Ontogeny and brooding strategy of the early Cambrian arthropod Isoxys minor from the Qingjiang biota

Abstract

†Isoxys is a worldwide distributed bivalved arthropod known almost exclusively from Cambrian Burgess Shale-type Lagerstätten. Outline analyses using 34 specimens of the iconic large bivalved arthropod †Isoxys minor from the Cambrian Stage 3 (~518 Ma) Qingjiang biota and the Cambrian Stage 4 Guanshan biota, interpret that they are the same species and there is a very slight difference in the shape of the outlines of the carapaces between the two biotas. This suggests that environment might be driving intraspecific variation. Quantitative analysis of shape changes during growth using 51 specimens of †I. minor from the Qingjiang biota, reveals that its valves gradually elongate and the ratio of cardinal spines and spherical eyes relative to the valve length significantly decreases during postembryonic development. †I. minor has proportionally large cardinal spines and eyes in the earliest stages, and this allometric growth is beneficial for self-protection and foraging, which may have improved the survival rate of individuals with these characters. In addition, two of the specimens document the evidence of brood care in †I. minor, and the egg cluster occupies almost the entire dorsal region under the carapace. Compared to other early Paleozoic egg-carrying arthropods, †I. minor broods have the highest number (~300 per clutch) of small (Ø, ~0.5 mm) eggs. Since the ovigerous individuals are almost half the size of the adults, †I. minor may have possessed reproductive ability during the early life stage. The results indicate that spines played an antipredatory role for †I. minor, and that it followed an r-strategy of reproducing with many individuals at an early stage. †I. minor also represents the earliest diverging arthropod from which brood care has been documented.

Large euarthropod carapaces from a high latitude Cambrian (Drumian) deposit in Spain

“…Differences in Palaeozoic euarthropod carapace outline shape, morphology, and ornamentation can be attributed to differences in preservation, including deformation (e.g. [ 65 , 72 ]) decay or effacement [ 65 , 73 ], changes during ontogeny [ 74 ], intraspecific variation and dimorphism [ 74 – 77 ], or can be used to diagnose and recognize taxa [ 50 , 54 , 65 , 78 ]. In other euarthropods, including large ones such as radiodonts, carapaces are composed of multiple elements with different shapes [ 10 , 65 – 71 ].…”

Intraspecific variation of early Cambrian (stage 3) arthropod Retifacies abnormalis revealed by morphometric analyses

“…Specimens were separated in to two categories, labelled 'Morph A' and 'Morph B' based on differences in the reticulation (more details in Results below). The violin plot (i.e., box plots made of Kernel density distributions, see Figure 3) of the body length was performed with Power BI visual tool Violin Plot 1.3.0 (Ma et al, 2023). Measurements of the length (sag.)…”

Discovery of diverse Pectocaris species at the Cambrian series 2 Hongjingshao formation Xiazhuang section (Kunming, SW China) and its ecological, taphonomic, and biostratigraphic implications

“…A number of non-trilobite arthropods initially found in the Chengjiang Lagerstätte, including Isoxys , Leanchoilia , Misszhouia , Naraoia , and Xandarella , have recently been found in contemporary Lagerstätten in South China including the Qingjiang ( Fu et al, 2019 ) and Fandian ( Du et al, 2020 ) Lagerstätten. Some bivalved arthropods, such as Isoxys , are found across the Cambrian period, spanning tens of millions of years ( Williams, Siveter & Peel, 1996 ; Hu et al, 2007 ; García-Bellido et al, 2009 ; García-Bellido, Vannier & Collins, 2009 ; Sun et al, 2022 ; Ma et al, 2023 ). In addition to retrieving bio-stratigraphical connections among Lagerstätten, these findings would also indicate that the above mentioned arthropods were ecologically tolerant, or had strong dispersal ability, or that the related lineage (in the case of Isoxys ) successfully evolved through time and adapted to the changing environment.…”