Cretapsara athanata Luque, 2021

Luque, Javier, Xing, Lida, Briggs, Derek E. G., Clark, Elizabeth G., Duque, Alex, Hui, Junbo, Mai, Huijuan & McKellar, Ryan C., 2021, Crab in amber reveals an early colonization of nonmarine environments during the Cretaceous, Science Advances 7, pp. 1-13 : 2-7

Cretapsara athanata Luque , gen. etsp. nov.

LSID. Genus group: http://zoobank.org/urn:lsid:zoobank.org:act: E6 F68190 -D2A7-4D6E-8CE8-EDD319E8DA13.

LSID. Species group: http://zoobank.org/urn:lsid:zoobank. org:act:B6E29B76-C37E-47D8-9FDE-228DEB3DB9BA.

Etymology: Thegenus name combines Creta (Latin), for chalk as in Cretaceous, and Apsara, a spirit of the clouds and waters; gender is feminine. The specific epithet is based on athanatos (Greek), immortal, referring to its lifelike preservation. The family name is based on the only known genus, erected herein.

Holotype: Specimen LYAM-9 ( Figs. 1 and 2 ) deposited inthe Longyin Amber Museum, Xishan District, Kunming 650228, Yunnan Province, China. Specimen collected by local miners in 2015.

Type locality and horizon: Angbamo site , Tanai Township, Myitkyina District, Kachin Province, Myanmar [~98.8 ± 0.6 Ma; lowermost Cenomanian ( 17 – 19)].

Diagnosis: The carapace is subhexagonal, nearly as long as it is wide; theorbitofrontalmarginis nearlyas wideas thebody and bears wide, shallow orbits that lack supraorbital fissures or intraorbital spines. The rostrum is bilobate, short, and wide, measuring about half the widthof the orbitofrontal margin. The anterolateralmargin is one-third as long as the posterolateral margin, and it bears a small outer orbital spine and two short subtriangular anterolateral spines; the posterolateral margin is straight to slightly convex, and it bears at least foursmall and equidistant tubercles. Theposterior margin is wide, nearly straight, rimmed, and apparently ornamented with a row of small tubercles. The cervical and branchiocardiac grooves are well developed, reach the lateral margins of the body, and are expressed ventrally; the dorsal regions are welldeveloped and delimited by grooves. The buccal cavern is wide, covered by a pair of operculiform mxp3 that lack a V-shaped incision in the occlusal margin. The thoracic sternites 1 to 3 are visible ventrally and are separated froma subtrapezoidal sternite 4 by a shallow groove. Sternites 5 and 6 are similar in shape; sternite 5 is the widest of all, andsternites 6 to 8 reduce in size posteriorly. The sterno-abdominal cavity is shallow, and there is no evidence of a linea media traversing the sternites. The pleon has a small subtriangular telson and, based on the stereomicroscope images and the micro-CT scanning, apparently has six free and unfused pleonites, from which at least the first two are fully visible dorsally; thereisno evidenceof uropodsoruropod remains. The eyes are as large as the orbits; the corneal eye is globular, as wide as itis long, and apparentlycovered by small hexagonal facets; the eyestalk is short and cylindrical. The antennulae and antennae are reduced. The claws or chelipeds areequal in shape and size, and thefour pairs of walking legs are well developed, slender, and similar in size and shape; they lack chelate, subchelate, or flattened distal podomeres (e.g., propodi and dactyli); the podomeres are semicircular in cross section. Excurrent openings are well developed, small, and circular.

Description: Dorsal carapace: The carapace is subhexagonal in outline and about as long as wide (~2 mm) ( Figs. 1D and 2B ). The orbitofrontal margin is nearly as wide as the maximum width of the carapace, and each orbit is wide, concave, and shallow, with a sinuous supraorbital margin that lacks supraorbital fissures or spines ( Figs. 2 , Ato C, and 3), apart from a small, anterolaterally diverging outer orbital spine ( Fig. 3 , Fand G, thick arrow). The rostrum is short and half the width of the orbitofrontal margin, downturned, apparently with a shallow axial sulcus and raised subparallel lateral margins grading into the inner orbital margins ( Figs. 2B and 3 , Aand F, and movie S1). The anterolateral and posterolateral carapace marginsare distinct and separatedby a shallow lateral expression of the cervical groove ( Fig. 3 , Fand G, dotted line). The anterolateral margin is slightly convex, about one-third the length of the posterolateral margin, and it bears two anterolateral spines that are short and subtriangular ( Figs. 2 and 3F ). The edge of the posterolateral margin is straight to slightly convex, and it bears at least four small tubercles that are somewhat equidistant from each other ( Fig. 3F , movie S1, and data file S1). The posterior margin of the carapace is about two-thirds of the maximum carapace width, lacks reentrants, and, as evidenced by the micro-CT scans, appears to be rimmed and bearing a row of small tubercles (movie S1 and data file S1).

