Huerzelerimys is a genus of the Murinae lived in Eurasia in Late Miocene.The genus has been known in Europe for a long time, but only recently has been found in Asia. However, most of the known fossils are isolated teeth, only a few are fragmentary jaws. In 2005, a skull with mandible and several cervicals of Huerzelerimys were collected from the Late Miocene Liushu Formation in Linxia Basin, Gansu Province by a field team of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences). The skull is the first one ever found of the Huerzelerimys fossils and provides us with important new information about this genus.
In a paper published in the journal Vertebrata PalAsiatica on 19 March 2020, Wang Ban-Yue and Qiu Zhan-Xiang of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and Li Lü-Zhou of the School of Earth and Space Sciences at Peking University, and the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, formally describe the Gansu skull as a new species of Huerzelerimys.
The new species is named Huerzelerimys asiaticus, in reference to the continent where the fossil was collected. The specimen from which it is described, IVPP V 16288, comprises a complete skull with mandible and 5 cervicals. It was collected from upper part of the Liushu Formation, which is assigned to the late Bahean Asian Land Mammal Age/Stage of the Late Miocene, making it about 8 million years old.
Huerzelerimys asiaticus is a small-sized Huerzelerimys. Skull broad and short with short and broad rostrum, narrow interorbital roof, more developed premaxillary laterodorsal crest, weak and subparallel frontal crests; incisive foramen long, terminated posteriorly near the anterior root of M1; paired posterior palatal foramina situated mesial to M2; posterior border of hard palate lying posterior to M3; interpterygoid foramen absent; alisphenoid canal bony; bulla large and inflated; internal carotid foramen located near the basilar tubercle; horizontal ramus of mandible low, having a deeply concave diastema; masseteric ridge extending anteriorly below the anterior margin of m1; the mental foramen situated anteroventral to m1, anterior to the masseteric ridge, and near to mandibular diastema.
I2 orthodont; M1 with slightly anteriorly located t1; M1 and M2 with t6 and t9 connected by distinct crest and crest-like t12; t1 and t3 of M2 and t1 of M3 connected with t5; M3 with t3 and a large isolated t8; m1 with small Acc connected with both Alc and Abc; m1 and m2 having distinct buccal cingula, larger c2 attached to protoconid, and low crest-like posterior heel; m2 and m3 having isolated Abc; c1 absent in m3.
The skull is well preserved, but the zygomatic arches are broken and the posterior part of the skull is damaged. The cranial bone sutures are obscure, especially those on the skull roof.
The skull is small-sized among the muroid rodents, with myomorphous zygomasseteric structure. It is relatively short and broad. The condylobasal length is subequal to that of a small-sized Rattus (Rattus rattus). The rostral part is rather short and broad, relative to the cranial part. The length of maxillary diastema is slightly shorter than the height of the middle part of the skull.
The skull is roughly oval in outline. As in Rattus, the rostrum is rather short and broad, with its two lateral sides slightly convergent forwards. Rostral end of the rostrum is nearly as wide as the interorbital part. The nasal is wedge-shaped. Caudally, it is lined up with the posterior end of the premaxillo-frontal sutures. The premaxillary laterodorsal crest is well developed, nearly parallel to the naso-premaxillary suture and separates the premaxilla into dorsal and lateral parts. The dorsal part of the premaxilla is narrow and long, forming a strip. The premaxillo-frontal suture is strongly serrated, extending anterolaterally to meet the premaxillo-maxillary and maxillo-frontal sutures. The posterolateral end of the maxillo-frontal suture extends to the lacrimal. The lacrimal is situated mainly in the orbit, exposed only a little on the dorsal side, forming a distinct lacrimal tubercle at the anteromesial corner of the orbit. The coronal suture is convex posteriorly. The dorsal parts of the parietals are slightly convex, much broader than those of the frontals. The parietals are displaced about 0.5 mm to the right side relative to the frontals, so that the parietal suture and the frontal suture are not in the same longitudinal line. The interparietal is a large bone, elliptic in outline, wider than long.
