[Paleontology • 2015] Aegirocassis benmoulae • Anomalocaridid Trunk Limb Homology revealed by A Giant Filter-feeder with paired Flaps from the Early Ordovician Fezouata Biota of Morocco

Reconstruction of the giant filter-feeding anomalocaridid Aegirocassis benmoulae from the Early Ordovician (ca 480 million years old) of Morocco feeding on a plankton cloud. Aegirocassis reached a length in excess of 2 m, making it one of the biggest arthropods to have ever lived, and foreshadows the appearance much later of giant filter-feeding sharks and whales.

Illustration: Marianne Collins

Exceptionally preserved fossils from the Palaeozoic era provide crucial insights into arthropod evolution, with recent discoveries bringing phylogeny and character homology into sharp focus. Integral to such studies are anomalocaridids, a clade of stem arthropods whose remarkable morphology illuminates early arthropod relationships and Cambrian ecology. Although recent work has focused on the anomalocaridid head, the nature of their trunk has been debated widely. Here we describe new anomalocaridid specimens from the Early Ordovician Fezouata Biota of Morocco, which not only show well-preserved head appendages providing key ecological data, but also elucidate the nature of anomalocaridid trunk flaps, resolving their homology with arthropod trunk limbs. The new material shows that each trunk segment bears a separate dorsal and ventral pair of flaps, with a series of setal blades attached at the base of the dorsal flaps. Comparisons with other stem lineage arthropods indicate that anomalocaridid ventral flaps are homologous with lobopodous walking limbs and the endopod of the euarthropod biramous limb, whereas the dorsal flaps and associated setal blades are homologous with the flaps of gilled lobopodians (for example, Kerygmachela kierkegaardi, Pambdelurion whittingtoni) and exites of the ‘Cambrian biramous limb’. This evidence shows that anomalocaridids represent a stage before the fusion of exite and endopod into the ‘Cambrian biramous limb, confirming their basal placement in the euarthropod stem, rather than in the arthropod crown or with cycloneuralian worms. Unlike other anomalocaridids, the Fezouata taxon combines head appendages convergently adapted for filter-feeding with an unprecedented body length exceeding 2 m, indicating a new direction in the feeding ecology of the clade. The evolution of giant filter-feeding anomalocaridids may reflect the establishment of highly developed planktic ecosystems during the Great Ordovician Biodiversification Event.

A dorsal view of the fossil alongside a drawing of the specimen. Note the double set of lateral flaps, a discovery that researchers overlooked in previous anomalocaridid fossils.

 Photograph by Peter Van Roy, Yale University; drawing by Allison C. Daley, University of Oxford

Phylum Arthropoda von Siebold, 1848

Order Radiodonta Collins, 1996

Family Hurdiidae Vinther, Stein, Longrich & Harper, 2014

Aegirocassis benmoulae gen. et sp. nov.

Etymology. Ægir: a giant in Norse mythology and god of the sea; cassis (Latin, helmet): referring to the huge size and elaborate cephalic shield; and in recognition of Mohamed ‘Ou Said’ Ben Moula, who discovered the Fezouata Biota and the specimens described here. Gender feminine.

Figure 1: Aegirocassis benmoulae, holotype YPM 237172, Early Ordovician, Fezouata Biota, Morocco.
a–d, Dorsal view: a, part, showing ventral flaps; b, part, separate block in place, showing dorsal flaps; c, interpretative drawing combining part and counterpart; d, part, matrix surrounding dorsal flaps digitally removed to show both sets of flaps. e–g, Lateral view: e, part; f, part, dorsal flaps added from counterpart; g, interpretative drawing combining part and counterpart. h, Part, transverse rods composed of hollow cones of third ventral flap. i, Counterpart, oblique view of anterior free end of setal blades showing lamellae laterally. Arabic numerals indicate trunk somites.

Peter Van Roy, Allison C. Daley and Derek E. G. Briggs. 2015. Anomalocaridid Trunk Limb Homology revealed by A Giant Filter-feeder with paired Flaps.

