So is Brontosaurus a valid genus or not?
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Vladimír Socha
Jul 25, 2025, 12:03:58 PM7/25/25
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Good day!
I have encountered differing opinions on the validity of the genus Brontosaurus during the past 10 years (from the date of publication of Tschopp et al. paper). It seems to me like there is generally not much willingness to use this taxon anymore. Can it be caused by the fact that for 112 years it was regarded as invalid genus and it might still appear unsuitable for common usage? Thank you for your thoughts, in advance! VS.
References:
Marsh, O. C. (1879). Notice of new Jurassic reptiles (PDF). American Journal of Science. 18 (108): 501–505.
Riggs, E. S. (1903). Structure and Relationships of Opisthocoelian Dinosaurs. Part I, Apatosaurus Marsh (PDF). Publications of the Field Columbian Museum Geographical Series. 2 (4): 165–196.
Tschopp, E.; Mateus, O. V.; Benson, R. B. J. (2015). A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ. 3: e857.
Mike Taylor
Jul 25, 2025, 12:07:36 PM7/25/25
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Tschopp et al. (2015) made a scientific case for the taxonomic validity of the name Brontosaurus in a peer-reviewed (and extremely comprehensive) paper. Since then, while there has been some grumbling from the wings here and there, no-one has published anything contradicting their argument. So as far as I'm concerned, they conclusion stands until someone takes it on in a fair fight — which is why I've been using it in my own recent papers, e.g. Wedel, Mathew J., and
Michael P. Taylor.
2023.
The biomechanical significance of bifurcated cervical ribs in apatosaurine sauropods.
VAMP (Vertebrate Anatomy Morphology Palaeontology)
11:91-100.
doi:
10.18435/vamp29394
-- Mike.
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Mickey Mortimer
Jul 28, 2025, 4:50:42 AM7/28/25
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My impression has been that almost every subsequent study has used Brontosaurus for excelsus, so I'd be interested to learn the references behind your assertion "there is generally not much willingness to use this taxon anymore."
Mickey Mortimer
John D'Angelo
Jul 28, 2025, 7:22:24 PM7/28/25
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The impressive scale and thoroughness of Tschopp et al.’s study has understandably meant that any study challenging their results has a high bar to clear in terms of effort and rigor, so it’s not surprising nobody has done so yet. However, I think that a closer examination of their results reveals that the case for considering Brontosaurus a valid genus is not nearly as well-supported as it seems at first glance.
Most significantly, Tschopp et al. do not consistently recover either Apatosaurus or Brontosaurus (as they define them) as monophyletic (even ignoring the position of the problematic Amphicoelias altus, which renders their Brontosaurus non-monophyletic in both analyses). Their equal weights topology recovers a clade of A. ajax and A. louisae as sister to a clade uniting B. excelsus, B. parvus, and B. yahnahpin, but their implied weights topology found A. louisae to be outside a clade uniting the other four species, with A. ajax as sister to B. parvus. Not only do they fail to consistently recover a topology consistent with their proposed taxonomy, the topology in which both genera are polyphyletic is arguably ever so slightly the more credible of the two topologies: it is more consistent with the only previous phylogenetic analysis of relationships within Apatosaurus sensu lato (Upchurch et al. 2004), it aligns with the general inclination of 20th-century studies to consider A. excelsus and A. louisae as clearly distinct but A. ajax and A. excelsus difficult to distinguish (e.g. Riggs 1903, Gilmore 1936, McIntosh 1990), and Tschopp et al. themselves suggest in their methods section that implied weighting may provide more accurate results for immature specimens (and thus the position recovered for the immature holotype of A. ajax may be more accurate under IW than EW). So while Apatosaurus sensu lato is unquestionably monophyletic, Apatosaurus sensu Tschopp et al. and Brontosaurus sensu Tschopp et al. are both potentially polyphyletic—and I think that alone is enough to seriously call into question their proposed taxonomy.