As in other murines, the frontal crest extends backwards to the crista supratympanica. The two slightly curved frontal crests are roughly parallel with each other as in Rattus, but not so well developed as in the latter. Though broken away, the zygomatic arches must be slender and their anterior roots are situated in front of the posterior ends of the nasal and premaxilla.
The skull roof is convex dorsally. The anterior ends of the nasal and premaxilla are situated almost in the same vertical line. The diastema is much longer than the length of the upper molar row. The anterior part of the skull (anterior to M1) is shorter and lower than the posterior part of the skull (including M1), only about 1/2 as long as the latter. The lateral part of the premaxilla, bordered anterodorsally by the arched pmldc, is broad and slightly concave. The premaxillo-maxillary suture runs roughly vertically in front of the infraorbital foramen. The infraorbital foramen lies within the maxilla, having an oval upper part and an infraorbital fissure below. The masseteric tubercle is located below the infraorbital fissure. The insertion of masseter profundus is confined within the infraorbial foramen. The zygomatic plate is located ventrolateral to the infraorbial foramen and its surface is concave facing anteroventrally and laterally.
The posterior border of the large orbit is formed by the squamosal. The distinct temporal crest extends to the crista supratympanica. The ventral surface of the large auditory bulla is lower than that of the occipital condyle. The external auditory meatus is formed by a very short bony tube. The stylomastoid foramen is located behind the external auditory meatus and above the mastoid process. The nuchal surface is vertical and convex posteriorly. The left petromastoid portion (os mastoideum) is preserved and nearly round in outline and has a convex surface. The mastoid foramen is situated at the mesosuperior corner of the petromastoid portion.
In the orbital area, several foramina can be observed. The optic foramen is large and located above the M3. The ethmoidal foramen is small, situated anterosuperior to the optic foramen and above the M2. The area below the ethmoidal foramen and anteroventral to opf is broken, thus the sphenopalatine foramen cannot be fixed. The foramen rotundum is located ventroposterior to the optic foramen and superoposterior to the M3. Differing from Rattus, but similar to Apodemys, the alisphenoid canal (canalis alisphenoidale) is a bony canal penetrating the pterygoid. The anterior foramen of alisphenoid canal opens on the lateral side of the external pterygoid process. The large foramen ovale penetrates the superoposterior part of the external pterygoid process, confluent with the posterior foramen of the alisphenoid canal.
The incisive foramen is very long, about 4/5 as long as the diastema, extending posteriorly to the level of the anterior root of the M1. The zigzag premaxillomaxillary suture intersects the incisive foramen at around the anterior 1/3 of the foramen. The zygomatic plate is broad and concave, extending anterodorsally and facing anteroventrally and laterally. The masseteric tubercle is located at the anteromesial corner of the zygomatic plate, behind the premaxillo-maxillary suture. The left and right cheek tooth rows are nearly parallel to each other. The posterior palatine foramen is situated within the palatine, lying mesial to the M2. The distinct palatine sulcus extends to the posterior palatine foramen. The posterior border of the hard palate is located behind the M3. The mesopterygoid fossa is subequal to the pterygoid fossa in width. In the pterygoid fossa, no interpterygoid foramen (or sphenopterygoid vacuity) is observed. The middle lacerate foramen is located at the mesioposterior corner of the pterygoid fossa. The glenoid fossa is formed by the zygomatic process of squamosal, and transversely concave. The postglenoid foramen is long and large, extending along the petro-squamosal suture on the posterior part of the glenoid fossa. The auditory bulla is very large (about 1/5 of the condylobasal length) and strongly inflated. No bony septum is present in the broken bulla. The foramen of the Eustachian canal opens anteromesially to the bulla. On the mesial side of the bulla, the internal carotid foramen is located on the anterior 1/3 of the bulla, nearly in the same transverse level of the basilar tubercle. The jugular foramen is located between the bulla and occipital. There is a distinct stapedial foramen at the mesioposterior corner of the bulla as in Rattus. The hypoglossal foramen is located anterolateral to the occipital condyle.
