does not preclude the specimens having "captured" parts of burrows that were passing through). Furthermore, the internal features do not include common burrowing structures such as backfill.
  • The specimens have the same composition as silica concretions from the same rocks, and are very comparable overall, with the same kind of weathering rings, lichen pitting, and random internal voids and tubes.

Nolan et al. concluded that Brooksella is no different from the local concretions except for the lobes, and should therefore "be considered a pseudofossil until proven otherwise." A consequence of this conclusion is that Brooksella, not being a glass sponge, would not have been a source of silica for preservation of fossils in the Conasauga. (It's not stated, but it seems that it would have been a sink instead.) It further goes to show that you shouldn't trust strange things in the Cambrian.

Brooksella (A–E) and concretions (F–K) collected from the Conasauga Formation by Nolan et al. (scale bar 1 cm, or 0.4 in); Figure 5. CC BY 4.0.


Bassler, R. S. 1941. A supposed jellyfish from the pre-Cambrian of the Grand Canyon. Proceedings of the United States National Museum 89(3104):519–522.

Ciampaglio, C. N., L. E. Babcock, C. L. Wellman, A. R. York, and H. K. Brunswick. 2006. Phylogenetic affinities and taphonomy of Brooksella from the Cambrian of Georgia and Alabama, USA. Palaeoworld 15:256–265.

Cloud, P. E., Jr. 1968. Pre-metazoan evolution and the origins of the Metazoa. Pages 1–72 in E. T. Drake, editor. Evolution and environment. Yale University Press, New Haven, Connecticut.

Fedonkin, M. A., and B. N. Runnegar. 1992. Proterozoic metazoan trace fossils. Pages 389–395 in J. W. Schopf and C. Klein, editors. The Proterozoic biosphere: A multidisciplinary study. Cambridge University Press, Cambridge, United Kingdom.

Ford, T. D. 1990. Grand Canyon Supergroup: Nankoweap Formation, Chuar Group, and Sixtymile Formation. Pages 49–70 in S. S. Beus and M. Morales, editors. Grand Canyon geology. Oxford University Press, New York, New York.

Ford, T. D., and W. J. Breed. 1977. Chuaria circularis Walcott and other Precambrian fossils from the Grand Canyon. Journal of the Palaeontological Society of India 20:170–177.

Glaessner, M. F. 1969. Trace fossils from the Precambrian and basal Cambrian. Lethaia 2(4):369–393.

Kauffman, E. G., and F. Fursich. 1983. Brooksella canyonensis: A billion year old complex metazoan trace fossil from the Grand Canyon. Abstracts with Programs - Geological Society of America 15(6):608.

Kauffman, E. G., and J. R. Steidtmann. 1981. Are these the oldest metazoan trace fossils? Journal of Paleontology 55:923–947.

Nolan, M. R., S. E. Walker, T. Selly, and J. Schiffbauer. 2023. Is the middle Cambrian Brooksella a hexactinellid sponge, trace fossil or pseudofossil? PeerJ 11:e14796. doi:

Van Gundy, C. E. 1937. Jellyfish from Grand Canyon Algonkian. Science 85(2204):314.

Walcott, C. D. 1896. Fossil jelly fishes from the Middle Cambrian Terrane. Proceedings of the United States National Museum 18:611–614.

Walcott, C. D. 1898. Fossil Medusæ. U.S. Geological Survey, Washington, D.C. Monograph 30.

Sunday, February 26, 2023

Your Friends The Titanosaurs: Chucarosaurus and (maybe) Ruixinia

We check in again with titanosaurs with one new genus and species that is definitely titanosaurian, and another that falls into the hazy uncertainty of potential Early Cretaceous titanosaurs.

Chucarosaurus diripienda

It's been a thing the past couple of years for dinosaurs to take the winter off, with few new names in December, January, and February, and 2023 has continued the tradition. Also, oddly enough, titanosaurs seem to strike during the winter; not that any time of the year is safe from them, but you can count on them taking up the slack left by tyrannosaurs and duckbills and so on.

Where was I? Anyway, the first publicized non-avian dinosaur of 2023 is our first guest, the titanosaur Chucarosaurus diripienda.

