From Kelley v. City of Albuquerque, 542 F.3d 802 (10th Cir. 2008):

[Unitherm.] The City contends that Ms. Kelley failed to establish a prima facie case of retaliation because the evidence is insufficient to show a causal connection between Ms. Kelley's participation in the EEOC proceedings and her termination. The City raised the causation issue in its Rule 50(a) motion to the district court, both at the close of Ms. Kelley's case-in-chief and at the close of all of the evidence. However, the City did not include the issue in its post-verdict Rule 50(b)/Rule 59 motion.... We conclude that this failure forecloses the City's challenge on appeal to the sufficiency of the evidence.

The Supreme Court has declared that a "failure to comply with Rule 50(b) forecloses [a party's] challenge to the sufficiency of the evidence." Unitherm Food Sys., Inc. v. Swift-Eckrich, Inc., 546 U.S. 394, 404, 407, 126 S. Ct. 980, 163 L. Ed. 2d 974 (2006). "[T]he precise subject matter of a party's Rule 50(a) motion--namely, its entitlement to judgment as a matter of law--cannot be appealed unless that motion is renewed pursuant to Rule 50(b)." ...

In Unitherm, the Court reasoned that district court's denial of a Rule 50(a) motion "cannot form the basis of . . . [an] appeal, because the denial of that motion [is] not error." ... More specifically, the denial does not decide anything; it just puts off resolution of the sufficiency-of-the-evidence question until after the verdict.... [S]ee Fed. R. Civ. P. 50(b) ("If the court does not grant a motion for judgment as a matter of law made under Rule 50(a), the court is considered to have submitted the action to the jury subject to the court's later deciding the legal questions raised by the motion." (emphasis added)). Thus, the City's failure to file a Rule 50(b) motion forecloses its appellate challenge on the causation issue. 15 See Unitherm, 546 U.S. at 406.

[Jurisdiction] [Footnote 15:]15 Unitherm stressed the mandatory nature of Rule 50(b) by repeatedly warning that a party's failure to file a Rule 50(b) motion left the appellate courts "powerless" to enter judgment in its favor.... At least one of our sister circuits has read this strong language regarding power as indicating that the a party's failure to file a Rule 50(b) motion deprives an appellate court of subject matter jurisdiction to review the sufficiency of the evidence. See Allison v. City of E. Lansing, 484 F.3d 874, 876 (6th Cir. 2007) ("Given Unitherm Food's holding, it is now clear that renewing the motion post-verdict is jurisdictional and cannot be waived."). The Sixth Circuit's view is open to question because of the Supreme Court's subsequent decision in Bowles v. Russell, 551 U.S. , 127 S. Ct. 2360, 2365, 168 L. Ed. 2d 96 (2007). The Court in Bowles concluded that Federal Rule of Appellate Procedure 4(a) is jurisdictional because it implements 28 U.S.C. § 2107.... We have interpreted Bowles to mean that only rules that implement statutory limits can be jurisdictional. United States v. Mitchell, 518 F.3d 740, 744 (10th Cir. 2008) ("Bowles . . . clarified that court-issued federal procedural rules not derived from statutes are not jurisdictional, but rather inflexible claim-processing rules."); United States v. Garduno, 506 F.3d 1287, 1290 (10th Cir. 2007) ("In Bowles v. Russell, the Supreme Court further clarified that whether a procedural rule's time limitations are claim-processing or jurisdictional hinges on whether the rule is grounded in a statute."); accord Metro. Life Ins. Co. v. Price, 501 F.3d 271, 278 (3d Cir. 2007) ("True 'jurisdictional' limitations are set by the Constitution and by Congress, not by rules of procedure or judge-made doctrine."). Rule 50(b) is not grounded in a statute. Accordingly, in a jurisdictional inquiry relating to it, the principles of Bowles would seemingly be implicated. However, we need not definitively decide this jurisdictional question--a matter of first impression--here. Therefore, we do not do so. Although "a federal court generally may not rule on the merits of a case without first determining that it has jurisdiction," the Supreme Court has recognized that "jurisdiction is vital only if the court proposes to issue a judgment on the merits." Sinochem Int'l Co. v. Malay. Int'l Shipping Corp., 549 U.S. 422, 127 S. Ct. 1184, 1191-92, 167 L. Ed. 2d 15 (2007) (alteration and internal quotation marks omitted) (quoting Intec USA, LLC v. Engle, 467 F.3d 1038, 1041 (7th Cir. 2006)). "[A] federal court has leeway to choose among threshold grounds for denying audience to a case on the merits." Id. at 1191 (internal quotation marks omitted...; see Arar v. Ashcroft, 532 F.3d 157, 172 (2d Cir. 2008). In denying the City an audience on its causation challenge under the rationale of Unitherm, we need not rule on the merits. Furthermore, we find additional support for a Unitherm-based resolution in our observation that employing Unitherm arguably is a "less burdensome course" than resolving the first-impression jurisdictional question, especially if that resolution potentially would cause us to part company with the Sixth Circuit. [Citations omitted.] Accordingly, we need not (and do not) decide here whether a party's failure to file a Rule 50(b) motion deprives us of jurisdiction over its sufficiency-of-the-evidence challenge.