The left and right hemimandibles are well-preserved, with only the tops of the coronoid processes broken away. The mandible is sciurognathous. The horizontal ramus is low, with its diastema deeply concave. As in Rattus and Huerzelerimys exiguus, the masseteric ridge extends anteriorly to below the anterior margin of m1. The mental foramen is located anteroventral to the m1, lying near the diastema and anterior to the masseteric ridge.
The ascending ramus of the mandible is long. The anterior border of the coronoid process (rises from the buccal side of the mandible below the anterior part of the m2, and its upper part curves slightly posteriorly. The condyloid process is slightly lower than the coronoid process. The articular facet of the condyle is narrow-ovoid in shape, much longer than wide. The mandibular notch is very shallow. On the buccal side of the ascending ramus, the masseteric fossa is broad. The bulge formed by the posterior end of the lower incisor alveolus is prominent and situated below the mandibular notch on the buccal side. The angular process extends posteriorly below the condyloid process. On the lower half of the lingual side of the ascending ramus, the internal pterygoid fovea is large, triangular in shape, with its anterior angle reaching to below the m2 and the large posterior portion deeply concave, which is sharply bordered by curved upper and lower ridges. The slightly concave temporal fovae is located in the lower part of the coronoid process and posterolateral to m3. The mandibular foramen is situated below the mandibular notch, and the external pterygoid fovea is slightly concave, lying anteroinferior to the condyloid process.
The dental formula is 1·0·0·3/1·0·0·3. The molars are brachyodont and rooted. The molars are moderately worn.
The M1 is oval-shaped. The t1 is rounded in outline and located slightly anteriorly, with the anterior border being nearly opposite to the posterior part of the t3, and connected with the larger t2 by a crest. The t3 is smaller than t1 and t2. No tlbis is present. The t1 and t3 have no spurs extending to the t5. The t5 is connected with t4 and t6 by distinct crests. The crest between the t4 and t5 is lower than that between t5 and t6. The t4 and t8 are connected by a low crest. No t7 is present. The t9 is well developed and connected with t6 by a distinct crest, and the t12 is crest-like.
The M2 is round-angled triangular in outline, with 3 roots. The t1 is rounded. The t3 is much smaller than the t1, and both connect with the t5. The t9 is slightly larger than t3, but smaller than t6. The other features are similar to M1.
The M3 is more triangular than M2 in outline, also with 3 roots. The t1 is smaller than t4, and connected to t5. The t3 is present but very reduced. The t8 is large and isolated. The t9 is absent.
The m1 is ovoid in outline. The anterolingual cusp and anterobuccal cusp are subequal in size, and are connected with each other. The anterolingual cusp extends slightly more anteriorly than the anterobuccal cusp. The small anterocentral cusp (medial anteroconid) is heavily worn and attached to both the anterobuccal cusp and anterolingual cusp. The anterolingual cusp is connected with the metaconid. The metaconid and entoconid extend transversely, while the protoconid and hypoconid extend buccoposteriorly. The posterior pair of tubercles (hypoconid and entoconid) is separated from the second pair of tubercles (protoconid and metaconid), without longitudinal crest or longitudinal spur connecting the two pairs. The buccal cingulum is developed, with distinct accessory cusps on it. Among them, the c1 is the largest and the c2 is larger than the c3 and attached to the protoconid. The posterior heel (posterior cingulum) is low and crest-like, joining with the entoconid and hypoconid.
On the m2, the anterobuccal cusp is oval-shaped and isolated. The anterolingual cusp is absent. There is no longitudinal connection between the two posterior pairs of tubercles. The buccal cingulum is broken off. The c2 is smaller than anterobuccal cusp, but larger than the c1, and attached to the protoconid. The posterior heel is low and crest-like, joining with the entoconid and hypoconid.
On m3, the Abc is reduced to a small and isolated cusp. The protoconid is connected with the metaconid to form a transverse crest. The c1 is absent.
The I2 is orthodont and bends strongly. Its anterior part is extending ventrally. The crosssection of the I2 is triangular in outline, longer than wide, with its labial side convex. The enamel layer covers the whole labial side and parts of the mesial and lateral sides. The labial surface is smooth without ridge on it. No gap is seen on the lingual side.
