View of the Kaiparowits Formation from Death Ridge in Grand Staircase-Escalante National Monument.
Figure 4. Reconstruction of UMNH VP 16266 (holotype, Bolg amondol gen. et sp. nov.). Gold: preserved skeletal elements. Grey: morphological hypotheses of reconstructed elements based on the morphology of preserved skeletal elements. Black: missing skeletal elements, based on publicly available rendered CT scans on morphosource.org of specimen UF:Herp:153328, Heloderma horridum.
We knew that today’s large-bodied lizards must share common ancestors, but the gap lay in the fossil evidence to support that view—what creationists dismissively call “conjecture” or claims made without evidence. Yet the Theory of Evolution always predicted that such ancestral and transitional forms must have existed.
Creationists, however, have missed that particular boat because the gap has now been filled. The prediction of evolutionary theory has, once again, been vindicated.
The gap-filling discovery was made by Dr Hank Woolley of the Dinosaur Institute while examining a jar of bones at the Natural History Museum of Utah, simply labelled “lizard bones.” Dr Woolley identified them as belonging to the Monstersauria—a group of lizards with a 100-million-year history, but until now an incomplete fossil record. A modern member of this group is the Gila Monster, one of only two venomous lizards alive today. The fossil, belonging to a large-bodied lizard which Dr Woolley has named Bolg amondol after a character in J.R.R. Tolkien’s The Lord of the Rings, was found in the Kaiparowits Formation of the Grand Staircase–Escalante National Monument, Utah—a palaeontological treasure trove and one of North America’s richest dinosaur-dominated records. Alongside dozens of new species, the site offers a vital window into the deep past. The age of this fossil places it squarely in the age of the dinosaurs, a crucial factor in lizard evolution.
Incidentally, the picture above shows the Kaiparowits Formation. Perhaps a creationist would like to explain how those vast sedimentary strata could have been deposited in a single global flood.
The evolutionary history of Monstersauria. Evolutionary Overview of Monstersauria A Deep, Yet Fragmentary Fossil RecordThe discovery was recently published in Royal Society Open Science and is explained further in a Natural History Museum of LA County news release.
Monstersauria—which includes bulky, carnivorous lizards like the Gila monster (genus Heloderma) — has a fossil record stretching back roughly 100–110 million years, from the Mid‑Cretaceous onward [1.1]. Yet, the record remains notably incomplete, with most specimens represented by isolated skull fragments, osteoderms, or sparse skeletal remains [2.1].
Early Monstersaurs Across Continents
Cretaceous-era species such as Estesia mongoliensis and Gobiderma pulchrum have been unearthed in Mongolia, providing glimpses into morphological traits and ecological niches. The former, which lacks osteoderms, may have had dynamics similar to modern Heloderma, while the latter shows foundational skull and armor features typical of early monstersaurs [2.1]. Additionally, Morohasaurus from Early Cretaceous Japan may represent one of the oldest known monstersaurian forms—suggesting a broader geographic range in the past [3.1].
Shifting Phylogenetic Understanding
Recent phylogenetic analyses have challenged the traditional view of Monstersauria as a unified group. While the clade was once considered to include both extant Heloderma and fossil genera like Gobiderma, more comprehensive studies combining molecular and morphological data suggest that modern Heloderma may be more closely related to the Anguidae than to other anguimorphs, making Monstersauria potentially polyphyletic [4.1].
A New Chapter: Bolg amondol
The 2025 discovery of Bolg amondol from the Kaiparowits Formation in Utah significantly enriches our understanding of monstersaur evolution. This Late Cretaceous species—about 76 million years old — was already predicted by evolution but only confirmed when Dr. Hank Woolley recognised its significance in museum collections [5.1].
Morphological and genomic analyses place Bolg within Monstersauria: one morphology-based phylogeny even recovers it as the sister taxon to Gobiderma, while genome-based analysis leaves its relationships within the broader group unresolved [6.1].
Moreover, Bolg adds new evidence of faunal richness and ecological diversity among large lizards during the Dinosaur Age. Its presence in the Kaiparowits ecosystem—alongside other large-bodied predators—implies a productive, varied environment in which multiple predators co-existed and exploited different microhabitats [5.1].
Biogeography and Dispersal Patterns
The discovery of Bolg also supports the idea that smaller terrestrial vertebrates, not just dinosaurs, participated in intercontinental migration—traversing what were then connected landmasses between Asia and North America [7.1].