The basis for Tschopp et al.’s proposal to consider Apatosaurus and Brontosaurus as separate genera is their quantitative approach in which they define genera by counting the number of character changes separating clades. They decided on the threshold of 13 steps based on the widely-accepted congeneric species pairs of A. ajax and A. louisae (separated by 12 steps) and Diplodocus carnegii and D. hallorum (separated by 11 steps; note that Diplodocus hallorum sensu Tschopp et al. is largely equivalent to most 20th-century usage of Diplodocus longus). In their implied weighting analysis (which, you will remember, recovered both genera as polyphyletic) A. louisae was separated from the excelsus+parvus+yahnahpin+ajax clade by 14 steps, and A. ajax was separated from A. parvus by 14 steps, so the threshold for genus separation was just barely cleared (albeit without supporting A. ajax and A. louisae as congeneric). In their equal weighting results, which recovered a topology more consistent with their proposed taxonomy, Apatosaurus and Brontosaurus were separated by only 11 steps, below their threshold for recognizing separate genera and less than the distance between A. ajax and A. louisae. It should also be asked how appropriate the 13-step threshold is in the first place. The diplodocine genera Barosaurus, Diplodocus, Galeamopus, and Kaatedocus are separated from one another by 20–21 steps (the more fragmentary conditions of Tornieria and Leinkupal make it harder to evaluate the distances between more basal diplodocine genera). So perhaps we could conclude that 11–12 steps are typical for species within a genus and 20–21 steps are typical for closely related genera, so Tschopp et al. could have reasonably set their threshold for recognizing separate genera anywhere between 13 and 19 steps. Therefore, the 13-step threshold they selected is actually setting the bar for recognizing Brontosaurus as a distinct genus almost as low as possible, and Brontosaurus cannot even consistently reach that threshold. So even if you buy into their approach to quantitative taxonomy (and many don’t, including one of the reviewers of the paper), their approach would seem to provide only equivocal support, at best, for separating the two genera, and it is significant that it is only by choosing apomorphy counts from one analysis and the phylogenetic topology from the other are they able to support Brontosaurus as a valid genus.
Additionally, their principal coordinate analysis and pairwise dissimilarity metrics show that A. ajax and A. excelsus are the two most similar apatosaurine species to one another. In fact, the pairwise dissimilarity between the two species (0.18) is lower than the pairwise dissimilarity within A. parvus (0.23). Given the phylogenetic instability of A. ajax, I don’t think we can rule out the possibility that it is sister to A. excelsus, or possibly even synonymous with it.
Moreover, the proposed diagnoses of Apatosaurus and Brontosaurus are inadequate to reliably distinguish the two genera. The horizontal lamina in the SDF reported as an autapomorphy of Apatosaurus appears to be more or less identical to the horizontal lamina in the SDF reported as an autapomorphy of B. parvus, and the SDF is not adequately preserved in A. excelsus to evaluate this character, so it seems more likely that this lamina is shared among most or all apatosaurines (I certainly have yet to see an adequately-preserved apatosaurine neck that lacks it). Although Tschopp et al. diagnose Apatosaurus as having a straight scapular blade, the scapular blade of the holotype of A. ajax appears to be slightly curved ventrally, and damage to the distal end of the blade suggests the scapula could have shown stronger curvature when complete. The bases of the posterior dorsal neural spines are not adequately preserved in A. excelsus to determine whether they exhibit the condition reported as diagnostic of Brontosaurus. The rounded expansion of the scapular blade reported as diagnostic of Brontosaurus appears to be ontogenetically variable (it is absent in the holotype of A. parvus), so its absence in the immature holotype of A. ajax is not necessarily taxonomically informative. I am not convinced that the difference in the proportions of the astragalus is taxonomically informative, either. Most genera with multiple specimens listed in their supplementary data exhibit specimens on both sides of the state boundary, and the differences between A. excelsus and A. louisae are comparable to the differences between left and right sides of the same individual in CM 94. Furthermore, the astragalus is unknown in A. ajax, and the optimization of this character is unclear because it is polymorphic in both Dicraeosauridae and Diplodocinae. I have so far not been able to determine what difference is meant by the presence or absence of a roughened lateral aspect of the cervical PRDL; I don’t know whether this means I’ve overlooked something or that the character is not well-defined. That leaves a single character—the presence or absence of a shallow fossa on the scapula posterior to the acromial ridge—that I can’t dispute, but I wouldn’t stake a genus on a single subtle character like that.
