vertebrate species richness change from the late miocene to … · 2017. 9. 13. · msu department...

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INTRODUCTIONTurkana Basin is a central location for discoveries and research in hominid evolution, beginning with Paranthropus bosei, first discovered in 1969 by Richard and Meave Leakey. These discoveries have been crucial to understanding human evolution and corresponding environmental trends in the recent geologic past. However, other mammalian and reptilian fauna found in the region have not been as extensively studied, though non-hominin fossils are found in much higher abundances. Much of the environmental research in the Turkana Basin area has been isotope based. Stable carbon isotopes in tooth enamel have been used to estimate the percentage of C3 and C4 vegetation in hominin and non-hominin diets (Cerling et al., 2011; Cerling, Manthic, et al., 2013; Cerling, Chritz, Jablonski, Leakey, & Manthi, 2013; Uno et al., 2011), while a clumped isotope study using soil carbonates indicated increased temperatures in the Pliocene (Passey et al., 2010).

The purpose of the present study is to ascertain if paleoenvironmental shifts from the late Miocene to early Pliocene are associated with changes in vertebrate (mammals, turtles, and crocodiles) diversity (taxonomic richness). Fossil data were centralized to one location, the Lothagam locality site within the Turkana Basin, to minimize variability. Species richness analyses are compared with paleoenvironmental interpretations from isotope studies to determine if the trends in richness are associated with changes in C4 biomass and/or associated climatic change.

RESULTS DISCUSSIONThe total rarefaction curve, with all taxa, displays a significant decrease between the Lower and Upper Nawata members and between the Apak (5.0 – 4.0 Ma) to Kaiyumung (3.0 – 2.5 Ma) members (Fig. 1A). However, the mammalian curve (Fig. 1B) displays a significant decrease from the Apak to Kaiyumung members, and the reptilian curve (Fig. 1C) displays a significant decline from the Lower (8.0 – 6.5 Ma) to Upper Nawata (6.5 – 5.0 Ma) members. The combination of the rarefaction curves (Fig. 1) and the species richness indices (Tables 1, 2, & 3) gives a clear indication that there was a general decrease of species from the late Miocene to early Pliocene with total species richness dropping noticeably between the Apak and Kaiyumung members. However, mammalian data heavily influenced the results observed in the total specimen dataset, and further discussion will focus only on separate reptilian and mammalian datasets.

• Diversity indices calculated for mammalian data display a significant decrease from the Apak to Kaiyumung members (Tables 1 & 2), which is consistent with the decline in the rarefaction curves (Fig. 1A & 1B). An increase in mammalian diversity is also observed from the Upper Nawata to the Apak members (Tables 1 & 2). Diversity indices for the reptilian data display a significant decrease from the Lower Nawata to the Upper Nawata (Table 3). This decline is also consistent with the decrease seen in the rarefied reptilian data (Fig. 1C).

• Dominance indices for mammalian and reptilian data display an inverse relationship with diversity indices. As diversity decreases, dominance increases resulting in a more uneven distribution. Mammalian dominance increases from the Upper Nawata to Apak members and then decreases from the Apak to Kaiyumung members, with one index observing an increase from the Apak to Kaiyumung data (Tables 1 & 2). Evenness increases from the Upper Nawata to Apak members for mammalian data (Tables 1 & 2). Reptilian dominance increases from the Lower to Upper Nawata members only, while evenness increases from the Apak to Kaiyumung.

Across all four geologic units, both reptilian diversity and dominance appear to shift in the opposite direction than the mammals. In the rarefaction curves, there is a significant decrease in reptile diversity from the Lower to Upper Nawata members but no significant difference in mammalian diversity. Also, a significant decrease in mammalian diversity occurs in the rarefaction curves between the Apak and Kaiyumung members, but no decrease occurs in reptiles (Fig. 1). Reptilian diversity decreases from the Lower to Upper Nawata members, while mammalian diversity decreases from the Apak to Kaiyumung members (Tables 1, 2 & 3). Reptilian dominance increases from the Lower to Upper Nawata members, while mammalian dominance increases from the Upper Nawata to Apak members then decreases from the Apak to Kaiyumung members (Tables 1, 2, & 3). The consistent inverse relationship between mammalian and reptilian metrics shows that reptiles responded differently to environmental changes than mammals in the Turkana Basin.