Summary: What the Evolution of Monstersauria Reveals Aspect Insight Age & Origins A long evolutionary lineage (~110 Ma), with early forms found in Asia and possibly Japan, though much of the record remains fragmentary. Phylogenetics The clade may be polyphyletic—modern Heloderma might not be as closely linked to fossil monstersaurs as once believed. New Discoveries Bolg amondol strengthens evolutionary ties within the group and highlights greater past diversity and complex ecosystems. Biogeography Evidence of cross-continental distribution, suggesting broader movement and connectivity in the Cretaceous.
A Monster “Goblin” at the Feet of Dinosaurs
Meet the newly discovered giant, armored lizard named for the Hobbit’s goblin prince
There are monsters—and treasures—lurking in the bowels of museum paleontology collections.
I opened this jar of bones labeled ‘lizard’ at the Natural History Museum of Utah, and was like, oh wow, there's a fragmentary skeleton here. We know very little about large-bodied lizards from the Kaiparowits Formation in Grand Staircase-Escalante National Monument in Utah, so I knew this was significant right away.
Dr. C. Henrik (Hank) Woolley, lead author
Dinosaur Institute
Natural History Museum of Los Angeles County
Los Angeles, CA, USA.
The lizard Dr. Woolley identified belongs to the Monstersauria, a group of lizards characterized by their large size and distinctive features like pitted, polygonal armor attached to their skulls and sharp, spire-like teeth. They have a roughly 100 million-year history, but their fossil record is largely incomplete, making the discovery of the new species dubbed Bolg amondol —a name ripped from the lore of J.R.R. Tolkien's Lord of the Rings universe—a big deal for understanding these charismatic lizards. Bolg would have been pretty monstrous to our eyes.
Three feet tip to tail, maybe even bigger than that, depending on the length of the tail and torso, so by modern lizard standards, a very large animal, similar in size to a Savannah monitor lizard; something that you wouldn’t want to mess around with.
Dr. "Hank" Woolley,
Woolley gave the newly described species a name befitting its large size, vicious teeth, and armored head.
A Moniker from Middle-earth
Bolg represents an evolutionary lineage that sprouted within a group of large-bodied lizards called monstersaurs, the most familiar example being the Gila monsters, which still roam the deserts where Bolg was recovered from. Woolley knew that a new species of monstersaur called for an appropriate name from an iconic monster creator: J.R.R. Tolkien.
Bolg is a great sounding name. It's a goblin prince from The Hobbit, and I think of these lizards as goblin-like, especially looking at their skulls.
Dr. "Hank" Woolley,
He used the fictional language Sindarin—created by Tolkien for his elves—to craft the species epithet.
“Amon” means “mound”, and “dol” means “head” in the Elvish language, a reference to the mound-like osteoderms found on Bolg’s and other monstersaur skulls. “Mound-Headed Bolg” would fit right in with the goblins—and it’s revealing quite a bit about monstersaurs.
Dr. "Hank" Woolley,
It’s unclear how much Bolg would have looked or acted like Gila monsters, who don’t quite live up to their imposing moniker. “Gila” refers to the Gila River Basin of New Mexico and Arizona, one place where the strikingly orange and black lizards are found, and the “monster” part comes from myths that they had toxic breath or could spit venom. They do in fact have toxic venom, and it does have a reportedly foul odor, but Gila monsters are far from monstrous.
They’re pretty slow-moving lizards. They rely on their bright colors and armor to avoid predation. You would have to get really close to actually be bitten by a Gila monster.
Neftali Camacho, not an author of the paper.
Collections Manager in NHM’s Herpetology Department.
A collection specimen of a Gila monster (Heloderma suspectum), one of only five living species of the Monstersauria clade. Specimens like this help researchers better understand these iconic lizards.
Hidden Gems in Collection Drawers
Bolg is a great example of the importance of natural history museum collections. Although we knew the specimen was significant when it was discovered back in 2005, it took a specialist in lizard evolution like Hank to truly recognize its scientific importance, and take on the task of researching and scientifically describing this new species.
,Dr. Randall B. Irmis, co-author.
Natural History Museum of Utah
University of Utah
Salt Lake City, UT, USA.
The new species was identified from an associated skeleton of fragmentary bones: tiny pieces of the jaw, skull, vertebrae, tail, and the bony armor called osteoderms.
Size comparison of the holotype specimen of Bolg amondol (UMNH VP 16266, left, Natural History Museum of Utah / Bureau of Land Management) and a modern Gila monster (Heloderma suspectum, right). Gold: preserved skeletal elements. Light gray: reconstructed elements based on preserved material. Dark gray: missing skeletal elements, based on publicly available rendered CT scans of Heloderma horridum available on morphosource.org (UF:Herp 153328What's really interesting about this holotype specimen of Bolg is that it's fragmentary, yes, but we have a broad sample of the skeleton preserved. There's no overlapping bones— there's not two left hip bones or anything like that. So we can be confident that these remains likely belonged to a single individual. That means more characteristics are available for us to assess and compare to similar-looking lizards. Importantly, we can use those characteristics to understand this animal's evolutionary relationships and test hypotheses about where it fits on the lizard tree of life.