Most significantly, Tschopp et al. do not consistently recover either Apatosaurus or Brontosaurus (as they define them) as monophyletic (even ignoring the position of the problematic Amphicoelias altus, which renders their Brontosaurus non-monophyletic in both analyses). Their equal weights topology recovers a clade of A. ajax and A. louisae as sister to a clade uniting B. excelsus, B. parvus, and B. yahnahpin, but their implied weights topology found A. louisae to be outside a clade uniting the other four species, with A. ajax as sister to B. parvus. Not only do they fail to consistently recover a topology consistent with their proposed taxonomy, the topology in which both genera are polyphyletic is arguably ever so slightly the more credible of the two topologies: it is more consistent with the only previous phylogenetic analysis of relationships within Apatosaurus sensu lato (Upchurch et al. 2004), it aligns with the general inclination of 20th-century studies to consider A. excelsus and A. louisae as clearly distinct but A. ajax and A. excelsus difficult to distinguish (e.g. Riggs 1903, Gilmore 1936, McIntosh 1990), and Tschopp et al. themselves suggest in their methods section that implied weighting may provide more accurate results for immature specimens (and thus the position recovered for the immature holotype of A. ajax may be more accurate under IW than EW). So while Apatosaurus sensu lato is unquestionably monophyletic, Apatosaurus sensu Tschopp et al. and Brontosaurus sensu Tschopp et al. are both potentially polyphyletic—and I think that alone is enough to seriously call into question their proposed taxonomy.
The basis for Tschopp et al.’s proposal to consider Apatosaurus and Brontosaurus as separate genera is their quantitative approach in which they define genera by counting the number of character changes separating clades. They decided on the threshold of 13 steps based on the widely-accepted congeneric species pairs of A. ajax and A. louisae (separated by 12 steps) and Diplodocus carnegii and D. hallorum (separated by 11 steps; note that Diplodocus hallorum sensu Tschopp et al. is largely equivalent to most 20th-century usage of Diplodocus longus). In their implied weighting analysis (which, you will remember, recovered both genera as polyphyletic) A. louisae was separated from the excelsus+parvus+yahnahpin+ajax clade by 14 steps, and A. ajax was separated from A. parvus by 14 steps, so the threshold for genus separation was just barely cleared (albeit without supporting A. ajax and A. louisae as congeneric). In their equal weighting results, which recovered a topology more consistent with their proposed taxonomy, Apatosaurus and Brontosaurus were separated by only 11 steps, below their threshold for recognizing separate genera and less than the distance between A. ajax and A. louisae. It should also be asked how appropriate the 13-step threshold is in the first place. The diplodocine genera Barosaurus, Diplodocus, Galeamopus, and Kaatedocus are separated from one another by 20–21 steps (the more fragmentary conditions of Tornieria and Leinkupal make it harder to evaluate the distances between more basal diplodocine genera). So perhaps we could conclude that 11–12 steps are typical for species within a genus and 20–21 steps are typical for closely related genera, so Tschopp et al. could have reasonably set their threshold for recognizing separate genera anywhere between 13 and 19 steps. Therefore, the 13-step threshold they selected is actually setting the bar for recognizing Brontosaurus as a distinct genus almost as low as possible, and Brontosaurus cannot even consistently reach that threshold. So even if you buy into their approach to quantitative taxonomy (and many don’t, including one of the reviewers of the paper), their approach would seem to provide only equivocal support, at best, for separating the two genera, and it is significant that it is only by choosing apomorphy counts from one analysis and the phylogenetic topology from the other are they able to support Brontosaurus as a valid genus.
Additionally, their principal coordinate analysis and pairwise dissimilarity metrics show that A. ajax and A. excelsus are the two most similar apatosaurine species to one another. In fact, the pairwise dissimilarity between the two species (0.18) is lower than the pairwise dissimilarity within A. parvus (0.23). Given the phylogenetic instability of A. ajax, I don’t think we can rule out the possibility that it is sister to A. excelsus, or possibly even synonymous with it.