Genus and Species: "Chucaro" is a Quechua word for a "hard and indomitable animal", while "diripienda" is Latin for "scrambled" (Agnolin et al. 2023), giving us something like "scrambled indomitable lizard".

Citation: Agnolin, F. L., B. J. Gonzalez Riga, A. M. Aranciaga Rolando, S. Rozadilla, M. J. Motta, N. R. Chimento, and F. E. Novas. 2023. A new gigant titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of northwestern Patagonia, Argentina. Cretaceous Research (preprint). doi:

(Thank you to Matias Motta for providing a pdf!)

Stratigraphy and Geography: C. diripienda is to date known only from the lower Upper Cretaceous Huincul Formation of the Pueblo Blanco Natural Reserve, northwestern Río Negro Province, Argentina (Agnolin et al. 2023). The site is rather better known for a variety of theropods representing several lineages (Tralkasaurus, Aoniraptor, Gualicho, Taurovenator, Overoraptor) (Agnolin et al. 2023).

Holotype: MPCA (Museo Provincial "Carlos Ameghino", Cipolletti, Río Negro, Argentina) PV 820, consisting of a left humerus, most of a left radius, left metacarpal II, left ischium, somewhat eroded left femur, the shaft of the left fibula, the proximal end of the right tibia, and the distal end of an anonymous metapodial, found disarticulated but associated and interpreted as representing one individual (Agnolin et al. 2023). A left femur and tibia (MPCA PV 821) represent another individual.

You might notice right away that, unlike most titanosaurs, we're not dealing with vertebrae here. This is a little inconvenient for comparative purposes, but on the other hand it does focus attention on areas of titanosaurian anatomy that do not receive as much attention. The limb bones indicate a large but relatively gracile form, with an estimated femur length of 200 cm (78.7 inches) (Agnolin et al. 2023).

Two other titanosaurs have been named from the Huincul Formation, the redoubtable Argentinosaurus huinculensis and the somewhat less famous Choconsaurus baileywilsoni. Limb bones of A. huinculensis and C. diripienda can be distinguished by several details, as well as A. huinculensis being more robust overall (although I suppose that shouldn't be too surprising). There is very little overlap with C. baileywilsoni, only metacarpal II, which is much more elongate in C. baileywilsoni. The phylogenetic analysis places C. diripienda as a colosssaurian but not quite a lognkosaurian, hanging out in the area of Bonitasaura salgadoi and Notocolossus gonzalezparejasi (Agnolin et al. 2023).

Ruixinia zhangi

Oddly enough again, the last non-avian dinosaur of 2022 was a potential titanosaur, Ruixinia zhangi. It falls into the grand tradition of Early Cretaceous East Asian titanosaur-ish sauropods, and as such is well worth an entry.

Genus and Species: The entire name is an allusion to Ruixin Zhang, a benefactor of the Erlianhaote Dinosaur Museum (Mo et al. 2023). As such, a translation isn't really applicable.

Citation: Mo, J., F. Ma, Y. Yu., and X. Xu. 2023 (preprint 2022). A new titanosauriform sauropod with an unusual tail from the Lower Cretaceous of northeastern China. Cretaceous Research 144:article 105449. doi:

(Cretaceous Research could almost be sub-headed as "The Journal of Titanosaurian Research" these days. They're all over it.)

Stratigraphy and Geography: R. zhangi hails from the famous Yixian Formation and was found at Batuyingzi, Beipiao, western Liaoning Province, northeastern China (Mo et al. 2023).

Holotype: ELDM (Erlianhaote Dinosaur Museum, Inner Mongolia, China) EL-J009, an articulated partial skeleton featuring most of the vertebrae (14 cervicals, dorsals, several sacrals, and 52 caudals), several dorsal ribs, 36 chevrons, left hip minus the ischium, left femur, left tibia, left astragalus, left metatarsal V, and a possible pedal phalanx (Mo et al. 2023).

