So, what exactly would happen if — as I suggested both here and mentioned in the comments here — we went down that slippery slope, and distinguished all species as their own unique, equivalent taxa? How many new “genus”-like names would we coin (calling them “praenomina”) so as to consider all species equivalent?
If we started with something popular and “near and dear,” we can do this with Dinosauria, and even extend that outwards to Pterosauria and fossil Crurotarsi. The number of species which are not actual type species (under the Linnaean System) in Dinosauria alone is not high. I combed through a few resources (Paleobiology Database, Wikipedia, and my own records) in order to evaluate valid non-type species for three Archosaurian subgroups — Crurotarsi (Pseudosuchia), Pterosauria, and Dinosauria — with the intention of finding out a gross, by-the-numbers perspective on the impact of treating all non-type species referred to various “genera” as binomina themselves (or, in Linnaean-speak, by raising each of these species to “genus” rank, thus coining a new “genus” for each one).
Taxa not considered synonymous with another species, but which aren’t type species:
Crurotarsi (excluding eusuchians)
Paleorhinus neukami [Ebrachosuchus]
Popsaurus langstoni [Lythrosuchus]
Protosuchus haughtoni [Lesothosuchus]
Germanodactylus rhamphastinus [n1]
Pteranodon sternbergi [Geosternbergia]
Ornithocheirus mesembrinus [Tropeognathus]
Tupandactylus navigans [n2]
Sinopterus jii [Huaxiapterus]
Psittacosaurus lujiatunensis [n3]
?Psittacosaurus sattayaraki [n4]
Dryosaurus lettowvorbecki [Dysalotosaurus]
Brachylophosaurus goodwini [n5]
Centrosaurus brinkmani [n6]
Chasmosaurus ?kaiseni [n7]
Anchiceratops longirostris [n8]
Omeisaurus jiaoi [n9]
Mamenchisaurus jingyanensis [n10]
Apatosaurus excelsus [Brontosaurus] [n11]
Apatosaurus parvus [Elosaurus]
?Apatosaurus minimus [n12]
Diplodocus hallorum [Seismosaurus] [n13]
Camarasaurus lentus [Morosaurus]
Camarasaurus lewisi [Cathetosaurus]
Brachiosaurus brancai [oh, wait, that's Giraffatitan...] [n14]
?Dilophosaurus sinensis [n15] (=? Sinosaurus triassicus)
?Chilantaisaurus zhiziangensis [n16]
Caenagnathus sternbergi [n17]
Elmisaurus elegans [n17]
Sinornithosaurus haoiana [n18]
Microraptor gui [n18]
Considering this list, 178 non-type species (excluding Avialae and Eusuchia) would require “praenomina,” of which 12 have already available names that can be coopted for use (although these taxa are not, like Brontosaurus excelsus, considered “valid” as equivalent to another “genus-species couplet,” like Apatosaurus ajax). This means, for 166 taxa, a new name would need to be coined. Now, mind, this excludes fossil birds, extant birds and crocs, fossil near-extant crocs (including mostly forms that resemble modern crocodilians). 97 of these are dinosaurs (and of those 7 which have names, 6 are sauropods), which means 54% of the noted taxa are dinosaurs. This implies that the bulk of taxa are dinosaurian, which already bear a significant upswing in modern taxonomy for getting a full binomen upon description, rather than being described as a new species to an already-established “genus.” These species represent significantly less than half of the total species described for the groups I’ve combed them from, implying that they will, while inflating the taxon list in accordance with other binomina, not change the actual diversity of species one whit. You end up only getting less species being described under the umbrella of some other taxon (“genus,” followed by type species, then discussion of other species).
As a further consideration, I have also excluded several species which are based solely upon teeth (which in Dinosauria total about 100 different taxa, and of those about 30% are non-type species — I’m estimating, as I must reconstruct my tooth-based taxon database). Tooth-based taxa are excluded largely because it is more or less impossible to evaluate them in comparison with regard to complete-dentition sets, heterodonty, etc.