Moreover, the proposed diagnoses of Apatosaurus and Brontosaurus are inadequate to reliably distinguish the two genera. The horizontal lamina in the SDF reported as an autapomorphy of Apatosaurus appears to be more or less identical to the horizontal lamina in the SDF reported as an autapomorphy of B. parvus, and the SDF is not adequately preserved in A. excelsus to evaluate this character, so it seems more likely that this lamina is shared among most or all apatosaurines (I certainly have yet to see an adequately-preserved apatosaurine neck that lacks it). Although Tschopp et al. diagnose Apatosaurus as having a straight scapular blade, the scapular blade of the holotype of A. ajax appears to be slightly curved ventrally, and damage to the distal end of the blade suggests the scapula could have shown stronger curvature when complete. The bases of the posterior dorsal neural spines are not adequately preserved in A. excelsus to determine whether they exhibit the condition reported as diagnostic of Brontosaurus. The rounded expansion of the scapular blade reported as diagnostic of Brontosaurus appears to be ontogenetically variable (it is absent in the holotype of A. parvus), so its absence in the immature holotype of A. ajax is not necessarily taxonomically informative. I am not convinced that the difference in the proportions of the astragalus is taxonomically informative, either. Most genera with multiple specimens listed in their supplementary data exhibit specimens on both sides of the state boundary, and the differences between A. excelsus and A. louisae are comparable to the differences between left and right sides of the same individual in CM 94. Furthermore, the astragalus is unknown in A. ajax, and the optimization of this character is unclear because it is polymorphic in both Dicraeosauridae and Diplodocinae. I have so far not been able to determine what difference is meant by the presence or absence of a roughened lateral aspect of the cervical PRDL; I don’t know whether this means I’ve overlooked something or that the character is not well-defined. That leaves a single character—the presence or absence of a shallow fossa on the scapula posterior to the acromial ridge—that I can’t dispute, but I wouldn’t stake a genus on a single subtle character like that.
So to sum up, Apatosaurus and Brontosaurus, as defined by Tschopp et al., are difficult to distinguish morphologically and may not be reciprocally monophyletic. The metrics Tschopp et al. used to justify separating the two genera only provide equivocal support, at best. I do not see the value in replacing one easily recognizeable monophyletic genus with two difficult-to-distinguish, potentially polyphyletic genera. Of all the important work that Tschopp et al. did, I think it’s unfortunate that this is the conclusion that their paper is best known for, as it is, in my opinion, one of the weakest parts of the paper.
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Gregory Paul
Jul 28, 2025, 7:53:46 PM7/28/25
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It is actually as easy as pie to distinguish Apatosaurus and Brontosaurus, they are very different apatosaurines. Why is something I will not be discussing at the time.
GSPaul
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Franco Sancarlo
Jul 28, 2025, 8:16:06 PM7/28/25
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It's absolutely true, just look at the fossils in peer-reviewed papers or the skeletal models made by GSP in his books. They have a number of differences that are not insignificant and are noticeable at first glance. Similar doesn't mean the same. Tyrannosaurus and Tarbosaurus are similar, but they are not the same genus.
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Jaime Headden
Jul 29, 2025, 12:37:12 AM7/29/25
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The issue really is subjective, and the authors admit this. Hence the arbitrary cutoff.
However, the distinction to be made is not Apatosaurus vs Brontosaurus, but ajax vs excelsus. This whole issue becomes mooted if you simply gave each species its own binomen, and not have a game of how to tune up or down one's genericometer. For some, a single autapomorphy is enough for an entire clade, yet for others you'd have to be pressed on [checks notes] 11-13 unique transformations in the lineage towards ajax/louisae to distinguish excelsus. That list gets larger or smaller as one's analysis gets more or less granular, as characters are combined, or as states are absorbed into others, or split, thus concealing or revealing diversity.
This issue will hardly ever be solved with rhetoric and lists of characters. The easiest solution is, in my opinion, to hold each taxon as equally distinct from each other taxon, and the solution ultimately requires unique binomial nomenclature.
"Oh, but Headden, won't that inflate---"
It doesn't matter. It stops the conversation by setting the arbitrary line at "there's no technical distinction between genus and species" and moves it to "are these two species distinct" -- which is what everyone in this thread is talking about. So if we can get back to having that conversation ("Is excelsus distinct?") then we'd actually have a meaningful conversation.