The cervical groove becomes faint axially, deepens laterally reaching the anterolateral margin, and is expressed ventrally ( Fig. 3 , C, F, and G); the groove is concave posterior to the mesobranchial region and sinuous toward the lateral margin. The branchiocardiac groove is well developed, somewhat deep, flanking the cardiac region and, apparently, the urogastric region mesially and the mesometabranchial region distally ( Fig. 3F ).

The epigastric region is wide, lacking spines or tubercles, laterally inflated, and axially sulcate; the laterallobes reach the frontalregion and are nearly continuous posteriorly with the protogastric lobes ( Fig. 3 , Aand F). The protogastric region is wide, lacking spines or tubercles, laterally tumid, and forming an almost ovate longitudinal lobe flanked by the epigastric region; axially, the protogastric depression separates the protogastric lobes and opens to the narrow anterior mesogastric region. The mesogastric region is relatively small, lacking spines or tubercles, defined anteriorly by two grooves that separate the protogastric lobes and posteriorly by a concave cervical groove. The metagastric region is well developed, inflated, and about three times as wide as long; it is bounded anteriorly by the cervical groove, laterally by grooves that separate it from the epibranchial regions, and posteriorly by a narrow and depressed urogastric region. The urogastric region is considerably shorter and narrower than the metagastric region, about twice as wide as it is long, depressed relative to the metagastric and cardiac regions, and separated from them by faint grooves, delimited laterally by the branchiocardiac groove. The cardiac region is almost subtrapezoidal, lacking spines or tubercles, is wider posteriorly, and is delimited laterally by the branchiocardiac groove. The intestinal region appears to be wide, shorter mesially, and slightly wider laterally, with twolateral swellingsparallel totheposterior margin ( Fig. 3 , Aand F, and movie S1).

The postorbital region is tumid, is separated from the protogastric lobes by a deep and wide groove, and is bounded posteriorly by the cervical groove and laterally by the hepatic region; the postorbital regionlacks spines or tubercles. The hepaticregion is small, is flatto slightly depressed, and is bounded laterally by the anterolateral margin and posteriorly by the cervical groove. The epibranchial region is wide and is delimited anteriorly by the cervical groove and posteriorly by a shallow groove that separates the epibranchial and mesometabranchial regions; the epibranchial region has a swollen lobe between the anterolateral margin and the metagastric region. The mesobranchialand metabranchial regionsare undifferentiated, smooth, and unornamented ( Fig. 3 , Aand F, and movie S1).

Ventral carapace: The buccal cavern is wide and covered by a pairof operculiform mxp3. Thecoxae of the mxp3 separate the sternum from the pterygostome. The pterygostome is curved, bulged, finely granulated, with a row of coarser tubercles parallel to the lineabrachyuraormoltingplane ( Figs.2 , Cand F, and 3, andmovie S1).

The thoracic sternum has fused sternites 1 to 3 that are triangular, about as long as wide, and are visible ventrally; sternite 4 appears tobe subtrapezoidal in outline; sternite 5 is the widest and measures approximately two-thirds of the maximum carapace width; sternite 6 is narrower than sternite 5 but nearly as wide as sternite 4; sternites 7 and 8 are small and barely evident in the micro-CT scan. The sterno-abdominal cavity is shallow, and there is no evidence of a linea media traversing the sternites.

The pleon, based on the micro-CT scan reconstruction, appears to comprise six free, unfused pleonites and a small telson. The pleonites are subrectangular in outline. The firstpleonite is smallest and visible dorsally, and the second is wider and, apparently, also visible dorsally. The third and fourth pleonites are the widest, with the anterior part of the third pleonite visible dorsally, but the fourth is concealed under the body. The fifth pleonite is slightly narrower than the fourth sternite, whereas the sixth pleonite is considerably narrower than the preceding pleonites. The telson is triangular and about as wide as long. There is no visible evidence of uropods.

Eyes and antennae: The eyes and eyestalk are large, with each eye approximately one-third the maximum carapace width. The corneal eye is globular, as wide as long, measuring about one-fifththe width of the carapace, and seemsto be covered with small hexagonal facets; theeyestalkisshorterandcylindrical ( Figs. 2 , Ato C, and 3, B, E, and G). The antennula and antenna are very small ( Figs. 2 , Ato C, and 3) and, due to their small size and limitations in the resolution of the micro-CT scans, no details of the flagella can be discerned.

Mouthpartsand thoracic appendages: Themxp3 consist of a long slender exopod and an endopod with a long semirectangular ischium, a squarish merus, and slender distal palp ( Figs. 2 , A, C, and F, and 3, B, E, and G). The coxae of the mxp3 are small and do not meet axially; the ischiomerus has straight occlusal margins lined with fine setae ( Fig. 2 , A, C, and F) and does not form a rhomboid gap with the other mxp3 that reveals the other mouthparts, neither does it bear a crista dentata ( Figs. 2 , Cand F, and 3, B, E, and G). The palp, which is formed by the propodus, carpus, and merus, is long, thin, and located in an inner-mesial position.

