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Comment on Schubert et al. 2020. Muknalia minima from the Yucatán of Mexico is synonymous with the collared peccary, Pecari tajacu (Artiodactyla: Tayassuidae). Open Quaternary, 6: 8, pp. 1–9. DOI: https://doi.org/10.5334/oq.84

Authors:

Sarah R. Stinnesbeck ,

State Museum of Natural History Karlsruhe, DE
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Wolfgang Stinnesbeck,

Heidelberg University, DE
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Eberhard Frey,

State Museum of Natural History Karlsruhe, DE
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Jerónimo Avilés Olguín,

Museo del Desierto, Saltillo-Coahuila, MX
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Arturo H. González González

Museo del Desierto, Saltillo-Coahuila, MX
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Abstract

Schubert and colleagues have recently criticized our assessment of the mandibular ramus of a small peccary from Muknal cave in Quintana Roo, Mexico, to a new genus and species, Muknalia minima Stinnesbeck et al. 2017. They considered this assignation as invalid and the unique morphologies of the taxon to be the result of breakage and human modification. We strongly disagree with this interpretation and maintain our original view of a new genus and species.

How to Cite: Stinnesbeck, S.R., Stinnesbeck, W., Frey, E., Olguín, J.A. and González, A.H.G., 2021. Comment on Schubert et al. 2020. Muknalia minima from the Yucatán of Mexico is synonymous with the collared peccary, Pecari tajacu (Artiodactyla: Tayassuidae). Open Quaternary, 6: 8, pp. 1–9. DOI: https://doi.org/10.5334/oq.84. Open Quaternary, 7(1), p.2. DOI: http://doi.org/10.5334/oq.89
  Published on 22 Jun 2021
 Accepted on 30 Apr 2021            Submitted on 06 Aug 2020

Schubert and colleagues have recently re-evaluated our assessment of the mandibular ramus of a small peccary from Muknal cave in Quintana Roo, Mexico, to a new genus and species, Muknalia minimaStinnesbeck et al. 2017. They considered this assignation as invalid and the unique morphologies of the taxon to be the result of breakage and human modification.

The mandibular ramus of a peccary discovered by us in the Muknal cave, was modified by early settlers of the northeastern Yucatán Peninsula. The presence of modifications (e.g., cut and scratch marks) was documented and discussed in detail by Stinnesbeck et al. (2017, 2018). Schubert et al. (2020) concur with our interpretation of an anthropologic handling of the peccary dentary discovered by us at only a few meters (m) distance of a human skeleton, at 33 m water depth of the submerged Muknal cave (Stinnesbeck et al., 2018). However, features regarded by Stinnesbeck et al. (2017) to be of anatomical nature and representing an intact morphology, were regarded by Schubert et al. (2020) to result from anthropological handling (breakage and polishing). We address those on a case-by-case basis as follows:

1) Schubert et al. (2020) argue that the angle of the mandibular ramus was removed (broken off) by humans. This area, between the mandibular condyle, ramus and angle, exhibits a notch, which has been documented by Stinnesbeck et al. (2017) as a main morphological feature unknown in other peccaries. This notch exhibits a 110° ventrocaudally open angle, of which the dorsal side is represented by the ventral facing subcondylar margin of the mandibular ramus, running nearly parallel to the long axis of the specimen, while the ventral side of the angle is represented by the caudal margin of the ramus. The notch was interpreted by Schubert et al. (2020) as a gap, because of the fragmentation of the caudal margin of the ramus shown in Figures 1D and E displayed in Schubert et al. (2020), and there highlighted by a black arrow pointing towards the breakage (see Figure 1a, this paper). Therefore, Schubert et al. (2020) fundamentally disagree with Stinnesbeck et al. (2017) that the caudoventral margin of the ramus, i.e., the dorsal side of the 110° angle, called subcondylar area, is intact (see Figures 1 and 2, this paper). The interpretation of the authors of a ‘rounded and polished’ area may be the effect of a dark brown shadow in Figure 1A of Schubert et al. (2020), but this is not natural as seen in Figure 2 (this paper). Schubert et al. (2020) support their interpretation by the remaining condyloid process (Figures 1D and G). The bone itself is light brown to red-colored.

Figure 1 

Muknalia minima from Muknal cave (a) including comparison with Pecari tajacu (b; SMNK-MAMM-1539), both in lateral view.

Figure 2 

Muknalia minima (a) in direct comparison with Pecari tajacu, left SMNK-MAMM-26575 and right SMNK-MAMM-5423 (b) in caudal view. Close up of the condylar area and mandibular angle of Muknalia minima in caudal view.

We argue, however, that the breakage of the mandibular ramus starts at the vertex of the angle (see green arrows in Figures 1 and 2, this paper), as pointed out by the black arrow in Schubert et al. (2020, Figure 1C, E), and from this point on, the breakage runs ventrally (see Figure 1a green arrows, this paper). There is no fragmentation on the dorsal side of the vertex (see the area to the left of the arrow in Figure 1E of Schubert et al. (2020)).