Of the remaining taxa, the lion’s share belong to parasuchian and aetosaurian stem crocodilians. These taxa are largely used as index species for Triassic biostratigraphy, and as such removing some of these forms may upset various index zones in which their species are found. One must ask, then, whether naming these non-type species as new “genera” will effect the established usefulness of these “paleofaunas,” or if their use is actually all that, well … useful?
The number of taxa noted here (105 for dinosaurs, out of potentially 1000 non-avialaean dinosaurs), represents only 10% of the taxa presented, meaning that the impact of these forms is limited, whether they are “elevated” to “generic” level — or, in this consideration, are established as equivalent, producing a potential 10% increase in equivalent species. The additional benefits also include being able to treat each taxon as equivalent, thus permitting more access to various species rather than conflating additional species into their parent “genera” when only considering the type species. Each species would merely be the name listed on the analysis, rather than having to make inferences on which precise combination of species would be used.
I caution that this process should be restricted to fossil taxa of high profile groups. It is further useful to extent this philosophical approach to taxonomy and nomenclature by treating all fossil taxa this way, and then even further by treating all living taxa this way. The latter point would be far more radical, for rather than inflating by 5-10% of the “equivalent” taxa, it would likely inflate the number by 5000% (or more), considering the number of multi-specific “genera” within taxon groups such as Coleoptera, Bacteria, and the little issue of what to do with “subspecies.”
One hurdle at a time.
Update: Turner et al. (2012) have just published an extensive reanalysis of Dromaeosauridae, and general systematics of Paraves in connection to this. In this analysis, they recognize fewer members of Dromaeosauridae (including Unenlagiidae, as “Unenlagiinae” — see here; Microraptoria, as “Microraptorinae” — see here; and Eudromaeosauria, which includes Velociraptorinae and Dromaeosaurinae), by recognizing that NGMC 91, “Dave“, is a juvenile of Sinornithosaurus millennii, and that Sinornithosaurus haoiana (Liu et al., 2004) is a junior synonym; similarly, Microraptor gui was sunk into Microraptor zhaoianus, on the basis of lack of consistency of putative autapomorphies: some specimens have it, some don’t, and some specimens of zhaoianus could also have it, or it varies among species. These names will be struck out above.
[n1] The name “Daitingopterus” has been mentioned in connection to this species, but only as an undefined term in Maisch et al. (2004), where it is a nomen nudum.
[n2] Ingridia was a name that was applied to a series of species formerly within Tapejara, imperator and navigans. However, when originally used by Unwin & Martill (2007), they designated the type species as imperator. Kellner & Campos (2007) had, earlier, also used imperator as the type species to Tupandactylus, and by the margin of time, theirs is the used name, and Ingridia (as useful it might be now for, say, navigans) is subsumed.
[n3] Recently, Hedrick & Dodson revised the taxonomy of the Lujiatun psittacosaurids and concluded that Psittacosaurus lujiatunensis, major, and Hongshanosaurus hui represented the same, ontogenetically variable species. Problematically, Hedrick & Dodson referred to these species as “Psittacosaurus lujiatunensis,” subsuming the earlier-named hui into lujiatunensis, which in the ICZN’s view is a no-no. The correct nomenclature, taking their referral for granted, would be Psittacosaurus hui for all three. However, this raises the additional specter of a new potential praenomen availble for “hui/lujiatunensis,” and so these names cannot be struck from the list as yet.
[n4] Named as a distinct, basal, and small form of Psittacosaurus by Buffetaut & Suteethorn (1992), it has recently been doubted to actually belong to Psittacosaurus, especially by Sereno (2010). It is, apparently, a ceratopsian of low maturity, and thus may not be diagnostic (Sereno, 2010), although age alone does not appear a consistent delimiter of diagnostic value.
[n5] Horner named Brachylophosaurus goodwini in 1988, but has since designated this species as a synonym of Brachylophosaurus canadensis (Horner et al., 2004).