ACKNOWLEDGEMENTS

I would like to thank Dave Swingle for the opportunity to start this research project as part of my internship with the MSU Department of History, Philosophy and Religious Studies and the Museum of the Rockies. I would also like to thank UNL Department of Earth and Atmospheric Sciences for providing travel funding to present at this conference.

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Devra Hock and Ross Secord

Department of Earth and Atmospheric Sciences, University of Nebraska-Lincoln

Vertebrate species richness change from the late Miocene to early Pliocene of Lothagam, Turkana Basin, Kenya

Fossil specimens were grouped by geologic members and subdivided into mammalian and reptilian groups (Bernor, Raymond L., and Harris, 2003; Feibel, 2003; Harris, John M. and Leakey, 2003; Harris, 2003a, 2003b, 2003c; Harris & Leakey, 2003; Leakey, Meave G., Teaford, Mark F., Ward, 2003; Milledge, 1987; Storrs, 2003; Tassy, 2003; Werdelin, 2003; Weston, 2003; Winkler, 2003; Wood, 2003). Rarefaction was performed using the software “PAST” (Version 3.15; Hammer, Harper, & Ryan, 2001). In addition to rarefaction curves, indices for species richness, diversity, and dominance were calculated for all three data groups and compared against existing environmental interpretations from isotope and geologic data.

METHODS

To gain insights in species richness across the Miocene-Pliocene boundary, the composite fossil fauna is from a single site, Lothagam. Lothagam is an isolated, relatively small, locality site in the southwestern corner of the Turkana Basin, and is the only late Miocene locality in the Turkana Basin (Feibel, 2011). Lothagam consists of the Nachukui (late Miocene) and the Nawata formations (early Pliocene), both composed of alternating mudstone and sandstone beds, with some lava flows present in the middle Nachukui (Fig. 2; Uno et al., 2011). The Nawata ranges in age from ca. 8 to 6.25 Ma using 40Ar/39Ar in feldspar crystals. The Nachakui sits directly on top of the Nawata and ranges in age from 6.25 to 2.5 Ma based on 40Ar/39Ar in feldspar and whole rock K-Ar dating (Uno et al., 2011). The Nawata Formation is divided into the Lower Nawata and Upper Nawata members, and the Nachakui Formation is divided into the Apak, Muruongori, and Kaiyumung members (Fig. 2). The MuruongoriMember is solely comprised of lacustrine strata containing primarily fish fossils, and as such, was excluded from this study.

GEOLOGY

Fig. 1A) Rarefaction (Total Specimens) Fig. 1B) Rarefaction (Mammalian Specimens) Fig. 1C) Rarefaction (Reptilian Specimens)

Figure 1: A) Rarefaction calculated using Total Specimen Data; B) Rarefaction calculated using Mammalian Specimen Data; C) Rarefaction calculated using Reptilian Specimen Data

Table 1

A) Diversity - Total

Upper Nawata Apak p-value Apak Kaiyumung p-value

Simpson's Diversity 0.9432 0.9595 0.0388 0.9595 0.9136 0.0001

Shannon Not significant 3.517 2.902 0.0002

Equitability J 0.8069 0.9037 0.0004 0.9037 0.8533 0.0384

B) Dominance - Total


Simpson's Dominance 5.68E-02 4.05E-02 0.0088625 0.040525 0.086407 0.0060881

Berger-Parker Not significant 0.09783 0.2165 0.0001

Dominance 0.05682 0.04052 0.0388 0.04052 0.08641 0.0001

Evenness 0.444 0.6874 0.0006 Not significant

C) Species Richness - Total


Margalef Not significant 9.204 6.339 0.0435

Table 1: A) Significant diversity indices calculated for the total specimen data; B) Significant dominance indices calculated for the total specimen data;C) Significant species richness indices calculated for the total specimen dataRed-shaded indicates an increase; blue-shaded indicates a decrease in values