Going from the description of the holotype, you'll note that we're dealing with a nearly complete vertebral column. Unfortunately, the other side of the coin in this case is that the precaudals are rather smooshed (Mo et al. 2023). From a visual inspection the cervicals seem kind of chunky, suggesting a relatively thick neck. R. zhangi has a rather odd tail; some of the highlights are as follows. The anterior caudals are strongly procoelous. The last six caudals are fused into a rod, simple in structure unlike the club of Shunosaurus and the cockscombs of Mamenchisaurus hochuanensis (Mo et al. 2023). (If it's just a rod, I wonder if it's just a one-off pathology.) The caudal neural spines are notably low and mildly split at the tops in the anterior caudals; the supporting neural arches are not cheated anteriorly in the first part of the tail like most titanosaurs but do have an anterior bias later on (Mo et al. 2023).

R. zhangi is not particularly large, with a femur 137 cm (53.9 inches) long and an overall body length on the order of 12 m (39 ft), but it is the current champion of the Yixian Formation sauropods (followed by Dongbeititan with a femur length of about 114 cm [44.9 inches] and Liaoningotitan slightly smaller at 108 cm [42.5 inches]). The phylogenetic analysis finds it well within Titanosauria (Mo et al. 2023), which is promising, but it's also closely associated with two taxa that are questionably titanosaurian per other authors (Daxiatitan and Xianshanosaurus), which is not so promising.


Agnolin, F. L., B. J. Gonzalez Riga, A. M. Aranciaga Rolando, S. Rozadilla, M. J. Motta, N. R. Chimento, and F. E. Novas. 2023. A new gigant titanosaur (Dinosauria: Sauropoda) from the Upper Cretaceous of northwestern Patagonia, Argentina. Cretaceous Research (preprint). doi:

Mo, J., F. Ma, Y. Yu., and X. Xu. 2023 (preprint 2022). A new titanosauriform sauropod with an unusual tail from the Lower Cretaceous of northeastern China. Cretaceous Research 144:article 105449. doi:

Sunday, February 12, 2023

Hunting the wild stromatolite at Vermillion Falls

The gorge at Vermillion Falls, as seen before, is cut through quite a bit of the Prairie du Chien Group. Therefore, it is entirely reasonable to suppose that somewhere in all of those oodles of outcrop, there are some stromatolites. It should be just a matter of walking down and having a look, right? We-e-ll, easier said than done. A few factors are pushing against going from "predicted" to "established":

1) The classic booby prize of vertical exposures—sure, there's a lot of surface, but you don't get to see most of it up-close;

1b) Furthermore, for many of the places where you *can* examine the walls, you can't change your vantage point because backing up a foot or two puts you a foot or two lower or in open space (and that never helps). What makes this annoying with stromatolites is they can be expressed in various scales, and what you can't see with your nose on the rocks may be perfectly apparent from a couple of arm's lengths away except for that whole "absence of footing" thing, or a promising feature may disappear as you clamber up the slope to inspect it;

2) The surfaces have been fried to a crisp by weathering, which for our purposes disguises the fine layering and other subtle features.

The existing literature is not especially illustrative. Stauffer and Thiel (1941) included a section taken at the railroad bridge (now a footbridge), which describes the walls as 58.5 ft (17.8 m) of Shakopee Dolomite over 1 ft (0.3 m) of Root Valley Sandstone over 47 ft (14 m) of Oneota Dolomite, with nary a stromatolite mentioned (to be fair, the lithological descriptions are also very slim). The site has made it into a few theses and other student papers (Shea 1960; Squillace 1979; Robins 2005), but that seems to be about it. Maybe this limited documentation is a recognition of the above limitations by wiser heads than mine, but I have the faith of a gambler that something will turn up if I look long enough, so down we go back into the gorge.

Besides, why would I want to sit around inside when I could see things like this?

My strategy involves *not* looking directly for stromatolites, at least not the classic laminated features. I would never see them here except on fresh breaks. Instead, I'm looking for a couple of other features: bedding planes that are undulating rather than horizontal, and anomalously recessive intervals (I've seen elsewhere that stromatolitic intervals may be relatively erosion-prone compared to non-stromatolitic intervals). These two features should be visible even through the stain of weathering that began before the most recent Ice Age.

Down near the falls, in some of the lowest accessible beds, something very promising is apparent.




