†Henningsmoenicaris scutula, †Sandtorpia vestrogothiensis gen. et sp. nov. and heterochronic events in early crustacean evolution

Abstract

A detailed account of the morphology and ontogeny of the late Middle Cambrian crustacean †Henningsmoenicaris scutula is presented. Ten successive ontogenetic stages could be recognised in the material collected from various localities in Sweden. Morphogenetic changes include the development of a pair of stalked lateral eyes and the increase in the number and size of appendages and their setal armature. Notably, early stages lack ‘proximal endites’ on all post-antennular appendages; such a spine-bearing endite has previously been thought to appear simultaneously on these limbs. In †H. scutula a single functional endite appears on the third limb in an advanced stage; an additional endite appears on the second limb and, subsequently, further endites appear on more posterior limbs. Furthermore, a single specimen of †Sandtorpia vestrogothiensis gen. et sp. nov. is described. Based on this new information and data of other ‘Orsten’ taxa, particularly those assigned already to the early evolutionary lineage of Crustacea, a small-scale computer-based phylogenetic analysis was performed. This resolved the basal branchings of Crustacea s. l. as follows: †Oelandocarididae (=†Oelandocaris oelandica+†H. scutula+†S. vestrogothiensis)+(†Cambropachycopidae (=†Goticaris longispinosa+†Cambropachycope clarksoni)+ (†Martinssonia elongata+Labrophora (=†Phosphatocopina+Eucrustacea))). Plotting ontogenetic data on the phylogram and comparing the ground pattern at every node led to the detection of three peramorphic heterochronic events in the evolutionary lineage towards Eucrustacea.

Three-dimensionally preserved minute larva of a great-appendage arthropod from the early Cambrian Chengjiang biota

“…The minute larva of L. illecebrosa resembles the late-stage metanauplius of certain eucrustaceans, especially anostracans (17), other branchiopods, and cephalocarids (14), in several aspects. First, the anterior (post-SGA segments 1-4) and posterior (post-SGA segments [5][6][7][8][9][10][11][12][13][14] portions of the body differ in outline (rounded vs. narrow and elongated; Fig. 3 and Fig.…”

“…This is not a result of flattening: The outline of soft-bodied fossils is not subject to distortion as they collapse during decay (18). Second, the anterior and posterior portions of the body also differ in the stage of development of the appendages (well developed and presumably fully functional vs. less developed (post-SGA appendages [5][6][7][8] and even rudimentary (post-SGA appendages 9-14); see Figs. 3 and 4 and Fig.…”

“…This hypothesis suggests that the ancestral euarthropod larva had an "antennula" plus three appendages, such as the condition in the early crustacean Henningsmoenicaris scutula (5), which became further derived to a nauplius-like larva in eucrustaceans and pycnogonids with antennula plus two appendages. An alternative, that the nauplius-like larva was ancestral in Euarthropoda, seems unlikely, as early noneucrustacean crustaceans also possessed a hatching larva with the antennula plus three pairs of appendages (5). In either case, however, the phylogenetic position of Leanchoilia (21) supports our conclusion that a larva with just a few anterior limb-bearing segments, which adds segments after hatching [anamorphosis (24)], not only occurs in eucrustaceans (8,14) but was also present in all other major arthropod groups (25)(26)(27), i.e., in the ground pattern of Euarthropoda.…”

“…Paleontology contributes to evodevo by providing information that is only available in fossil organisms (2). Studies of evolutionary development in fossil arthropods, which have dominated faunas from the early Cambrian (∼520 million years ago) to the present, have focused on trilobites (3), "Orsten"-type fossil crustaceans (4)(5)(6), and Mesozoic malacostracan crustaceans (7). Due to their small size and low preservation potential, fossil evidence of the appendages of early developmental stages of arthropods are rare, and known mainly from those with the special "Orsten" type of preservation (8), i.e., with the cuticle secondarily phosphatized, from the mid-Cambrian (500-497 million years ago) (9).…”

The morphology and affinities of Skania fragilis (Arthropoda) from the middle Cambrian Burgess Shale

“…A similar hypothesis is proposed herein. The presence of a posteriorly expansive carapace in other members of the mandibulate stem-lineage, such as Henningsmoenicaris (Haug et al 2010) may indicate this represents the primitive condition of Marrellomorpha. If this is the case then the marrellids have secondarily lost their carapace.…”