Abstract
We report quantitative analyses of drilling predation on the free-living, tube-dwelling serpulid polychaete Ditrupa arietina from the Cope Cabo marine succession (Pliocene, Spain). Tubes of D. arietina are abundant in the sampled units: 9 bulk samples from 5 horizons yielded ∼5925 specimens of D. arietina. Except for fragmentation, tubes were well preserved. Complete specimens ranged from 3.1 to 13.4 mm in length and displayed allometric growth patterns, with larger specimens being relatively slimmer. Drilled Ditrupa tubes were observed in all samples. Drillholes, identified as Oichnus paraboloides, were characterized by circular to elliptical outline (drillhole eccentricity increased with its diameter), parabolic vertical profile, outer diameter larger than inner diameter, penetration of one tube wall only, narrow range of drill-hole sizes, and non-random (anterior) distribution of drillholes. A total of 233 drilled specimens were identified, with drilling frequencies varying across horizons from 2.7% to 21% (3.9% for pooled data). Many tube fragments were broken across a drillhole suggesting that the reported frequencies are conservative and that biologically-facilitated (drill-hole induced) fragmentation hampers fossil preservation of complete serpulid tubes. No failed or repaired holes were observed. Multiple complete drillholes were present (3.9%). Drilled specimens were significantly smaller than undrilled specimens and tube length and drill-hole diameter were weakly correlated. The results suggest that drillholes were produced by a size-selective, site-stereotypic predatory organism of unknown affinity. The qualitative and quantitative patterns reported here are mostly consistent with previous reports on recent and fossil Ditrupa and reveal parallels with drilling patterns documented for scaphopod mollusks, a group that is ecologically and morphologically similar to Ditrupa. Consistent with previous studies, the results suggest that free-dwelling serpulid polychaetes are preyed upon by drilling predators and may provide a viable source of data on biotic interactions in the fossil record.
Highlights
This study documents drilling predation patterns on the freeliving, tube-dwelling serpulid polychaete Ditrupa arietina from the Pliocene of southeastern Spain
Free-living serpulids occur in many marine benthic ecosystems and are known from the fossil record
Drillers have produced a rich fossil record of ecological interactions, with quantifiable data spanning from the Ediacaran [4,5] to the Holocene [6,7,8]
Summary
This study documents drilling predation patterns on the freeliving, tube-dwelling serpulid polychaete Ditrupa arietina from the Pliocene of southeastern Spain. Free-living serpulids occur in many marine benthic ecosystems and are known from the fossil record. Currently underutilized, the shellbearing serpulids are potentially an important source of quantifiable data on biotic interactions between polycheates and drilling predators, past and present. Morton and Harper [43] provide diverse qualitative and quantitative data that represent a valuable baseline for paleontological studies because D. arietina (and morphologically similar species) are known from multiple localities in the Cenozoic fossil record [41,44,45]. We present qualitative and quantitative analyses of a series of bulk samples collected vertically along the Cope Cabo outcrop (Murcia Region, SE Spain), which represents a well-developed succession of PlioPleistocene marine and continental sediments
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