Unexpected Resilience of Species with Temperature Dependent Sex Determination at the
Cretaceous-Paleogene Boundary
Sherman Silber
1
, Jonathan H. Geisler
2*
, Minjin Bolortsetseg
3
1 Infertility Center of St. Louis, St. Luke’s Hospital, St. Louis MO 63017, USA
2
New York College of Osteopathic Medicine, Old Westbury, NY 11568, USA
3
Institute for the Study of Mongolian Dinosaurs, Ulaanbaatar, 14201, Mongolia
*Author for correspondence (jgeisler@nyit.edu)
Supplementary Background on Extinction Studies Using the Hell Creek Formation
Previous attempts to test hypotheses for the K-Pg mass extinction using taxa from the Hell Creek and Tullock Formations have typically focused on the rate and timing of extinctions. The impact hypothesis predicts that extinctions should appear at or immediately after geochemical markers of the impact, except for confounding factors such as the Signor-Lipps effect (Signor and Lipps, 1982). By contrast, the volcanic hypothesis and particularly the hypothesis that invokes a global regression in sea level predict that the extinctions occurred over a protracted period of time and initiated before the bolide impact (Archibald, 1996). If both the impact of a bolide and eruptions associated with continental flood basalts played a role in the mass extinction, as was supported in a recent study on all mass extinctions in the Mesozoic and Cenozoic (Arens and West, 2008), then one would expect a period of elevated extinctions followed by an brief but intense period of even more extinctions immediately after the impact.
Non-avian dinosaurs show no apparent decline in diversity throughout the Hell
Creek Formation when sampled at the familial (Sheehan et al., 1991) or at the species level (Pearson et al., 2002), and the uppermost 3 meters of the Hell Creek Formation in some, but not all areas, has a similar abundance of dinosaurs as lower parts of that formation (Sheehan et al., 2000; Pearson et al., 2002). Likewise, other groups of vertebrates from the Hell Creek Formation show no decline in diversity (Pearson et al.,
2002), and this basic pattern is also found in the time equivalent Ferris Formation of
Wyoming (Lillegraven and Eberle, 1999). A more detailed study on mammals from the
Hell Creek Formation (i.e. Wilson, 2005) indicates small changes in mammal diversity through time that correlate with global temperatures, but similar to other studies, there was no overall decline in mammal diversity throughout the temporal range of the Hell
Creek Formation.
Archibald and Bryant (1990) and Archibald (1996) took a different approach to using the fossil record of the Hell Creek Formation to test extinction hypotheses. They focused on differential survival to distinguish among alternate extinction hypotheses, which requires that one estimate the environmental tolerance of extinct taxa as well as the environmental effects of various postulated causes (e.g. volcanic eruptions, impacts). In his extensive treatment of the Hell Creek Formation, Archibald (1996) determined that a marine regression explains about twice as many extinctions and survivals as the impact or volcanic hypothesis. However, as he noted, his results are complicated by the fact that many of the environmental effects of a large impact or the effects of massive volcanic

eruptions remain speculative. For example, in their original description of the impact hypothesis, Alvarez et al. (1980) focused on how dust generated by an impact would block much sunlight, leading to a collapse of many ecosystems. In a more recent study,
Pope (2002) found that very little of the material in the K-Pg boundary clay can be attributed to dust-size, clastic debris, suggesting that that there was not enough dust in the atmosphere after the impact to shut down photosynthesis. Although we think that the killing mechanisms of impacts and other environmental catastrophes need to be better understood to more fully test extinction hypotheses, in the present study, we focus on intrinsic properties of Cretaceous taxa, not recorded or speculated changes in their environment.
Supplementary Detailed Methods
The basic method employed in the current study is similar to that of Archibald
(1996). The overall explanatory power of the TSD (Temperature Dependent Sex
Determination) hypothesis is determined by counting the number of taxa whose extinction or survival is correctly predicted by the TSD hypothesis. Taxa that have TSD are predicted to have gone extinct, whereas taxa that have GSD (Genetic Sex
Determination) are predicted to have survived. Unlike Archibald (1996) who focused class, suborder, or similarly inclusive clade, in the present study, taxa were tabulated at the species level. Whereas at first glance this may seem problematic because closely related species are more likely to be similar in biology and behavior, and thus may not represent truly independent tests, the fact that higher-level taxa did not have a uniform response at the K-T boundary (Archibald, 1996) and that SDM's (Sex Determining
Mechanisms) have had a complicated evolutionary history within some higher-level taxa
(e.g. Squamata; Pokorná and Kratochvíl, 2009), supports our approach.