[n6] While designating a new ceratopsian, Xenoceratops foremostensis, Ryan et al. (2012) has designated brinkmani to its own “genus,” Coronosaurus.
[n7] Determined by Longrich (2010) to be nondiagnostic with respect to either Chasmosaurus or Pentaceratops, while also remarking that various features of the frill place this form close to Pentaceratops sternbergi (rather than within Chasmosaurus); he further considered that the material is also consistent with his new taxon, Mojoceratops perifania, along with Eoceratops canadensis, but refrained for synonymizing them. There are difficulties with this, including designation of “nomina dubia” runs into problems when you can also determine it is synonymous with another species, but further than these two taxa have names older than Mojoceratops perifania, in which case priority would seem to designate Eoceratops canadensis as the correct name, designated by Lambe in 1914 (as Chasmosaurus canadensis) and 1915 (as Eoceratops canadensis).
[n8] Mallon et al. (2011) has argued, through extensive sampling, that Anchiceratops longirostris represents variation in Anchiceratops ornatus.
[n9-10] The entire complex of species referred to Omeisaurus and Mamenchisaurus represent, for the most part, single skeletons. No systematic analysis analyzing variation among the skeletons has yet been produced in sorting out what species, or specimens, belong to each of these containers. It is possible they represent a broad grade (or several grades) and may even represent individual variation of smaller sets of species.
[n11] Yeah, that’s right.
[n12] Currently in the process of being redescribed by Drs. Mike Taylor and Matt Wedel (of SV-POW!). A new “genus” may be designated for this.
[n13] Lovelace et al. (2007) differentiated Seimosaurus hallorum from Supersaurus vivianae, but determined that the variation appears to fall into variation between specimens referred (generally) to Diplodocus. While it seems that the complex of Diplodocus includes hallorum, Lovelace et al. (2007) also suggested hallorum could be referred to longus, but did not qualify this.
[n14] Yeah, that’s also right. Arguably, this is the argument I wish Mike Taylor had made in designating Paul’s (1988) “genus” valid, but c’est la vie.
[n15] This one has been in the works so long, it’s not funny anymore. sinensis has been more or less clearly distinguished from Dilophosaurus wetherilli (and in fact, most “coelophysoid” grade theropods) for the greater part of three decades, but systematic evaluation has languished.
Edit: As an additional note, this species has now been synonymized into Sinosaurus triassicus. I am reluctant to agree with this as the basis for the referral is the supposed similarity of a small fragement of bone and teeth.
[n16] The type species of Chilantaisaurus is tashuikouensis (Hu, 1964), and was redescribed by Benson & Xu (2008), then later as a “megaraptoran” (Benson et al., 2010); Hu also described Chilantaisaurus maortuensis, and this was later described as Shaochilong maortuensis as a carcharodontosaurid (Brusatte et al., 2009). “Chilantaisaurus” zheziangensis (Dong, 1974) has yet to be definitely assigned, but it appears to be a therizinosauroid (Barsbold & Maryańska, 1990) and thus quite, quite distinct from other species of Chilantaisaurus.
[n17] These species were assumed to be synonymous with each other by Varricchio (2001), but also in more convoluted arrangements, where Currie & Russell (1988) referred sternbergi to Chirostenotes pergracilis, and elegans as Chirostenotes elegans (while maintaining distinction with Elmisaurus rarus). I maintain (as I do here and here, and following Currie, 1990) that while probable, it is not currently possible to determine wether these two species represent the same form, especially given geographical and stratigraphic separation. Moreover, as Cracraft (1970) argues, sternbergi represents a distinctly different form from that of Caenagnathus collinsi, so if collinsi and pergracilis are synonymous, it represents a wholly distinct taxon, and this may be true for elegans as well, which differs from all pes morphologies referred to Chirostenotes pergracilis to date, and even to rarus.
[n18] referred by Turner et al. (2012) to their type species as variations on a theme.
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