The i2 is slightly curved in longitudinal direction, with its anterior portion turning anterodorsally. The i2 originates in the ascending ramus of the mandible, forming a bulge on the buccal side. The cross-section of the i2 is narrow-ovoid, with convex labial and lateral sides, but a narrow and rounded lingual angle. The enamel covering is similar to that of the I2.
The 5 cervical vertebrae (C3‒C7) are in quasi-articulated states and well preserved, except C3 and C7 which are more or less damaged. They are similar to each other in general morphology: the body assumes a flattened cylinder in shape; the dorsal part of the vertebral arch is bow-shaped, with the spinous process low, its pedicle short, the pre- and postzygapophyses originated from the pedicle, and the transverse process from the lateral side of the pedicle. In C3‒C6, the transverse processes have two roots and a large transverse foramen, while in C7 the transverse process has only one large root and lacks the transverse foramen. The transverse process extends posterolaterally in C3–C5, but transversely with its enlarged lateral part being separated into two laminae (lamina ventral vertebrae cervical VI (and lamina dorsal vertebrae cervical VI) in C6.
Total length of vertebral bodies of C4‒C7: 6 mm; vertebral fossa of C7 is 0.8 mm in height and 1.8 mm in width; width of C7 at transverse process: 5 mm; width of C4 at prezygapophysis: 4.6 mm.
The skull of IVPP V 16288 has myomorphous zygomasseteric structure, with the infraorbital foramen specialised into a wide upper portion and a narrow infraorbital fissure. The zygomatic plate is broadened and located ventrolateral to the infraorbital foramen and tilted upward. The frontal crest is extending backwards to the crista supratympanica and the mandible is sciurognathous. The cheek teeth are brachyodont with roots, and the crown cusps arranged in three longitudinal rows. All these characters show clearly that V 16288 belongs to the Murinae.
Within the murines which have skulls the V 16288 skull is more similar to Rattus than to any other Murines, such as Apodemys, Micromys, Leopoldamys, Maxomys, Niviventer etc., as evidenced in the following characters: rostrum relatively broad and short, interorbital constriction moderate, braincase rather broad, incisive foramen very long, extending to the level of the anterior part of M1, posterior part of hard palate extending slightly behind M3, and bulla large and strongly inflated, etc. However, V 16288 differs from Rattus as well in having more developed premaxillnary laterodorsal crest, weaker frontal crest and bony alisphenoid canal, but lacking interpterygoid foramen in the pterygoid fossa. In dentition, V 16288 further differs from Rattus in molars being more brachyodont, M1 and M2 having crests linking t4 with t8 and t6 with t9, and having t12, and m1 having anterocentral cusp and subequal anterolingual cusp and anterobuccal cusp.
Obviously V 16288 represents a distinct genus, more primitive than Rattus.
Some Murine genera are known only by teeth. Compared with those genera, IVPP V 16288 is more similar to Progonomys and Huerzelerimys than to others in the following characters: molars being more brachyodont, upper molars with a high connection between t4 and t5, and lacking t7, M1 having more anteriorly located t1 but without t1bis, M1 and M2 having low crest between t4 and t8 and having t12, and m1 and m2 without longitudinal connections between the two posterior pairs of tubercles (protoconid-metaconid and hypoconid-entoconid), and anterocentral cusp of m1 being reduced etc.
Furthermore, in V 16288 M1 and M2 having well developed t9 and distinct connection between t6 and t9, M3 lacking t9, m1 having a distinct anterocentral cusp, connection between the two anterior pairs of tubercles (anterobuccal-anterolingual cusps, and protoconid-metaconis), and moderately developed cingular margin. All these features of V 16288 are similar to those of Huerzelerimys rather than to Progonomys.
In addition, the incisive foramen extends only to the level mesial to the anterior root of M1 in Huerzelerimys minor, but it can extend farther backward beyond the anterior root of M1 in Progonomys. In this feature V 16288 resembles Huerzelerimys minor rather than Progonomys. Meanwhile, the mandible of V 16288 is also similar to that of Huerzelerimys exiguus. Therefore, V 16288 should be referred to the genus Huerzelerimys.