Dr. "Hank" Woolley,
Most of the fossil lizards from the Age of Dinosaurs are even more fragmented—often just single isolated bones or teeth—so despite their fragmentary nature, the parts of Bolg’s skeleton that survived contain a stunning amount of information.
The identification of a new species of monstersaur highlights the likelihood that there were many more kinds of big lizards in the Late Cretaceous. Additionally, this find shows that unexplored diversity is waiting to be dug up in the field and in paleontology collections.
Identified bones belonging to Bolg amondol (Specimen UMNH VP 16266, Natural History Museum of Utah / Bureau of Land Management). They are not much to look at, but they are jam-packed with valuable information on the anatomy and lifestyle of Bolg amondol.
Bolg’s closest known relative hails from the other side of the planet in the Gobi Desert of Asia. While dinosaurs have long been known to have traveled between the once connected continents during the Late Cretaceous Period, the discovery of Bolg reveals that smaller animals also made the trek, suggesting there were common patterns of biogeography across terrestrial vertebrates during this time.
Dr. Woolley began this research as a PhD student at the Dinosaur Institute and has continued it as a National Science Foundation Postdoctoral Fellow in the department, underscoring the value of funding scientific research and the unique role the Dinosaur Institute plays as a source of mentorship for young paleontologists.
The Natural History Museum and Dinosaur Institute has been proud to lead the way in empowering early career scientists.
Dr. Nathan Smith, co-author
Gretchen Augustyn Director & Curator of the Dinosaur Institute
Natural History Museum of Los Angeles County, Los Angeles, CA, USA.
In addition to Dr. Woolley, co-author Dr. Keegan Melstrom (now an Assistant Professor at the University of Central Oklahoma), was also a Postdoctoral Fellow in the Dinosaur Institute.Stairway to Monstersaurs
LEFT IMAGE: Publicly available rendered CT scan of Heloderma horridum (UF:Herp:153328) from Morphosource.org used for comparison to Bolg amondol, with mound-like, polygonal osteoderms in green. RIGHT IMAGE: Skull of the modern monstersaur Heloderma horridum (LACM 159136), from the herpetology collection at NHMLAC, used for comparison with Bolg amondol.
Bolg was discovered in the Kaiparowits Formation of Grand Staircase-Escalante National Monument, a paleontological treasure trove with one of the most valuable dinosaur-dominated records in North America. Along with dozens of new species, the site provides a vital window into the ancient past, underscoring how vital it is to conserve and protect public lands for future research. [This] collaborative research, led by the Natural History Museum of Los Angeles County’s Dinosaur Institute, reveals hidden treasures awaiting future paleontologists in the bowels of museum fossil collections, and the vast potential of paleontological heritage preserved in Grand Staircase-Escalante National Monument and other public lands.
Publication:
AbstractOnce again, we see how the scientific method works in practice. Evolutionary theory made a clear prediction: that transitional and ancestral forms of large-bodied lizards must once have existed. For years, the absence of fossil evidence was a gap waiting to be filled—not a weakness in the theory, but an opportunity for future discoveries. And when the evidence finally turned up, tucked away in a jar of “lizard bones” in a museum collection, it fitted the prediction perfectly. This is how real science works: gaps get smaller, knowledge grows, and the overall picture becomes ever clearer.
Monstersauria (Squamata, Anguimorpha) fossils are present in most Upper Cretaceous sedimentary basins in western North America, but despite almost a century of collection, their record remains extremely fragmentary. Here, we describe new material belonging to large-bodied monstersaurs, including a new taxon, Bolg amondol gen. et sp. nov., based on a fragmentary associated skeleton and co-occurring specimens from the middle unit of the upper Campanian Kaiparowits Formation of Grand Staircase–Escalante National Monument in southern Utah, USA. Phylogenetic analyses recover B. amondol within Monstersauria, with two unique anatomical features: fused osteoderms on the jugal and the presence of autotomy septa on the distal caudal vertebrae. Critically, B. amondol is morphologically distinct from the problematic Late Cretaceous North American monstersaur Palaeosaniwa canadensis, whereas co-occurring monstersaur vertebrae and parietals from the Kaiparowits Formation (cf. P. canadensis) highlight a pressing need for a reassessment of this important, widespread taxon. These results offer new evidence that at least three lineages of distinct, large-bodied monstersaurian lizard were present on the palaeolandmass of Laramidia during the Campanian Stage. Importantly, B. amondol represents the most complete squamate recovered from late Campanian southern Laramidia and reveals key anatomical characteristics for future identification of isolated lizard fossil elements.