Furthermore, the area Schubert et al. (2020) are referring to is actually the short neck of the condylar process, which in caudal view has the shape of an inverted triangle (Figure 2a, this paper), not the articular condyle itself (the latter has indeed been removed, which is also seen in Figure 1A of Schubert et al. (2020), and bone compacta is identified there evidencing this breakage (see Figures 1A and F of Schubert et al. (2020)). This area shows abundant bone pores and foramina, which Schubert et al. (2020) interpret as trabecular bone. However, trabecular bone along the condylar neck is actually also seen in modern Pecari; it is natural and not caused by modification (see Figure 2b). The ventral border of the condylar neck (named subcondylar area by Stinnesbeck et al. (2017) and dorsal side of the vertex in Schubert et al. (2020)), i.e. the transition between the condylar process with the ramus, also shows an intact morphology (Figures 1 and 2), without any signs of breakage e.g. no trabecular bone is visible in the transition between the condylar neck and mandibular ramus (lower left quadrant of Figure 2c).

The dorsal side of the vertex (Figure 2a and c) differs morphologically to Pecari (Figure 2b) by the uniform thickness of the subcondylar area and its width. This anatomical difference is well visible in the right mandible of a Pecari in Figure 2b, showing that in a breakage, this area is significantly narrower than in Muknalia, in Pecari this area is 2 mm thick, while 5 mm thick in Muknalia. According to Schubert et al. (2020) the dorsal side of the vertex has been polished by humans, however, a polishment would not have led to an increasing of width. This indicates that the notch identified in the subcondylar area is an anatomical feature of the mandibular ramus (Figures 1 and 2, this paper), and not a gap; it thus supports our interpretation of a new genus and species.

The interpretation of Schubert et al. (2020) is mainly based on “striae” along the ramus, supporting the modification of the bone to create a notch of perforation in this region. However, these surface irregularities are nevertheless present on the entire mandibular ramus and appear to be of natural origin, e.g. due to weathering, rather than formed by human manipulation. This scenario is supported by the abundance of irregular apertures and breakage on the lateral and median surface of the bone.

Furthermore, the cut marks described by Stinnesbeck et al. (2018) spread like a fan from the vertex of the angle in both lateral and medial direction, indicating that the peccary was scraped directly at the vertex. If the mandibular angle would have looked like in Pecari, the cut marks would not run fan-like, directly from the vertex on. Rather, the spreading of cut marks would be visible as widely spaced sections and therefore far apart from each cut, but this is not the case.

We therefore maintain our original interpretation that a concave notch is present along the caudal edge of the ascending ramus and a vertically directed angular process, and that this is a morphological feature and not a modification.

2) We agree with Schubert et al. (2020), that the angular process is fragmented. Nevertheless, at closer inspection of the area (Figure 1) it is apparent that the ventral-most small tip of the mandibular angle belongs to its external margin, thus giving the mandibular angle a convex semi-circular shape. This feature strongly differs from all other peccaries, including Pecari tajacu. Furthermore, the lateral surface of the mandibular angle is convex in the Muknal specimen (Figure 2a, this paper), while flat to even concave in Pecari, due to the masseteric fossa. Also, the ventral tip of the mandibular angle shows medially in the Muknal specimen and not ventrally as in Pecari. The convexity of the lateral surface in the mandibular ramus of the Muknal specimen contrasts with its laterally pointing condyle, while in Pecari the entire ramus is straight.

3) The lateral outline of the coronoid process is trapezoidal in shape in the Muknal specimen and therefore differs from most Pecari individuals, but we agree with Schubert et al. (2020) that this feature might be variable. Nevertheless, the flat and almost horizontal dorsal margin of the Muknal ramus is not visible in the specimens of P. tajacu figured by Schubert et al. (2020, Figure 2B, C and D) for comparison with the holotype of Muknalia minima, nor was it seen by us in the Pecari collection (n = 15) of the State Museum of Natural History at Karlsruhe, Germany (SMNK) and the Museo del Desierto (MUDE). In caudal view, the coronoid process is slightly twisted in the Muknal specimen, and its caudal-most tip is medially inclined, which contrasts with Pecari in these collections, in which the coronoid process of the specimens is straight in dorsal view. For mechanical reasons this cannot be a variation because the coronoid process governs the direction of the forces produced by mm. masseter et temporalis and thus loads the mandibular articulation in both species in different ways.

Schubert et al. (2020) argue that the size of the Muknal lower jaw is comparable to that of two small subspecies of extant Pecari tajacu, i.e. P. tajacu yucatanensis from Yucatan and P. tajacu nanus from Cozumel island. These are notably smaller than Pecari tajacu which inhabits the rest of Mexico, Central and South America.