Survivorship at the K-Pg boundary in Eastern Montana was based primarily on table 5.1 of Archibald (1996), which is the latest version of the Cretaceous/Paleocene dataset published by Archibald and his colleagues (Archibald and Bryant, 1990). Minor changes were made to table 5.1 of Archibald (1996) to incorporate later work on turtles
(Holroyd and Hutchison, 2002; Lyons and Joyce, 2009a, b; Joyce et al., 2009), mammals
(Clemens, 2002) and dinosaurs (Currie et al., 2003; Horner and Goodwin, 2009).
In the present study, we inferred the SDM of Hell Creek tetrapods by mapping three alternate character states for SDM (XX/XY, ZZ/ZW, and TSD) onto a phylogeny including extant taxa and Hell Creek species in MacClade vers. 4.08 (Maddison &
Maddison, 2003). All most parsimonious reconstructions were considered for each internal branch, and SDM was treated as unordered character. Phylogenetic relationships of extant tetrapods were based on a modified version of the tree employed by Organ and
Janes (2008). Unlike their tree, Aves and Serpentes were treated as single operational taxonomic units, amphibians were added based on the phylogeny of Frost et al. (2006), turtles were placed as basal sauropsids as supported several studies (e.g. Hill, 2005;
Werneburg and Sanchez-Villagra, 2009), squamate phylogeny was based on Townsend et al. (2004), and Gekkonidae and Pygopodidae were collapsed into the terminal taxon
Gekkota because of uncertainty regarding the phylogeny within this group (Pokorna and
Kratochvil, 2009). Like Organ and Janes (2008), the SDM of extant taxa was based primarily on the Chromorep database (Olmo, 2005), although we followed Pokorna and

Kratochvil (2009) in disregarding poorly supported reports of TSD in the squamates from
Lacertidae, Varanidae, and Scincidae and Valenzuela (2004) in ignoring reports of TSD in Iguanidae. SDM data for amphibians came from Hillis and Green (1990). Extinct taxa were placed on the phylogeny of extant taxa based primarily on morphological studies, and where possible studies that combined morphological and molecular data
(Supplementary Table 1).
Of the 84 tetrapod taxa listed for the Hell Creek Formation, the SDM’s of 62 of them could be reasonably reconstructed. The SDM of the remaining taxa is ambiguous, and their placement on the tree of extant taxa is shown in the supplementary figure. We also investigated the sensitivity of our findings to alternate tree topologies as well as to the inclusion of reports of TSD in extant taxa that we excluded in our primary analysis
(see above). Table 2 indicates how our results vary on 6 alternate topologies, including the tree of Organ and Janes (2008). We also mapped the Organ and Janes (2008) SDM dataset on their tree, and those results are also shown in Table 2. The other trees we investigated include different placements for turtles (see Werneburg and Sanchez-
Villagra, 2009 for studies that support these alternate topologies). As Table 2 illustrates, including additional reports of TSD in squamates or mapping SDM data on alternate tree topologies has little effect on our overall conclusion: the TSD hypothesis does not adequately predict the extinctions or survivals of the majority of Cretaceous tetrapods from the Hell Creek Formation across the Cretaceous-Paleogene boundary.
Phylogeny
# of …
TSD taxa
XX/XY taxa
Sister-group to Turtles is…
Mammalia
17
27
ZZ/ZW taxa
Unspec. GSD
13
5
Uncertain SDM 21
Conclusive 63
Tests
Correct
Predictions
Correct
Predictions (%)
21
33.3
Lepidosauria
16
27
13
5
22
62
20
32.3
Aves
16
27
13
5
22
62
20
32.3
13
5
22
62
Crocodylia Other
Amniotes
16
27
17
27
13
5
21
63
20
32.3
21
33.3
Organ
Janes
(2008) tree
16
31
13
1
22
62
20
32.3
Same with their SDM dataset
16
31
8
1
27
57
18
31.6
Table 2: Effect of alternate tree topologies and the Organ and Janes (2008) SDM dataset on our analysis. In our preferred analysis, which is described in the main text and displayed in supplementary Table 1, the TSD hypothesis only explains 34% of the extinctions and survivals of Hell Creek taxa across the Cretaceous-Paleogene boundary.
As seen above, this number is nearly the same when SDM’s are mapped on alternative trees or if the Organ and Janes (2008) SDM dataset is used without modification.
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