Diversity Index Description

Simpson's DiversityValues approaching 0 indicate very low diversity of taxa; Values approaching 1 indicate high diversity of taxa

Shannon's HValue range from 0 to 1; lower values indicates very few taxa; higher values indicate many taxa

Equitability JValues range from 0 to 1; lower values indicate low diversity; higher values indicate high diversity

BrillouinHigher values indicate higher diversity of taxa; lower values indicate lower diversity of taxa

Dominance Index Description

Simpson's DominanceValues approaching 1 indicate a single very dominant taxon; Values approaching 0 indicate no single very dominant taxon

Berger-ParkerHigher values indicate maximal dominance; lower values indicate minimal dominance

Dominance Values approaching 1 indicate a single very dominant taxon; Values approaching 0 indicate no single very dominant taxon

EvennessValues approaching 1 indicate an uneven spread of taxa; Values approaching 0 indicate an even spread of taxa

Species Richness DescriptionMargalef Estimate of number of species present based off logarithm of sample size

Throughout the late Miocene to early Pliocene at Lothagam, mammalian responses appear to drive the trends of the total taxa. During this interval, there is a decline in mammalian species richness from the Apak to Kaiyumung members that was likely driven by the spread of C4 grasslands, with evenness in mammals also decreasing. In reptilian taxa, a sharp decrease in species richness is observed from the Lower to Upper Nawata members, along with a decrease in evenness. We conclude that progressively drier conditions through this interval resulted in lower reptile species diversity and lower evenness. Mammals also show a decline in species diversity through from the Lower to Upper Nawata, but one that is more moderate than in reptiles, and evenness in mammals also declines. These changes were probably caused by drier conditions and the spread of C4 grasslands.

CONCLUSION

MIO

CEN

EP

LIO

CEN

E

Figure 2: Lothagam stratigraphic column displaying geologic epochs, rock units, and associated ages. Modified from Uno et al., 2011

Table 2A) Diversity - Mammals


Simpson's Diversity 0.9307 0.9535 0.0238 0.9535 0.9035 0.0001

Shannon Not significant 3.385 2.779 0.0002

Equitability J 0.7866 0.9 0.0001 0.9 0.8433 0.0292

B) Dominance - MammalsUpper Nawata Apak p-value Apak Kaiyumung p-value

Simpson's Dominance 0.069342 0.046451 0.0031218 0.046451 0.096486 0.0074385

Berger-Parker Not significant 0.1084 0.2308 0.0001

Dominance 0.06934 0.04645 0.0238 0.04645 0.09649 0.0001

Evenness 0.4174 0.6865 0.0003 Not significant

Table 3A) Diversity - Reptiles

Lower Nawata Upper Nawata p-value Apak Kaiyumung p-valueSimpson's Diversity 0.8546 0.7323 0.0298 Not significantShannon 2.111 1.565 0.0104 Not significant

B) Dominance -Reptiles

Lower Nawata Upper Nawata p-value Apak Kaiyumung p-valueSimpson's Dominance 0.14543 0.26768 0.01168 Not significantBerger-Parker 0.2368 0.4394 0.0558 Not significantDominance 0.1454 0.2677 0.0298 Not significantEvenness Not significant 0.7172 0.9165 0.0193

C) Species Richness - ReptilesLower Nawata Upper Nawata p-value Apak Kaiyumung p-value

Margalef 2.749 1.432 0.0033 Not significantMenhinick 1.784 0.8616 0.0032 Not significant



Fig. 2)

vertebrate species richness change from the late miocene to … · 2017. 9. 13. · msu department...

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