We've definitely got an undulating surface that is appropriately stromatiform* in appearance, as well as being within an interval that is recessed. (In fact, the interval above is recessed in comparison to the interval above that, so this is not for people who don't like rocks poised above their heads [or, for that matter, people who have a phobia of birds pooping on them, because there are pigeons roosting in cavities above].)

*Google says it can come up with about 328,000 hits for "stromatiform". What it doesn't tell you is how many are useful and how many are just dictionaries, rhyming words, anagrams, pronunciations, and SEO things seeking to boost traffic with lists from dictionaries.

A little closer, between two apparent mounds

So stromatiform...

Obviously the next thing to do is to is to get close and look for finer details. That's where things become less clear. There are, conveniently, some fresh surfaces (not made by me!), and they do show alternating compositions. They are, however, rather thick layers.

Layers in one fresh surface.

Not quite as fresh, but you get the idea.

Layers on a spalled piece, frozen to the underlying moss.

I'd been hoping to find nice stacks of small columns beneath the undulating layers, but they aren't appearing (yet, at least). If we compare to Logan et al. (1964) and May et al. (2012), the latter also dealing with large Prairie du Chien stromatolites, it's as if we've skipped the columnar "Cryptozoon" stage and gone straight to broad "Collenia". This may say something about the local environment, or it may say that I'm just getting excited about some inorganic stromatiform-producing process.

Then there's whatever the heck is this. Bedded sedimentary rocks don't do things like this without a good reason.

If the issue is not immediately apparent, check the annotated version below and click to expand as necessary.

Too bad it was on the other side of the gorge. There's probably a way down somewhere...


Logan, B.W., R. Rezak, and R. N. Ginsburg. 1964. Classification and environmental significance of algal stromatolites. The Journal of Geology 72(1):68-83.

May, S. L., L. E. Davis, and D. G. Brown. 2012. Algal stromatolites in the Willow River Member of the Lower Ordovician Shakopee Formation near Chatfield, Minnesota, USA. The Compass: Earth Science Journal of Sigma Gamma Epsilon 84(1, Article 6):42–48.

Robins, C. 2005. The geology of the New Richmond Sandstone. Senior Integrative Exercise. Carleton College, Northfield, Minnesota.

Shea, J. H. 1960. Stratigraphy of the Lower Ordovician New Richmond Sandstone in the Upper Mississippi Valley. Thesis. University of Wisconsin, Madison, Wisconsin.

Squillace, P. J. 1979. The geology of the New Richmond Member of the Shakopee Formation (Lower Ordovician), Upper Mississippi Valley. Thesis. University of Minnesota, Minneapolis, Minnesota.

Stauffer, C. R., and G. A. Thiel. 1941. The Paleozoic and related rocks of southeastern Minnesota. Bulletin 29. Minnesota Geological Survey, St. Paul, Minnesota.

Tuesday, January 31, 2023

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:

Monday, January 16, 2023

On the concept of "100 Dinosaurs from A to Z"

Back in the 1980s, there were books aimed at the kid market that promised 100 (or 101) alphabetical dinosaurs. This was about the last time in dinosaurological history when a book with that premise could still boast a fairly comprehensive selection of reasonably well-known dinosaurs. To be honest, it's a little unclear in hindsight why books with 100 dinosaurs would be compelling when there were competing books on the market that gave the reader more dinosaurs and were not significantly more difficult reading. Maybe they were seen as more kid-friendly, but then again, someone who's up for "100 Dinosaurs" is probably going to age out of it and go for "A Field Guide To Dinosaurs" pretty quickly. The market seemed to agree, because the books turned into just "Dinosaurs A-Z" by the end of the decade and then disappeared.

Most of them were not actually complete alphabets, simply because the dinosaur alphabet wasn't completed until 1983 (Quaesitosaurus and Xiaosaurus), and for several other letters, the choices were pretty dismal. F would always be Fabrosaurus, which may well have helped it hang around after taxonomic obsolescence by lodging in the brains of impressionable children. (Otherwise there was Fulgurotherium, but it was poorly known, space was limited, and Muttaburrasaurus and perhaps Rhoetosaurus were already there to represent Australia.) U would be "Ultrasaurus", because it was huge and in the news and the only other option was Unquillosaurus. J? It's a choice between the two immortals Jaxartosaurus or Jubbulpuria, or just skipping it. (In related news, I've had a soft spot for Jubbulpuria for decades. Us "J"s have to stick together!)

