Macrochelys apalachicolae, Thomas, Travis M., Granatosky, Michael C., Bourque, Jason R., Krysko, Kenneth L., Moler, Paul E., Gamble, Tony, Suarez, Eric, Leone, Erin, Enge, Kevin M. & Roman, Joe, 2014

Thomas, Travis M., Granatosky, Michael C., Bourque, Jason R., Krysko, Kenneth L., Moler, Paul E., Gamble, Tony, Suarez, Eric, Leone, Erin, Enge, Kevin M. & Roman, Joe, 2014, Taxonomic assessment of Alligator Snapping Turtles (Chelydridae: Macrochelys), with the description of two new species from the southeastern United States, Zootaxa 3786 (2), pp. 141-165 : 151-160

Macrochelys apalachicolae sp. nov.

Common name. Apalachicola Alligator Snapping Turtle

Holotype. UF 3998, partial skeleton from the Apalachicola River, Gadsden County, Florida, on 4 April 1953 by the Florida Museum of Natural History (see Figures 13 , 14 ). (Central lineage; Figure 9 ).

Paratypes. UF 52676, partial skeleton from Waddells Mill Creek, Jackson County, Florida, on 10 April 1978 by L. Richard Franz et al.; UF 152479 skull from Econfina Creek, Bay County, Florida (30.15274o N, 85.55748o W, elev. 2 m, 13.1 m depth), on 21 August 1982 by Joseph P. Ward and Joseph J. Ward.

Diagnosis. Macrochelys apalachicolae is distinguished by the following: carapacial caudal notch narrow and triangular or narrow and U-shaped ( Figure 13 ), relatively shallow, and reduced; posterior projection of the squamosal globular and obtusely angled in lateral aspect (Figure 5,14); pygal with two serrations, with medial suture; peripheral 11 with one serration; distal rib end of costal 1 enters posterior third of peripheral 3; pleural scute set 1 with slight to no overlap onto the nuchal; processus trochlearis oticum relatively straight with a single distal protuberance; posterior margin of squamosal-opisthotic contact relatively straight in dorsal aspect.

Comments. Although there is a general pattern of small triangular pygal regions of the carapace, there is observable variation within the species. All cranial specimens are characterized by large, globular squamosal projections that are intermediate between those of M. suwannensis and M. temminckii . Although M. apalachicolae is genetically most similar to M. temminckii , in some ways it is morphologically more similar to M. suwanniensis ; they share the unique synapomorphy of a sutured pygal. Macrochelys apalachicolae is somewhat morphologically intermediate between M. temminckii and M. suwannensis with regard to carapacial caudal notch proportions. The degree of overlap of pleural 1 onto the nuchal also suggests this (usually lying on or just anterior to the nuchalcostal 1 suture), as does a pygal that possesses two serrations (a western character) that is typically sutured medially (a character found in M. suwannensis ).

Distribution. Restricted to river drainages bounded by the Choctawhatchee and Ochlockonee rivers in Florida, Georgia, and Alabama.

Etymology. Specific epithet refers to the new Latin apalachicol– (referring to the Apalachicola River) and the Latin –ae (treating the name of the river as a Latin cognate in the First Declension, genitive case), combined to form the composite noun apalachicolae .

Fossil record. The earliest fossil representatives of Macrochelys in Florida are from the early Miocene, early Barstovian NALMA, ca. 15–16 Ma. These fossils are fragmentary and consist of a partial costal 8 (UF-Vertebrate Paleontology [VP] 259076) and partial hyo- and hypoplastron (UF-VP 259077). Although difficult to ascribe to the species level, they are contemporaneous with Macrochelys stricta ( Matthew 1924) from the early Barstovian of Nebraska.

Additional Macrochelys specimens are not observed in Florida until the late Miocene, early Hemphillian NALMA, ca. 8–9 Ma, with the occurrence of Macrochelys auffenbergi ( Dobie 1968) , which is represented by fairly complete material from the McGehee Farm locality in Alachua County. We reexamined these type specimens, as well as previously undescribed specimens of M. auffenbergi , to diagnose the species based on shell and skull characters and to distinguish it from extant Macrochelys . In M. auffenbergi , the nuchal and cervical are relatively narrow; pleural 1 does not contact the nuchal (shared with western M. temminckii and Chelydra ); pygal is much longer than wide, with two serrations and a very narrow caudal notch; pygal lacks medial suturing and is keeled along vertebral 5; epiplastra are relatively wide and lobate (these are long and slender in extant Macrochelys ); and sulcal impressions for scutes on the plastron are distinct and deeply incised (these scales are very thin and their impressions faint to lacking in extant Macrochelys ). In extant Macrochelys , the nuchal and cervical are wide; Pleural 1 does (in both M. apalachicolae and M. suwanniensis ) and does not (in M. temminckii ) contact the nuchal; the pygal is much wider than long (in both M. apalachicolae and M. suwanniensis ); the epiplastra are very narrow; and the plastron lacks well-defined scute sulcal impressions. The skull of M. auffenbergi , although relatively large, does not exhibit the extraordinary megacephaly expressed in extant Macrochelys . The relative head size is much smaller in M. auffenbergi than in modern Macrochelys , and in that way the fossil taxon is plesiomorphic. The triturating surfaces of the skull and mandible are slender and not as expanded as seen in extant Macrochelys , perhaps an indication that the fossil taxon was less durophagous (eating fewer hard-shelled organisms) than the extant Macrochelys . Increase in head size through time appears to correlate with a decrease in plastral forelobe width within Macrochelys .

