Decomposition of dinosaurian remains inferred by invertebrate traces on vertebrate bone reveal new insights into Late Jurassic ecology, decay, and climate in western Colorado

Abstract

A survey of 2,368 vertebrate fossils from the Upper Jurassic Mygatt-Moore Quarry (MMQ) (Morrison Formation, Brushy Basin Member) in western Colorado revealed 2,161 bone surface modifications on 884 specimens. This is the largest, site-wide bone surface modification survey of any Jurassic locality. Traces made by invertebrate actors were common in the assemblage, second in observed frequency after vertebrate bite marks. Invertebrate traces are found on 16.174% of the total surveyed material and comprise 20.148% of all identified traces. Six distinct invertebrate trace types were identified, including pits and furrows, rosettes, two types of bioglyph scrapes, bore holes and chambers. A minimum of four trace makers are indicated by the types, sizes and morphologies of the traces. Potential trace makers are inferred to be dermestid or clerid beetles, gastropods, an unknown necrophagous insect, and an unknown osteophagus insect. Of these, only gastropods are preserved at the site as body fossils. The remaining potential trace makers are part of the hidden paleodiversity from the North American Late Jurassic Period, revealed only through this ichnologic and taphonomic analysis. Site taphonomy suggests variable, but generally slow burial rates that range from months up to 6 years, while invertebrate traces on exposed elements indicate a minimum residence time of five months for carcasses with even few preserved invertebrate traces. These traces provide insight into the paleoecology, paleoclimate, and site formation of the MMQ, especially with regards to residence times of the skeletal remains on the paleolandscape. Comprehensive taphonomic studies, like this survey, are useful in exploring patterns of paleoecology and site formation, but they are also rare in Mesozoic assemblages. Additional work is required to determine if 16.174% is typical of bulk-collected fossils from Jurassic ecosystems in North America, or if the MMQ represents an unusual locality.

A European giant: a large spinosaurid (Dinosauria: Theropoda) from the Vectis Formation (Wealden Group, Early Cretaceous), UK

“…Several other necrophagous insect groups can be excluded from consideration ( Bader, Hasiotis & Martin, 2009 ; Cruzado-Caballero et al, 2021 ; Xing et al, 2013 ): hymenopterans and isopterans typically produce star-shaped features and isopterans tend to cause more widespread, irregular damage, rather than tunnels ( Hutchet, 2014 ); tineid moths (Lepidoptera) specialise in keratinous tissues and traces made by them have yet to be identified in the fossil record; and the burrows of mayfly (Ephemeroptera) larvae are typically narrow, U-shaped, thin walled, and limited to aquatic environments. Damage by other aquatic organisms such as burrowing bivalves are also improbable given the taphonomic circumstances and the curved form of the structures (such molluscs usually produce clavate-shaped borings; McHugh et al, 2020 ), whilst the parallel-sided morphology with lack of splitting makes plant root damage unlikely ( Rogers, 1992 ).…”

Assessing Paleoecological Data Retention Among Disparate Field Collection Regimes: A Case Study at the Mygatt-Moore Quarry (Morrison Formation)

“…Prior to 2016Prior to (1984Prior to -2015, collection protocols varied among the multiple collectors who ran the site, some with taphonomically focused collection priorities and others emphasizing selective sampling of more complete, taxonomically informative specimens. The 2016-2019 collection by the lead author was part of an assemblage-wide survey of bone surface modifications at the MMQ, the results of which have been published elsewhere (Drumheller et al 2020;McHugh et al 2020). Here, we evaluate the BSM data recovery between these two collection periods and propose a new methodology to better quantify frequencies of taphonomic activity at a vertebrate fossil site.…”

“…However, other data are also affected. Trace fossils found on the vertebrate bone material, or bone surface modifications (BSMs), are a key source of paleoecological data that are potentially vulnerable to selective sampling (Drumheller et al 2020; McHugh et al 2020). Modified bone surfaces record species interactions and environmental conditions prior to fossilization, providing critical sources of taphonomic and paleoecologic data (Behrensmeyer and Kidwell 1985; Fisher 1995; Behrensmeyer et al 2000).…”

“…Modified bone surfaces record species interactions and environmental conditions prior to fossilization, providing critical sources of taphonomic and paleoecologic data (Behrensmeyer and Kidwell 1985; Fisher 1995; Behrensmeyer et al 2000). However, the widespread practice of preferentially collecting more complete, less modified, and aesthetically better-preserved vertebrate fossils can bias sampling against the recovery of BSM data (Tang 2000; Drumheller et al 2020; McHugh et al 2020). This potentially undermines large-scale, synthetic analyses of ancient ecological interactions that are based on modified bone surfaces.…”

“…More highly modified, or seemingly damaged, fossils may be disregarded in the field or, if collected, subsequently are rejected for permanent accession into research collections. This practice, while often logistically and financially unavoidable, skews initial data collection and further undermines future analyses (Tang 2000; Drumheller et al 2020; McHugh et al 2020). With over 30 years of collection under multiple different researchers at the Mygatt-Moore Quarry, it is unclear how periods of selective sampling at the site influenced the overall composition of the resulting collection.…”