No categorical ranks on Wild Prehistory

As the editor-in-chief, I am responsible for the way we present information over at Wild Prehistory. The principles, that I do my best to follow, seem to be quite simple – the articles must be easy to understand, as jargon-free as possible, and without ambiguous and contradicting statements.

Sometimes, it’s really hard to meet all these principles. Indeed, even the best professionals occasionally rely on questionable terminology (just because the terms are deeply rooted in the language of their field, for instance). In some cases, nevertheless, problems can be easily avoided by adhering to a simple rule.

One such rule will be discussed here. Let’s call it “No categorical ranks on Wild Prehistory”.

What is categorical rank?
Have you heard of the subphylum Vertebrata, the class Mammalia, or the order Primates? And what about the family Hominidae? Sure, we have all heard of the categorical ranks but what do we actually know about them?

You probably get the idea – the ranks are used to suggest the hierarchy and to help with the orientation in the system (in this case, the zoological classification). But, does it really help?

No one remembers the ranks
Let’s pretend for a moment that the world is inhabited just by animals (yes, this is the first problem – we do not have a single nomenclatural code governing the use of categorical ranks). We have six main (or primary) categorical ranks that we can use within the animal kingdom: phylum, class, order, family, genus, and species.

Obviously, six ranks are far from enough if we want to use them for all the groups we have named. Thus, the scientists invented the secondary and further ranks. These include superphylum and subphylum, superclass and subclass, superorder and suborder, superfamily and subfamily, supergenus and subgenus, and subspecies. Other ranks are less standardized and are not often used. For instance, a classification of the mammals, published in 1997 by Malcolm C. McKenna and Susan K. Bell, used eight different “order-group” ranks (magnorder, superorder, grandorder, mirorder, order, suborder, infraorder, parvorder).

Needless to say, almost no one bothers to remember all the ranks. And the list is still not exhaustive (for example, we didn’t mention the relatively popular “family-group” ranks tribe and subtribe).

The human classification
If we wanted to put humans within the rank-based system we mentioned above, the classification could look like this:

RankScientific name
SubspeciesHomo sapiens sapiens
SpeciesHomo sapiens
GenusHomo
SubtribeHominina
TribeHominini
SubfamilyHomininae
FamilyHominidae
SuperfamilyHominoidea
ParvorderCatarrhini
InfraorderSimiiformes
SuborderHaplorrhini
OrderPrimates
MirorderPrimatomorpha
GrandorderEuarchonta
SuperorderEuarchontoglires
MagnorderBoreoeutheria
SubclassTheriiformes
ClassMammalia
SuperclassTetrapoda
SubphylumVertebrata
PhylumChordata
SuperphylumDeuterostomia
KingdomAnimalia
DomainEukaryota

The use of particular ranks differ. As noted above, our subspecies is named Homo sapiens sapiens. The names of species and subspecies are always written using lowercase letters and connected with a genus name (Homo), forming a binomen (when only the genus and species names are connected) or a trinomen (when all three names, the genus, species, and subspecies, are connected). The name of our species can also be abbreviated as H. sapiens (the subspecies name would then be H. s. sapiens). In the rank-based animal nomenclature, governed by the International Code of Zoological Nomenclature (or, ICZN), only the names with ranks up to the genus group are italicized. Higher ranks are written regularly.

… And still not enough
The human classification, as presented above, looks pretty impressive and exhaustive, right? I mean, doesn’t it look like it perfectly puts humans within the animal system? Well, maybe, but it’s still heavily incomplete! Let’s have a look at an expanded list, with other frequently used “levels”:

RankScientific name
SubspeciesHomo sapiens sapiens
SpeciesHomo sapiens
GenusHomo
SubtribeHominina
TribeHominini
SubfamilyHomininae
FamilyHominidae
SuperfamilyHominoidea
ParvorderCatarrhini
InfraorderSimiiformes
SuborderHaplorrhini
OrderPrimates
MirorderPrimatomorpha
GrandorderEuarchonta
SuperorderEuarchontoglires
MagnorderBoreoeutheria
CohortPlacentalia
Eutheria
SupercohortTheria
InfralegionTribosphenida
SublegionZatheria
LegionCladotheria
SuperlegionTrechnotheria
InfraclassHolotheria
SubclassTheriiformes
ClassMammalia
Mammaliaformes
Mammaliamorpha
Prozostrodontia
Probainognathia
Eucynodontia
Epicynodontia
Cynodontia
Eutheriodontia
Theriodontia
Neotherapsida
Eutherapsida
Therapsida
Sphenacodontoidea
Sphenacodontia
Eupelycosauria
Synapsida
Amniota
Reptiliomorpha
SuperclassTetrapoda
Stegocephalia
Elpistostegalia
Eotetrapodiformes
Tetrapodomorpha
Rhipidistia
Sarcopterygii
Osteichthyes
Gnathostomata
SubphylumVertebrata
Craniata
Olfactores
PhylumChordata
SuperphylumDeuterostomia
Nephrozoa
Bilateria
Eumetazoa
KingdomAnimalia
Apoikozoa
Filozoa
Holozoa
Opisthokonta
Obazoa
DomainEukaryota

The first table has 24 items, the expanded one has 68. And it still cannot be considered exhaustive as the number changes following important studies or discoveries.