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published. timeline of characteristics Paleontological identifying and, 15. age FIGURE geological Macrochelys next appears in Florida from Polk County, from the Widden Creek and Palmetto faunas of the Bone Valley Formation, late Hemphillian, latest Miocene-earliest Pliocene ( Meylan 1995). These fossils are fragmentary and occur well south of the current range of Macrochelys . The isolated fossil elements are difficult to identify at the species level; however, the Bone Valley taxon appears larger than M. auffenbergi and more comparable in size to extant specimens. A few features indicate that the fossil taxon is somewhat intermediate in morphology between M. temminckii and M. auffenbergi . These features include having a pygal that is only slightly longer than wide or almost as wide as long (in M. auffenbergi , the pygal is much longer than wide, and in the extant clade, it is much wider than long), and relatively long slender dentaries without overlying expanded triturating surfaces (also seen in M. temminckii and M. auffenbergi ).

Macrochelys fossils are relatively common in late Blancan to Recent (from ca. 2.5 Ma) fluvial and estuarine deposits in Florida. Most of these fossils are fragmentary. Records include:

1) Late Blancan (ca. 2.5 Ma) US 19 bridge site from the Suwannee River, Gilchrist County. A pygal (UF-VP 247166) from this locality is wider than long with two serrations and unsutured medially. The dentaries (UF-VP 247163–247165) are generally slender as in M. auffenbergi and M. temminckii .

2) Late Blancan (ca. 2.5 Ma) Haile 15A locality, Alachua County. A nuchal (UF-VP 259613) possesses no Pleural 1 sulci dorsally, the same condition as in M. auffenbergi and M. temminckii .

3) Latest Blancan (ca. 2 Ma) De Soto Shell Pit locality (pits 1 and 3A), De Soto County. Records from this locality are farther south than the current range of the genus. A pygal (UF-VP 240915) from De Soto 3A is wider than long with two serrations and unsutured medially.

4) Early Irvingtonian (ca. 1.6-1.0 Ma) ( Morgan & Hulbert 1995; Meylan 1995) Leisey Shell Pits (sites 1, 1B, and 3B), Hillsborough County. Both Macrochelys and Chelydra occur at this locality, and we feel that there is some confusion with regard to Meylan’s (1995) Macrochelys vouchers. Some of the vouchers represent other taxa (e.g., UF-VP 84005 from pit 1A is half of an emydid bridge peripheral), including a giant Chelydra species (e.g., a partial peripheral UF-VP 81198 from pit 1A and a partial shell UF-VP 125099 from pit 2). Meylan (1995:285) regarded some of his chelydrid identifications as tentative, being aware of an unnamed contemporaneous giant Chelydra (see below for discussion of Chelydra species). Also, the Macrochelys left dentary (UF-VP 116093) reported from pit 3A is actually from pit 3B. The Macrochelys fossils occur south of the extant range and are significant in that the dentaries are very robust with expansive triturating surfaces like those in modern M. suwanniensis .

5) Latest Irvingtonian to earliest Rancholabrean (ca. 0.3 Ma) Oldsmar locality, Pinellas County ( Meylan 1995). UF-VP 135629 represents a partial posterior carapace with the pygal region well preserved. As with the Leisey specimens, this specimen most closely resembles M. apalachicolae and M. suwanniensis . The pygal is much wider than long, is sutured medially, and possesses two serrations, the condition most typically seen in the central assemblage.

6) Numerous late Pleistocene Rancholabrean NALMA records exist from Florida, including: Jug Springs, Ichetucknee River, Columbia County ( Auffenberg 1957); Suwannee River sites; Hornsby Springs, Santa Fe River, Alachua County; and Aucilla River 1A, Taylor County; as well as extralimital occurences from Wekiva Spring, Levy County; Rock Springs, Orange County; Oklawaha 1, Oklawaha River, Marion County; and Buzzard Island, Putnam County ( Meylan 1995). Most of the Rancholabrean fossils are difficult to assess at the species level due to their incompleteness. Specimens from the Ichetucknee River (Suwannee River drainage), including some previously discussed by Auffenberg (1957), consist of nearly complete shells and skulls (e.g., UF-VP 259848, UF- VP 259849, and UF-VP 259842). However, the squamosal and pygal regions are not preserved in these fossils. In UF-VP 259848 and UF-VP 259849, peripheral 11 possesses only one serration, indicating the pygal was also serrated.

Figures

FIGURE 9. Bayesian inference phylogeny for extant chelydrids (Chelydra and Macrochelys). Note that representative skeletal synapomorphies in skull and carapace (above and below, respectively, next to lineage name) are provided for each lineage of Macrochelys; values above major nodes represent posterior probabilities (≥ 95 %); values below major nodes represent the mean divergence time estimation of the most recent common ancestor (MRCA); and bars at major nodes represent 95 % Highest Posterior Density (HPD).

FIGURE 13. Photograph of Macrochelys apalachicolae holotype (UF 3998) demonstrating external (A) and internal (B) carapace morphology.

FIGURE 14. Photograph of Macrochelys apalachicolae holotype (UF 3998) demonstrating a superior (A), inferior (B), cranial (C), caudal (D), and left (E) and right (F) lateral view of skull morphology.