The genus Huerzelerimys is known to include 5 species: Huerzelerimys vireti, Huerzelerimys turoliensis, Huerzelerimys oreopitheci, Huerzelerimys minor, and Huerzelerimys exiguus.
The cheek teeth of V 16288 are much smaller than those of Huerzelerimys turoliensis and Huerzelerimys oreopitheci, but larger than those of Huerzelerimys exiguus, and subequal to those of Huerzelerimys vireti and Huerzelerimys minor.
Morpghologically, V 16288 is different from Huerzelerimys minor and Huerzelerimys vireti in M1 and M2 having distinct crest connecting t6 with t9 and having a crested t12, m1 having nearly centrally located anterocentral cusp uniting with both anterolingual cusp and anterobuccal cusp, m1 and m2 having larger c2 attached to protoconid and having lower and crested posterior heel, and m3 lacking c1.
V 16288 differs from Huerzelerimys minor in M3 having isolated t8, and anterobuccal cusp of m2 and m3 being isolated. V 16288 further differs from Huerzelerimys vireti in t1 and t3 of M2 and t1 of M3 being connected to t5. Therefore, V 16288 cannot be attributed either to Huerzelerimys minor, nor to Huerzelerimys vireti.
V 16288 differs from Huerzelerimys exiguus in M1 having slightly anteriorly located t1, and t6 and t9 being distinctly connected; m1 having anterocentral cusp attached to both the anterolingual cusp and anterobuccal cusp; m1 and m2 having more developed accessory cusps, and lower and weaker posterior heel.
V 16288 differs from Huerzelerimys turoliensis in M1 and M2 having more developed t12, m1 having distinct anterocentral cusp, m1 and m2 having more distinct buccal accessory cusps and having lower crested-like posterior heel.
V 16288 differs from Huerzelerimys oreopitheci in molars being more brachyodont, M1 and M2 having low t12 and smaller t9, M3 having t3, and m1 having more developed anterocentral cusp and m1 and m2 having larger c2 attached to protoconid, and lower crested-like posterior heel.
V 16288 therefore represents a new species of Huerzelerimys, which Wang et al. name as Huerzelerimys asiaticus.
Judging from the previously known species of Huerzelerimys, the overall evolutionary tendencies of Huerzelerimys can be summed up as follows: increase in size (a), increase in crown height (b), forward shifting of t1 in M1 (c), gradual convergence and union of t6 and t9 (d), reduction of t12 in M1 (e), reduction of Acc in m1 (f), and development of accessory cusps in m1 (g).
Huerzelerimys asiaticus differs from Huerzelerimys exiguus in having such advanced features as (a), (c), (d) and (g). This shows that Huerzelerimys asiaticus is more advanced than Huerzelerimys exiguus.
Although it is subequal to Huerzelerimys minor and Huerzelerimys vireti in size, Huerzelerimys asiaticus has some more advanced features, (d) and (g), than the latter two. It seems that Huerzelerimys asiaticus may represent a slightly more advanced species than Huerzelerimys minor and Huerzelerimys vireti.
Although it has more developed accessory cusps in m1, an apomorphic character in Huerzelerimys (g), Huerzelerimys asiaticus is less advanced than Huerzelerimys turoliensis in features (a), (e) and (f), and is different from Huerzelerimys oreopitheci in lacking such advanced features as (a), (b), (e) and (f). Obviously Huerzelerimys asiaticus may represent a more primitive species than the latter two.
To sum up, Huerzelerimys asiaticus is more advanced than Huerzelerimys exiguus, Huerzelerimys minor and Huerzelerimys vireti, but more primitive than Huerzelerimys turoliensis and Huerzelerimys oreopitheci.
Since the age of Huerzelerimys exiguus is early Late Miocene, roughly corresponding to upper MN10 or lower MN11 in European time scale, Huerzelerimys minor is known from upper MN10, H. vireti from MN11, and Huerzelerimys turoliensis from upper MN11 to MN12, and Huerzelerimys oreopitheci is known from middle Turolian (middle MN12), the age of the upper part of the Liushu Formation yielding Huerzelerimys asiaticus, is estimated by Wang et al. as late Bahean, corresponing to upper MN11 or lower MN12.
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