| Decapoda | ||
| Arthropoda | Meiura |
| Arthropoda | Decapoda
└─► |
|||
| Malacostraca | None |
Hexapoda |
|
Abbreviated Dendrogram
Malacostraca
│
└─Decapoda
├─Dendrobranchiata
└─┬─Caridea
└─Reptantia
├─Eryonoidea
└─┬─Eureptantia
├─Palinuroidea
└─Meiura
├─Platykotta
└─┬─Eocarcinus
└─┬─Crown Anomura
└─Brachyura
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Contents
Overview |
Taxa on This Page
Meiura is one of only two new decapod clades agreed on by the only two comprehensive morphological cladistic analyses of the group to date (Scholtz & Richter 1995 and Dixon et al 2003), the other being Eureptantia. The Meiura is the cade that includses both the anomurans or crab-like decapods and the true crabs. Molecular analysis doesn't support this, which shows that the anomura and the brachyura diverged very soon after evolving from a lobster-like ancestor; in too short a time to be resolved by molecular sequencing phylogeny.
Two taxa are currently known which together span the Triassic-Jurassic boundary and are very close to the Meiuran origin. These are Platykotta the Eocarcinus. Depending on your choice of hypotheses, they are either stem crabs, stem anomura, or (no-one seems to have suggested this so we are admittedly being rather presumptious here, but being editor of a huge paleo website does confer certain privelages) representatives of an early meiuran lineage or lineages that don't belong to either, but continued alongside them, at least to the early Jurassic. If this latter is the case, than Triassic and early Jurassic decapods were far more diverse than existing forms, with many transitional types existing alongside the more specialised or derived taxa that continued to the present. This owuld be a somewhat smaller repat of teh Cambrian and even the early to mid Paleozoic, when there wrere many transitional ecdysozoans around that were intermediate between the extant phyla. MAK120529
Meiura Scholtz & Richter, 1995
From the Triassic
Phylogeny: Either Eureptantia : Astacura + Palinuroidea + Thalassinidea + * : Stem Anomura + Brachyura
or Eureptantia : Astacura + Palinuroidea + Thalassinidea + * : Platykotta + (Eocarcinus + (Brachyura + Anomura))
Comments:. The first alternative phylogeny implies a stem and crown Anomura, the second a stem and crown Meiura MAK120529.
From the Triassic
Phylogeny: Meiura : Brachyura + * : Platykotta + (Eocarcinus + Crown Anomura)
Comments:. The extra taxon is to accommodate stem taxa Platykotta and Eocarcinus, assuming these taxa are basal Anomura (Feldmann & Schweitzer 2010). Of course they could equally turn out to be basal Meiura . Hence a few of these entries have a choice of phylogenies MAK120528.
Horizon: Late Norian to Rhaetian from the Ghalilah Formation on the Musandam Peninsula, United Arab Emirates (Latest Triassic)
Phylogeny: Either Stem Anomura : (Eocarcinus + Crown Anomura) + *
or Meiura : (Eocarcinus + (Brachyura + Anomura)) + *
Comments: broad, triangular sternum, unlike the narrow sternum of Astacidea and Glypheoidea, but similar to that of Palinuroidea (which would support Dixon et al 2003's hypothesis of Achelata as stem Meiura (clade Eurysternalia)), and to lobster-like Anomura. Unlike other anomurans the carapace is considerably longer than wide (Wikipedia). Somewhat more "lobster-like" (hence more basal?) than Eocarcinus, but otherwise very similar to it, and placed with it in the Superfamily Eocarcinoidea (Chablais et al 2011). If however one taxon is more basal than another, than the Eocarcinoidea becomes a paraphyletic assemblage of basal Anomura (or even basal Meiura) defined by various mosaic traits. MAK120528.
Eocarcinus praecursor Withers, 1932
Early Jurassic (Pliensbachian) of England
Phylogeny: Either Stem Anomura : Platykotta + (Crown Anomura + * )
or Meiura : Platykotta + ((Brachyura + Anomura) + * )
Comments: Depending on your preferred hypothesis, this is either the oldest true crab (e.g. Förster, 1985, Guinot & Tavares 2001, De Grave et al 2009), transitional between the Glypheoidea (specifically the Middle Triassic Pseudopemphix) and the Middle Jurassic crabs in the Homolodromioidea, especially the earliest known species, Eoprosopon klugi of the Late Pliensbachian (Krobicki & Zaton 2008), or else it is not be a crab at all, but an early member of the Anomura (Feldmann & Schweitzer 2010; Chablais et al 2011). Is one right and one wrong, or is it a matter of inputting different data resulting in different results. Or are they in a sense both right, and Eocarcinus (and hence also Platykotta) is a basal Meiuran rather than an early Anomuran or Brachyuran. MAK120529
Links: Wikipedia
page MAK120529