Nevertheless, there are still morphological features which differ: All Pecari individuals exhibit a concave diastema (see red line in Figure 1b, this paper). The concavity starts directly anterior of p1, resulting in a deep depression in Pecari, in contrast to the straight diastema margin in the Muknal specimen (red line in Figure 1a).

We agree that the measurements taken from the Muknal specimen fit within the range of these endemic and, in the case of P. tajacu nanus, insular subspecies. However, the ratio identified by us between e.g. the diastema length and tooth row length from p2 to m3 does not. This includes the dwarf individuals presented by Schubert et al. (2020):

Schubert et al. (2020) further argue that the diastema value presented by Stinnesbeck et al. (2017) of the Muknal specimen is an estimation (page 5). This is not, because the alveolus of the canine is preserved, the complete tooth-row length can be measured with precision.

Muknalia minima has a diastema length of 21 mm, a tooth row length of 105, measured from c1 to m3, and a cheek-tooth length of 69 mm (p2 to m3) (see Stinnesbeck et al. 2017 and Schubert et al. 2020). A minimum value of 21.63 mm has been documented in a single individual of P. tajacu nanus, an endemic subspecies of a Cozumel Island individual (Schubert et al. 2020); this value can be compared with the 21 mm diastema length of the Muknal specimen. However, the mean diastema length in P. tajacu nanus is 22.82 mm, which means that the diastema in the Muknal specimen is 8% smaller than in most Cozumel Island subspecies. The diastema length in P. tajacu yucatanensis is also significantly longer than in Muknalia, even though this latter subspecies has also been regarded as smaller than P. tajacu (Schubert et al. 2020). We therefore consider the diastema length to be a diagnostic feature.

The diastema-tooth-row ratio thus differs significantly between the Muknal specimen and all other peccaries. In the Muknal individual the diastema only reaches 20% of the tooth-row length, while this value is >25% in the Cozumel island (P. tajacu nanus) and Yucatan peccaries (P. tajacu yucatanensis). This indicates that the Muknal individual had a smaller and shorter muzzle than Pecari, including the small Yucatan or Cozumel island subspecies. Otherwise, how could you explain that the diastema of the Muknal specimen is as small as seen in the smallest subspecies P. tajacu nanus, but the tooth row is as long as in large continental individuals of Pecari? This shows, that the teeth or cheek-tooth-row in Muknalia corresponds to P. tajacu in size, while other anatomical features, such as diastema length are only comparable to single individuals of the subspecies P. tajacu nanus and P. tajacu yucatanensis, and here to the end of range of measured size. The anatomical proportions and mandibular shape in Muknalia therefore differ strongly from P. tajacu including its subspecies P. tajacu nanus and P. tajacu yucatanensis.

The scenario of a smaller muzzle is also supported by the converging dorsal and ventral borders of the mandible seen in lateral view (Figure 1a, this paper). In Muknalia minima the mandibular height decreases by 15% between m3 and pm1, while in the smaller-sized Pecari individuals (e.g. P. tayacu yucatenensis) the mandibular height decreases by <10% and even in the Cozumel dwarf peccary the factor differs, there reaching 12%. In most Pecari, however, the height along the tooth-row is continuously even, dorsal and ventral mandibular margins run almost parallel to each other (Figure 1b).

Schubert et al. (2020) also emphasize that the dentition of the Muknal specimen is comparable to that of the genus Pecari and use this argument for their interpretation that Muknalia should be invalidated. However, the mandibular dentition of peccaries is remarkably similar in Pecari and Tayassu and barely distinguishable (see Woodburne 1968), while the genus Pecari is variable concerning the tooth crown morphology, as also noted by Schubert et al. (2020).

Also, the condyloid neck in the Muknal specimen is at the same height as the cheek tooth row; it is therefore one-third lower than in the individuals figured by Schubert et al. (2020) in Figure 2.

We therefore maintain and emphasize our view that Muknalia minimia is a valid genus and species of peccary. Muknalia minima has been compared by Stinnesbeck et al. (2017) with all three living tayassuids, the collared peccary, white-lipped peccary and Chacoan peccary, based on the collection of extant specimens housed in the SMNK, but also several individuals of Pleistocene and Holocene age from the submerged Tulum cave system.

Competing Interests

The authors have no competing interests to declare.

References

  1. Stinnesbeck, SR, et al. 2017. A new fossil peccary from the Pleistocene-Holocene boundary of the eastern Yucatán Peninsula, Mexico. Journal of South American Earth Sciences, 77: 341–349. DOI: https://doi.org/10.1016/j.jsames.2016.11.003 

  2. Stinnesbeck, SR, et al. 2018. The Muknal cave near Tulum, Mexico: An early-Holocene funeral site on the Yucatán peninsula. The Holocene, 28(12): 1992–2005. DOI: https://doi.org/10.1177/0959683618798124 

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