With a minimal knowledge of 1980s dinosaurs, it's easy enough to figure out most of the genera that would be in these books.

  • First you have the superstars: Tyrannosaurus, Triceratops, Stegosaurus, Apatosaurus (with obligatory comments about Brontosaurus). Today, that inner ring would include Spinosaurus and Velociraptor (even if Deinonychus did the heavy lifting), and Brontosaurus would swap for Apatosaurus.
  • Then there's the B-listers, names like Allosaurus, Parasaurolophus, Styracosaurus, Brachiosaurus, and Iguanodon, and lots of entries from the Morrison and Late Cretaceous of North America.
  • Then you descend into a grab-bag of C-listers, geographic and stratigraphic tokens and oddities, and flavors-of-the-month, some of which would go on to better things, while others didn't. (Yaverlandia! "Ultrasaurus"!)
  • Finally, there's Geranosaurus and Altispinax. (Altispinax never had it so good as it did in the 1980s.)

Today, with more than 1600 formally described species of non-avian dinosaur that a reasonable person (i.e., me) would recognize (with some sleeping dubious dogs being left to lie to avoid inconvenient issues of priority), promising "100 dinosaurs" is not impressive. At the moment, one could write "100 dinosaur" books just on specific countries. Argentina, China, Mongolia, the United Kingdom, and the United States of America all qualify (granted, the UK edition would have a distinctly scrappy and Victorian flavor), with Canada almost there. In fact, with China and the US both well over 300 species, "100 dinosaur" books for them would be omitting more than 2 out of 3 species. Surprisingly, complete alphabets of dinosaurs from these prolific countries are not possible at this time; Argentina (missing "Y") and China (missing "U") get the closest, with the U.S. a distant third with no "Q", "W", or "X".

It is also possible to have entire "100" books on certain groups. Maniraptors? No sweat, at just over 200 (which gives some wiggle room for reclassification of entire lineages). Old-timey prosauropods? Yep, they just make it. Titanosaurs? Room to spare (as we observed in great detail). Ankylosaurs just make it, too (you can throw in stegosaurs for more thyreophoran charm and bump some of the underachievers). Ceratopsians cross easily, and while hadrosaurids by themselves don't quite make it, going down to Hadrosauromorpha to pick up traditional basal hadrosaurs also crosses the threshold. Complete alphabets are still a problem, though. (You can do it with titanosaurs if you make Wintonotitan an honorary member.) Why someone should *want* to write a book on 100 dinosaurs of a country or a specific group, I'm not sure (if you were going to go to the trouble, you might as well cover *all* of them and not just an arbitrary number), but someone certainly *could*.

When I started drafting this post, I thought about coming up with my own list of 100 dinosaurs, for the fun of it. Then I realized I would basically pick 100 dinosaurs I remembered as important from the 1980s. While classics are classics for a reason, it isn't any fun to come up with the same old list that any of us could have picked. Nor was the idea of consciously selecting some kind of quota (geographic, stratigraphic, taxonomic, year of description, etc.). So, I decided to leave it up to fate. I copied the genera and species from The Compact Thescelosaurus, then took some pity and eliminated every nomen dubium, which removed 315 entries. (Before I did this, I tried it once and got all three entries for Cionodon. Yay?) The remaining 1,313 were run through the Google Sheets random sort function and the first 100+ entries were grabbed to produce the following lightly curated list (the entries were at the species level but the books always worked at the genus level, so I scratched multiple entries from the same genera). If you don't like Dravidosaurus or would rather have 101, the genus on deck is Tyrannotitan. If both conditions apply, then Rugocaudia is up next.

Behold, 100 Random Dinosaurs From A To Z:

AdasaurusEomamenchisaurus"Mamenchisaurus" sinocanadorumSoriatitan
AlbertonykusEuhelopusMonolophosaurus"Syntarsus" kayentakatae

You know what? I feel pretty good about this list. It's got an interesting spread in time, space, lineage, and fame. It gets 25 out of 26 letters, too (no Q, which didn't show up until 257 with the much-beloved Qinlingosaurus, but that's randomness for you).