ANALYSIS OF MEASURED THALASSINOIDES ISP. DIAMETERS ACROSS THE CRETACEOUS-PALEOGENE BOUNDARY AT SEWELL, NJ (USA) USING GAUSSIAN CURVE-FITTING

The Inversand Quarry at Sewell (New Jersey, U.S.A.) contains a heavily bioturbated, glauconite-rich marine succession of the Navesink (NF) and overlying Hornerstown (HF) Formations and preserves one of the best exposed Cretaceous-Paleogene (K-Pg) sections along the Atlantic coastal plain. The lower part of the Hornerstown Formation contains a bonebed, the Main Fossiliferous Layer (MFL), with remains of Cretaceous marine reptiles, turtles, and birds, interpreted as a thanatocoenosis. Thalassinoides isp. morphology and dimensions were assessed across the K-Pg boundary. Mean burrow diameters decrease abruptly by 26 - 29% (n=1767) and provide an example of the Lilliput effect. "Lilliput effect" refers to extinction selectivity for small sizes (phyletic dwarfism), as well as small-sized organisms preferentially surviving the extinction and replacing larger-sized taxa. Our new examination of histograms suggests that there were two size populations of trace makers in the Cretaceous NF, lower HF and MFL, but only one size population in the post-extinction portion of the HF. As a proxy for thalassinid body size, the burrow diameters were binned at 0.5 cm intervals and analyzed using a Gaussian curve fitting program to fit the histograms assuming one or two populations and burrow diameters of populations being normally distributed. Results of Gaussian fitting indicate that there were two burrow populations (Population 1 diameter: 13.3 ± 3.4 mm, 60% of total population; Population 2: 20.6 ± 3.0 mm, 40%, R² = 0.90) in the NF, lower HF and MFL, but only one (11.0 ± 3.6 mm, R² = 0.96) in the upper HF. This confirms the conclusion that Paleocene Thalassinoides tracemakers burrowed into the underlying Cretaceous units. This analysis also demonstrates that, for this particular dataset, the Lilliput effect was driven by smaller tracemakers preferentially persisting after the event.