1. Introduction
Monstersauria [1] is a long-lived, extant (Cretaceous–Present) clade of Northern Hemisphere anguimorph lizards, with taxa distributed across North America and Eurasia [1–22]. The oldest fossils attributed to Monstersauria are found in Lower Cretaceous sediments in Japan (Morohasaurus kamitakiensis, lower Albian Ohyamashimo Formation [15]) and the western United States (Primaderma nessovi, Cenomanian Mussentuchit Member, Cedar Mountain Formation [10]). Today, Monstersauria is represented by five species belonging to the genus Heloderma that are distributed throughout the deserts of the American Southwest and northwestern Mexico [23]. Because of the complex history of squamate phylogenetics and the presence of multiple competing hypotheses of squamate evolutionary relationships based on morphology and genomics, monstersaurian taxa have been placed in various combinations of anguimorph clades [11,12,24–29]. In some morphology-based hypotheses, Monstersauria is a polyphyletic group that includes taxa deemed to be more closely related to Varanus than to anguids [11,25]. Recent molecular and combined-evidence phylogenetic analyses [12,24,26–29] recovered Monstersauria/Helodermatidae (taxa more closely related to Heloderma than Varanus or Anguis) as sharing a more recent common ancestor with Anguidae than with Varanidae. Despite the disagreement over outgroup relationships, the monophyly of Monstersauria has been recovered repeatedly with new analyses and the addition of new taxa (e.g. [11–14]; this analysis).
Our current understanding of the clade Monstersauria contains fewer than 15 described fossil and extant species [6,12,23,30]. Despite this limited diversity, they attained an array of body sizes and morphological disparity throughout the clade’s evolutionary history. The most recent diagnosis for Monstersauria [6] lists a combination of 34 unambiguous synapomorphies that distinguish the clade from other anguimorphans (electronic supplementary material). Combinations of these character-states give the clade its distinctive general body plan: bulky, osteoderm-covered lizards with blunt snouts and widely spaced, spire-like conical teeth.
Across the approximately 100 Myr fossil record of Monstersauria, the clade achieved its highest taxonomic diversity and widest geographic distribution during the Campanian Stage (approx. 83.6−72.1 Ma) of the Late Cretaceous Period, with occurrences of taxa in present-day Mongolia [1,2,5,6,9,12], and in sedimentary basins in the Western Interior of North America [3,4,7,8,30–34]. The Campanian Djadokhta and Baruungoyot formations of Mongolia preserve the most complete and best-studied fossil monstersaurians: the smaller-bodied Gobiderma pulchrum [2,12] and the larger-bodied Estesia mongoliensis [1,5,6]. These two taxa are part of an assemblage of at least 50 co-occurring species of lizards [2,9], illustrating the richest-known squamate diversity in a Mesozoic faunal assemblage [35]. Additionally, the potential monstersaur taxon (see §5) Asprosaurus bibongriensis is known from an incomplete, associated skeleton from the Campanian Seonso Formation of South Korea [14], and could represent the largest member of the clade yet known.
The exceptionally complete Campanian monstersaur fossils from Asia contrast with those of North America, which are represented almost exclusively by numerous but fragmentary specimens [3,4,7,8,30–34,36]. Only two monstersaurian species have thus far been erected from the late Campanian of North America: Palaeosaniwa canadensis [31] and Labrodioctes montanaensis, the latter assigned to Helodermatidae [7]. Other large-bodied anguimorph taxa include Parasaniwa cynochoros [32] from the Kaiparowits Formation of Utah, as well as the tentative referral of specimens from the Oldman Formation (Alberta, Canada) to a new species of Parasaniwa (Parasaniwa, new species, cf. P. wyomingensis [7]), referred to herein as Oldman Fm Parasaniwa sp. nov. To date, most described and catalogued fossil remains of material exhibiting monstersaurian and/or varanoid/platynotan features from the late Campanian of North America have either been referred to Pal. canadensis (usually material belonging to larger individuals) or to Paras. wyomingensis (usually material belonging to smaller individuals). These historical referrals span northern (Wapiti Formation [30]; Oldman/Dinosaur Park formations [7]; Judith River Formation [37]; Two Medicine Formation [8,30]; central (Mesaverde Formation [38]), and southern geologic units from the Western Interior of North America (Aguja Formation [34]; Cerro del Pueblo Formation [39]) that comprise more than 5 million years of the Campanian Stage. The referral of fragmentary Campanian lizard material to Pal. canadensis, a taxon erected on the basis of a fragmentary vertebra from the Oldman Formation [31], and Paras. wyomingensis, a taxon erected on the basis of material from the upper Maastrichtian Lance Formation [31], over such a wide palaeolatitudinal (approx. 35−60° N) and temporal range suggests that the taxonomic diversity of ‘platynotans’/monstersaurians on the late Campanian palaeolandmass Laramidia [40] has been historically underestimated.