Nature. (2015) doi: 10.1038/nature14256

Prehistoric 'Sea Monster' Had More Legs Than Thought  http://shar.es/1fvzhZ via @LiveScience

YaleNews | Giant sea creature hints at early arthropod evolution http://news.yale.edu/2015/03/11/giant-sea-creature-hints-early-arthropod-evolution  via @Yale

[Paleontology • 2014] Lyrarapax unguispinus • Brain Structure resolves the Segmental Affinity of anomalocaridid Appendages

Lyrarapax unguispinus

Cong, Ma, Hou, Edgecombe & Strausfeld. 2014

a, b, Dorsal view of Lyrarapax unguispinus YKLP13305 (left side slightly tilted downwards) resolving straight midgut (mg) and sinusoidal alimentary tract (alt). Four neck and eleven trunk segments, the first providing paired oar-like flaps (fl between arrowheads), the last providing the tail fan (tf). Dark areas in the head indicate paired frontal appendage ganglia (frg), optic tract (opt) linking retinas (re) in eyes (ey) to flattened lateral protocerebral lobes (lpr in h) flanking an approximately bilaterally symmetric protocerebrum (pr). Metameric striate areas indicate muscle (m). c–e, Raised and indented grooves of muscle blocks (enlargements of boxed areas in b). f–h, Neural traces: blue digital filter (f) cancels colours in fossil except dark neural regions (for example, medial protocerebrum, mpr) that are resolved by scanning electron microscopy and energy-dispersive X-ray spectroscopy (g), as carbon-rich domains, and shown by oblique illumination relative to eye and head margins (h); bm, basement membrane and first optic neuropil. Raised neck segments gradually obscure caudally directed descending tracts (dt). Scale bars: a, b, 1 cm; c–e, 0.5 mm; f (also for g) and h, 5 mm.

Despite being among the most celebrated taxa from Cambrian biotas, anomalocaridids (order Radiodonta) have provoked intense debate about their affinities within the moulting-animal clade that includes Arthropoda. Current alternatives identify anomalocaridids as either stem-group euarthropods, crown-group euarthropods near the ancestry of chelicerates, or a segmented ecdysozoan lineage with convergent similarity to arthropods in appendage construction. Determining unambiguous affinities has been impeded by uncertainties about the segmental affiliation of anomalocaridid frontal appendages. These structures are variably homologized with jointed appendages of the second (deutocerebral) head segment, including antennae and ‘great appendages’ of Cambrian arthropods, or with the paired antenniform frontal appendages of living Onychophora and some Cambrian lobopodians. Here we describe Lyrarapax unguispinus, a new anomalocaridid from the early Cambrian Chengjiang biota, southwest China, nearly complete specimens of which preserve traces of muscles, digestive tract and brain. The traces of brain provide the first direct evidence for the segmental composition of the anomalocaridid head and its appendicular organization. Carbon-rich areas in the head resolve paired pre-protocerebral ganglia at the origin of paired frontal appendages. The ganglia connect to areas indicative of a bilateral pre-oral brain that receives projections from the eyestalk neuropils and compound retina. The dorsal, segmented brain of L. unguispinus reinforces an alliance between anomalocaridids and arthropods rather than cycloneuralians. Correspondences in brain organization between anomalocaridids and Onychophora resolve pre-protocerebral ganglia, associated with pre-ocular frontal appendages, as characters of the last common ancestor of euarthropods and onychophorans. A position of Radiodonta on the euarthropod stem-lineage implies the transformation of frontal appendages to another structure in crown-group euarthropods, with gene expression and neuroanatomy providing strong evidence that the paired, pre-oral labrum is the remnant of paired frontal appendages.

A spectacularly preserved creature, dubbed Lyrarapax unguispinus, was unearthed in China. The 520-million-year-old sea creature was so well-preserved that parts of its brain and nervous system were clearly defined.

photo: Peiyun Cong

Arthropoda von Siebold, 1848

Radiodonta Collins, 1996

Amplectobeluidae Vinther et al., 2014

Lyrarapax unguispinus gen. et sp. nov.

Etymology. lyra (Latin): referring to an overall lyre-like body shape; rapax (Latin): predator; unguis (Latin): claw; spinus (Latin): thorn, alluding to the spinose, claw-like frontal appendages.

Holotype. Holotype YKLP 13304a, b (Fig. 1 and Extended Data Figs 1a and 2a–d), part and counterpart.

Referred material. Paratypes YKLP 13305 (part only, Figs 2 and 3b, c), YKLP 13306 (part and counterpart, Extended Data Fig. 3).

Locality. Ercaicun (YKLP 13304, 13306) and Mafang (YKLP 13305) in Haikou, Yunnan Province, China.

Horizon. Heilinpu Formation, Cambrian Series 2, Stage 3, Yu’anshan Member (Eoredlichia–Wutingaspis assemblage zone).

Peiyun Cong, Xiaoya Ma, Xianguang Hou, Gregory D. Edgecombe & Nicholas J. Strausfeld. 2014. Brain Structure resolves the Segmental Affinity of anomalocaridid Appendages. Nature. doi: dx.doi.org/10.1038/nature13486

http://www.livescience.com/46830-cambrian-creature-preserved-brain.html