But, at least the rank-based system does help, right?
As you can see above, the expanded list includes some unranked names. There are two reasons for this. First, some of the names were not given any rank. And second, some of the names have ranks already given to others. For example, Therapsida is sometimes given the rank of order, Cynodontia the rank of suborder, Eucynodontia – the infraorder. It’s because the names have long history of differing classifications created by different authors with different approaches. Unfortunately, the ICZN does not govern the names above the family group so it’s unable to force a unified use of categorical ranks.

When one’s looking at the human classification above, it seems to be pretty long and quite complicating but probably still understandable. Let’s not get fooled that the rank-based system has any utility. Comparing to the systematic position of some other groups, the human classification is very simple.

Obviously, all organisms should fit in the system, even those with complicated or unstable evolutionary connections. Let’s consider birds (Aves) this time. For a long time, the position of birds on the Tree of Life was problematic. The only relatively stable thing about their systematics were the ranks. Traditionally, Aves is considered a class. The problems start when we put them within the evolutionary context. The last decades have conclusively proven that birds are deeply nested within theropod dinosaurs that are, on the other hand, deeply nested within “reptiles”. Again, look at an excerpt of the vertebrate rank-based system (Aves to Amniota; compare with Mammalia to Amniota above):

RankScientific name
ClassAves
Ornithurae
Ornithuromorpha
Euornithes
Ornithothoraces
Pygostylia
Avebrevicauda
Euavialae
Avialae
Paraves
Pennaraptora
Maniraptora
Maniraptoriformes
Tyrannoraptora
Coelurosauria
Avetheropoda
Orionides
Tetanurae
Averostra
Neotheropoda
Theropoda
Eusaurischia
Saurischia
Dinosauria
Dinosauriformes
Dinosauromorpha
Ornithodira
Archosauria
Eucrocopoda
Archosauriformes
Crocopoda
Archosauromorpha
Sauria
Neodiapsida
Diapsida
Romeriida
Eureptilia
Sauropsida
Amniota

As can be noticed, the Aves-to-Amniota segment is more than twice as long as the Mammalia-to-Amniota (even though the bird species are more than twice as numerous as the mammals). And again, the list cannot be considered exhaustive…

This time, the ranks – except for the class – were entirely omitted because they are rarely used for this part of the system. It might be interesting to know, nevertheless, that Dinosauria is often considered a superorder, the Saurischia an order, and the Theropoda a suborder. One can ask why are the ranks still in use. Well, the reasons have nothing to do with science. First, such large-scale considerations that easily expose the flaws of the rank-based system are rare. Scientists usually work on relatively small groups of organisms within which the flaws aren’t that obvious. Second, ranks are being used for centuries and people are usually not particularly keen to change their habits. Third, phylogeneticists, the scientists working on the interrelationships of groups of organisms, see and use the system as trees rather then tables as presented above. And so, they don’t actually pay attention to the use of ranks.

Then there are people who state that the system doesn’t need to reflect the actual relationships so they see no problem with placing mammals on the same line as the birds. Today, however, such an argument is incomprehensible. Ranks were invented long before we learnt that all living organisms are related and that we all evolved from a common ancestor. In other words, the ranks were invented to sort the life based on the similarities rather than the evolutionary relationships. As such, the use of ranks can seriously distort our perception of evolution.

So, what’s the approach adopted here on Wild Prehistory?
Only two categories of biological entities are used here: the species and clades. It’s important to note that species is not used as a rank here but rather “a segment of a population-level lineage that is evolving separately from other such lineage segments as indicated by one or more lines of evidence (e.g., distinguishability, reproductive isolation, monophyly, etc.)”. The clades, on the other hand, represent groups of organisms that contain an ancestor and all of its descendants. For example, birds are evolutionarily nested within theropod dinosaurs. Thus, if we don’t consider birds to be dinosaurs, then Dinosauria is not a clade (note that the term is underlined when it appears in each article for the first time; when you move your mouse or put a finger on it, it shows the definition). Naturally, if we are not sure whether or not the group is a clade, we simply call it “group”.

  •  
  •  
  •  
  •  
  •  
  •  

Leave a Reply

Your email address will not be published. Required fields are marked *