Over the past 30 years, survey efforts in the upper Campanian Kaiparowits Formation of Grand Staircase–Escalante National Monument, southern Utah, USA, have recovered an abundant and diverse squamate assemblage [32,41,42]. This assemblage contains taxa found nowhere else on Laramidia, bolstering dinosaur-based [43] and plant-based [44] hypotheses of sedimentary basin-level, or regional, endemism during the Campanian Stage. Despite intensified research and the abundance of squamate fossils, relatively few specimens have been referred to monstersaurian taxa and/or other clades that may include monstersaurian taxa under different hypotheses of squamate evolutionary relationships (i.e. Platynota, Varanoidea). Nydam [32] assigned UMNH VP 21180, a partial maxilla, to a new species of Parasaniwa, Paras. cynochoros. Nydam [32] also described several isolated marginal tooth-bearing elements and teeth that exhibit morphology distinctive enough to assign to different ‘morphotypes’ of playnotan lizards (Kaiparowits morphotypes H–J). Though fragmentary, the fossils described in Nydam [32] demonstrate the presence of at least two large-bodied anguimorph taxa in the Kaiparowits Formation.
Here, we describe several new specimens from the Kaiparowits Formation that can be attributed to large monstersaur lizard taxa, including one associated partial skeleton assigned to a new taxon, Bolg amondol gen. et sp. nov. Using descriptive comparisons and the results of parsimony-based phylogenetic analyses, we discuss the implications of this new material for understanding monstersaur taxonomic richness, morphological disparity and body-size distribution during the late Campanian of Laramidia.Figure 1.
Geologic map of Upper Cretaceous sediments in Grand Staircase–Escalante National Monument, Utah, modified from Titus et al. [45]. Stars indicate field areas containing localities of specimens described in this study (blue star: The Blues field area; red star: Fossil Ridge field area). Yellow star indicates the approximate location of the Parasaniwa cynochoros [32] type locality. Shaded relief layer of Grand Staircase–Escalante National Monument contains information from TessaDEM, which is made available here under the Open Database License (OdbL): https://tessadem.com.
Figure 2.
Composite stratigraphic column of the Kaiparowits Formation, with stratigraphic positions of localities of described material marked by stars (blue star: The Blues field area; red star: Fossil Ridge field area). Vertical red line indicates the stratigraphic uncertainty range for these localities. Yellow star indicates the position of the Parasaniwa cynochoros [32] type locality. Black horizontal arrows indicate stratigraphic position of radiometrically dated bentonite layers, indicated in purple. Stratigraphic column modified from Roberts [49], Beveridge et al. [53] and Ramezani et al. [62].
Figure 3.
Fossil locality UMNH VP Loc. 733, where the holotype specimen of Bolg amondol (UMNH VP 16266) was discovered in 2005.Photo taken by author C.H.W. in October 2023.
Woolley C. Henrik, Sertich Joseph J. W., Melstrom Keegan M., Irmis Randall B. and Smith Nathan D. 2025
New monstersaur specimens from the Kaiparowits Formation of Utah reveal unexpected richness of large-bodied lizards in Late Cretaceous North America
R. Soc. Open Sci. 12250435 http://doi.org/10.1098/rsos.250435
Copyright: © 2025 The authors.
Published by The Royal Society. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Contrast that with creationism. Creationist dogma thrives on gaps in knowledge because their entire argument amounts to pointing at the unknown and inserting their chosen god as an explanation. But each new find like Bolg amondol demonstrates why this strategy is doomed. The gaps keep closing. The “god of the gaps” keeps shrinking. The discovery of a transitional monstersaur is not just another win for evolutionary science—it is another blow to the credibility of creationism, which can only survive in ignorance.
It is telling, too, that creationists never predict these discoveries. They never tell us what sort of fossils should exist, in what strata they should be found, or what they should look like. Instead, they insist that these fossils will never be found because there aren't any, so they just sit on the sidelines, jeering at science until science once again proves them wrong. In the case of Bolg amondol, evolutionary biology passed the test with flying colours. Creationism, once again, had nothing to offer but silence.
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