Tribrachidium is a monospecific genus of the Ediacara biota that is found globally. In the Flinders Ranges of South Australia, the spatial distribution of Tribrachidium across the seafloor is best described as patchy. Although Tribrachidium is the dominant fossil on two of the twenty-six beds currently excavated and is present in large numbers on another, the genus most commonly occurs as no more than a handful of specimens on a bed. Tribrachidium size frequency distributions of each of the three beds with more than 5 specimens are all statistically distinguishable from one another. Additionally, the size range on any given bed is smaller than the overall size range observed for the genus. These patterns suggest that these organisms lived in populations composed of cohorts. The beds with numerous Tribrachidium come from different facies and are characterized by the presence of dissimilar mixes of taxa and textured organic surfaces, indicating that Tribrachidium was a generalist, able to adapt to a variety environments. Uniquely, the base or internal structure of Tribrachidium is also found preserved in both positive and negative relief as a sequence of concentric ridges on beds where Tribrachidium is the dominant genus. The most parsimonious explanation for the presence of these concentric ridge fossils is that they are the fossilized form of a Tribrachidium preserved when the organism was buried upside-down, flipped over, or partially decayed prior to burial.

The effects of changes to environments on Earth in the past have been well documented, and we have fossil evidence of how organisms responded to these changes. In the past, possible responses to environmental change have ranged from extinction and origination to migration to physical morphological changes. As we are currently facing the prospect of dramatic alterations to Earth’s environments, understanding how life responded in the past may be one of our best opportunities to gauge how future scenarios might play out.

Here I present new data detailing how two different groups of organisms were living in fluctuating environments at two times in Earth’s history. During the Ediacaran there was a unique subset of organisms with three-fold symmetry that went extinct before the beginning of the Cambrian. Although they were relatively abundant and diverse, representing ~12-15% of the taxonomic diversity of the second (White Sea) Ediacaran assemblage, no animals at any point afterwards have had tri-radially symmetric body plans. I show that most of these tri-radial taxa share many morphological and ecological similarities beyond their distinctive body plan and were likely related members of a tri-radial clade. Furthermore, there is evidence that multiple tri-radial taxa were generalists, able to survive in a wide variety of environments, indicating that a relatively sharp environmental change would have been necessary to eliminate this group.

An almost opposite scenario is true for deep-sea ostracodes during Eocene Thermal Maximum 2 (ETM2) in the equatorial and North Atlantic. Using high-resolution sampling from ocean drill cores (ODP Site 1258 and IODP Site U1409) I show that, although this was a relatively smaller event than the preceding Paleocene-Eocene Thermal Maximum, ostracodes respond in noticeable ways. Ostracode diversity and abundance (as measured by accumulation rate) remain low throughout the duration of ETM2. Additionally, the dominant species in these cores exhibit a size decrease that is also correlated with the event. Despite showing these clear signals of having been impacted by ETM2, no ostracode genera disappear completely from either of the sites, indicating that they were able to adapt and survive the environmental changes they faced.