Cheers,
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Jaime A. Headden
The Bite Stuff: http://qilong.wordpress.com/
"Innocent, unbiased observation is a myth" - P. B. Medawar (1969)
Ethan Schoales
Jul 29, 2025, 12:42:33 AM7/29/25
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That may well be the case scientifically. In people’s perception, though, the genus makes more of a difference. It’s not like any cartoons and comics ever mentioned Apatosaurus excelsus or that it was basically synonymous with “paleontologically outdated” for years.
Jaime Headden
Jul 29, 2025, 12:55:52 AM7/29/25
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But that's where the beauty of the monotypical genus/species couplet, the singular effective binomial: When you talk about excelsus, you're talking about Apatosaurus; and vice versa. One becomes shorthand to the other.
Cheers,
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Ethan Schoales
Jul 29, 2025, 12:59:43 AM7/29/25
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If Apatosaurus = excelsus, what would Ajax and louisae be?
Milo Gaillard
Jul 29, 2025, 1:02:43 AM7/29/25
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Not to mention there’s also what Brian Curtice said after examining the Brontosaurus excelsus holotype. And well…let’s just say that things are looking real bad for Brontosaurus.
Sent from my iPhone
On Jul 28, 2025, at 16:22, John D'Angelo <dangel...@gmail.com> wrote:
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Mickey Mortimer
Jul 29, 2025, 1:33:20 AM7/29/25
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Very thorough and interesting post, John. Sounds similar to whenever I look at these kinds of issues in theropods (Allosaurus, Microraptor, Archaeopteryx, etc.) where for example, yes the lithographica type and the seimensii type have differences and will look different when reconstructed, but once you get other specimens involved good luck getting any robust topology in a cladogram, especially with ontogeny in play. But in order to maintain their result (even to the poor support with which they did), Tschopp et al. used the extreme splitter mentality which I do not follow.
Mickey Mortimer
Riley Shero
Jul 30, 2025, 8:17:18 AM7/30/25
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As usual, we are left to untangle the mess left behind by Cope and Marsh. My own personal views are similar to Jamie. Since genera are universally used as the primary nomenclatural unit, it is most efficient and least confusing to assign each new species to its own genus (at least for poorly-sampled taxa, of which almost all non-avian dinosaurs are). This has a degree of future-proofing that can prevent a Mamenchisaurus situation from developing (look it up, it's a mess). Genera also do not have a generally-applicable rigorous definition that is reflective of any sort of biological reality. Whether or not two species belong to the same genus is a determination that must be made subjectively in most cases. Tschopp et al. (2015), as has been mentioned, suggest a specific statistical demarcation for delineating genera, but theirs is not necessarily applicable to other taxa and has not been widely used in subsequent publications, as far as I know. Ultimately, despite all the hard work Tschopp and colleagues did to produce the nearly 300-page manuscript which resurrected Brontosaurus, there is still more work to be done. There was a recent paper published (van der Linden et al., 2025 - in which Tschopp is one of the co-authors) on diplodocoids, and more work is likely forthcoming.
Dawid Mazurek
Jul 30, 2025, 8:44:15 AM7/30/25
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Cope and Marsh, again! Someone should e-mail these guys? 😉
Stephen Poropat
Jul 30, 2025, 8:47:28 AM7/30/25
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Cope had nothing to do with Apatosaurus / Brontosaurus though. All Marsh.
Dr Stephen F. Poropat
Deputy Director
Western Australian Organic and Isotope Geochemistry Centre
School of Earth and Planetary Sciences
Curtin University
Bentley, Western Australia
Australia 6102
On Wed, 30 Jul 2025 at 20:44, Dawid Mazurek <dawidma...@gmail.com> wrote:
Cope and Marsh, again! Someone should e-mail these guys? 😉
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Milo Gaillard
Jul 30, 2025, 10:35:26 AM7/30/25
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To Riley,
I think I pretty much almost completely agree with you on these matters. But there is still much more work that needs to be done on diplodocoids as you said.
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