The first of the five pairs of pereopods (P1) constitutes the claws (or chelipeds), which aresymmetricalor homochelous ( Figs. 1 and 3 ). Their mobile fingers (or dactyli) are long and slender, barely curving downward, and parallel to the fixed finger or pollex of the propodus, which is also long and slender and apparently bears six to eight very small teeth on the occlusal margin ( Fig. 2D , movie S1, and data file S1). The other four pairs of thoracic appendages are slender walking legs (P2 to P5), similar in size and shape, with slender, acute dactyli and lacking chelate or subchelate terminations ( Figs. 1 and 3 ). The podomeres are semicircularin transverse section rather than flattened ( Figs. 1 to 4, movie S1, and data file S1). The claws and the legs have several small, fine setae scattered over their surface ( Fig. 2D and E ).

Gills and excurrent openings: The micro-CT scan of the holotype of C. athanata revealed the presence of at least six pairs of phyllobranchiate gills, bearing apparently up to 20 series of well-developed, flattened branchial lamellae that are perpendicular to the main gill shaft ( Fig. 4 , Dto F, and movies S2 to S4). Apair of conspicuous, small, and subcircular excurrent openings is evident beneath the antennalsockets adjacent tothe epistomialregion ( Figs. 2 , A, C, and F, and 3, Band G; movie S1; and data file S1).

Figures

Fig. 1. Cretapsara athanata Luque gen. et sp. nov., a modern-looking eubrachyuran crab in Burmese amber. (A to D) Holotype LYAM-9. (A) Whole amber sample with crab inclusion in ventral view. (B) Close-up of ventral carapace. (C) Whole amber sample with crab inclusion in dorsal view. (D) Close-up of dorsal carapace. White arrows in (B) and (D) indicate the detached left fifth leg or pereopod.Photos by L.X.Figure by J.L.

Fig. 2. Cretapsara athanata Luque gen. et sp.nov. from Burmese amber. (A to G) Detailed views of Holotype LYAM-9.(A) Close-up of antero-ventral carapace with eyes and eyestalks, antennae, and the excurrent openings beside the palps of the third maxillipeds (mxp3).(B) Close-up of dorsal carapace showing the orbits lacking orbital fissures and the anterolateral margin with three spines including the outer orbital spine. (C) Overview of specimen in ventral view with legs,pleon,sternum,mouthparts, eyes,and left cheliped.(D) Close-up of left cheliped in ventral oblique view,showing a straight dactylus and pollex,the latter bearing small acute teeth on occlusal margin. (E) Ventral view of pereopods 2 to 5, showing all legs of similar size and slender shape, with small, thin,spaced setae,and an acute,long dactyli.(F) Close-up of the mxp3, showing the palp and the fine lining of setae in the occlusal margin, the two hollow openings beside the palp of mxp3 (white arrow), and small granulations in the pterygostome. (G) Close-up of posterior sternites and pleon tucked under the carapace, typical of a eubrachyuran. White arrows in (A) and (F) indicate the excurrent openings. an1,antennula; an2,antenna; ce,corneal eye;es,eyestalk; ex,exopod; is, ischium; me,merus; pa:distal palp of mxp3. Photos by L.X.Figure by J.L.

Fig. 3. 3D mesh of Cretapsara athanata Luque gen. et sp. nov. holotype LYAM-9. (A to E) 3D mesh extracted from reconstructed micro-CT data in VGSTUDIO MAX, remeshed in MeshLab, and visualized using Autodesk Maya: (A) dorsal, (B) ventral, (C) right lateral,(D) oblique postero-dorsal,(E) oblique antero-ventral views,showing the claws of equal size and four pairs of slender legs similar in shape and size,with P5 slightly smaller than the other legs.(F and G) Details of the dorsal (F) and ventral (G) carapace,showing details of the large eyes and orbits,small antennae,and a small,acute outer orbital spine [(F) thick arrow],two small anterolateral spines (F,thin arrows), a posterolateral margin bearing at least four small and equidistant tubercles (F,small arrows),straight posterior margin,slender coxae of the pereopods,a typical heterotreme eubrachyuran sternum (G),and a reduced and folded pleon with the first pleonites dorsally exposed.Left fifth pereopod digitally reattached.bcg,branchiocardiac groove; ca,carpus;cg, cervical groove;cx,coxa;da,dactylus;ib,ischiobasis;ma,manus or palm of claw;P1,claws or chelipeds;P2 to P5,pereopods or walking legs 2 to 5; po,pollex or fixed finger cheliped propodus; pr,propodus.Images by E.G.C. Figure by J.L.

Fig. 4. 3D reconstructions of Cretapsara athanata Luque gen. et sp. nov. holotype LYAM-9 and its gills. (A) A single micro-CT slice,coronal section near the base of eyestalks.(B) 3D mesh extracted from reconstructed micro-CT data in VGSTUDIO MAX,remeshed in MeshLab,and visualized using Autodesk Maya (see Materials and Methods); dorsal view showing the position of the gills.(C) Right lateral oblique view.(D) Close-up of (C) showing the right (bottom) and left (top) gills seen from the right side. (E and F) Close-up of the right gills seen from inside (E) and outside (F).Notice the branchial lamellae and the afferent/efferent vessels.Images by E.G.C.Figure by J.L.