The trilobite upper limb branch is a well-developed gill

Abstract

Whether the upper limb branch of Paleozoic “biramous” arthropods, including trilobites, served a respiratory function has been much debated. Here, new imaging of the trilobite Triarthrus eatoni shows that dumbbell-shaped filaments in the upper limb branch are morphologically comparable with gill structures in crustaceans that aerate the hemolymph. In Olenoides serratus, the upper limb’s partial articulation to the body via an extended arthrodial membrane is morphologically comparable to the junction of the respiratory book gill of Limulus and differentiates it from the typically robust exopod junction in Chelicerata or Crustacea. Apparently limited mechanical rotation of the upper branch may have protected the respiratory structures. Partial attachment of the upper branch to the body wall may represent an intermediate state in the evolution of limb branch fusion between dorsal attachment to the body wall, as in Radiodonta, and ventral fusion to the limb base, as in extant Euarthropoda.

The first Silurian trilobite with three‐dimensionally preserved soft parts reveals novel appendage morphology

“…These threads are in the same position and similar in size to the upper and lower marginal channels of the filaments of the Ordovician Triarthrus , as described by Hou et al . (2021), and are interpreted as such. The orientation of the three sabre‐like filaments indicates that they, and the other filaments, are inclined at an angle to the ovoid portion of the exopod (Figs 2M, P, Q, 3B).…”

“…Bruton & Haas (1999), following Manton (1977) and Whittington (1980), interpreted the function for the trilobite exopod as respiratory, as did Hou et al . (2021). The last authors described a closed, chainsaw‐like circulatory system within each filament of Triarthrus , with marginal efferent (dorsal) and afferent (ventral) channels for oxygenated and de‐oxygenated haemolymph, respectively, and diffusion occurring centrally on the filament between the marginal channels.…”

“…Such a connecting membrane was not reported in Triarthrus (Hou et al . 2021). This may reflect the contrast in preservation between replacement by pyrite in Triarthrus versus void filling by calcite in Herefordshire concretions, but in the current material the connecting membrane appears similar in thickness to the central (membranous) part of each filament, and if this were also the case in Triarthrus , the connecting membrane might be expected to have been preserved had it been present.…”

“…The fan‐shaped exopod of Dalmanites is similar to those of phacopide genera and differs from that in Cambrian trilobites such as Olenoides and Eoredlichia (Bergström 1969; Whittington 1975, 1980; Bruton & Haas 1999), and Ordovician Triarthrus (Whittington & Almond 1987; Hou et al . 2021), in which subparallel filaments fall in a more comb‐like fashion from a long exopod shaft that supports a small filamentless, setae‐bearing distal lobe.…”

Exites in Cambrian arthropods and homology of arthropod limb branches

“…The likely attachment of the exopodite of the chelicerates Dibasterium and Offacolus to the body wall rather than to the protopodite noted above may be approximated in trilobites. It has recently been argued that the exopodite of the Cambrian trilobite Olenoides serratus attaches partly to the body wall and partly to the limb base 40 . In conjunction with previous work, which suggests that exopodite and endopodite form by the splitting of the main leg axis 5 , 6 and that exopodites of different arthropod groups originate on different podomeres 7 , these findings reinforce the argument that the trilobite exopodite may not be homologous with the pancrustacean exopodite.…”